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Title: The Factors of Organic Evolution

Author: Herbert Spencer

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                              THE FACTORS

                                   OF

                           ORGANIC EVOLUTION.


                                   BY

                            HERBERT SPENCER.

                    REPRINTED, WITH ADDITIONS, FROM
                       _THE NINETEENTH CENTURY_.

                               NEW YORK:
                        D. APPLETON AND COMPANY,
                        1, 8, AND 5 BOND STREET.
                                 1887.




                                PREFACE.


The two parts of which this Essay consists, originally published in _The
Nineteenth Century_ for April and May 1886 respectively, now reappear
with the assent of the proprietor and editor of that periodical, to whom
my thanks are due for his courtesy in giving it. Some passages of
considerable length which, with a view to needful brevity, were omitted
when the articles first appeared, have been restored.

Though the direct bearings of the arguments contained in this Essay are
biological, the argument contained in its first half has indirect
bearings upon Psychology, Ethics, and Sociology. My belief in the
profound importance of these indirect bearings, was originally a chief
prompter to set forth the argument; and it now prompts me to re-issue it
in permanent form.

Though mental phenomena of many kinds, and especially of the simpler
kinds, are explicable only as resulting from the natural selection of
favourable variations; yet there are, I believe, still more numerous
mental phenomena, including all those of any considerable complexity,
which cannot be explained otherwise than as results of the inheritance
of functionally-produced modifications. What theory of psychological
evolution is espoused, thus depends on acceptance or rejection of the
doctrine that not only in the individual, but in the successions of
individuals, use and disuse of parts produce respectively increase and
decrease of them.

Of course there are involved the conceptions we form of the genesis and
nature of our higher emotions; and, by implication, the conceptions we
form of our moral intuitions. If functionally-produced modifications are
inheritable, then the mental associations habitually produced in
individuals by experiences of the relations between actions and their
consequences, pleasurable or painful, may, in the successions of
individuals, generate innate tendencies to like or dislike such actions.
But if not, the genesis of such tendencies is, as we shall see, not
satisfactorily explicable.

That our sociological beliefs must also be profoundly affected by the
conclusions we draw on this point, is obvious. If a nation is modified
_en masse_ by transmission of the effects produced on the natures of its
members by those modes of daily activity which its institutions and
circumstances involve; then we must infer that such institutions and
circumstances mould its members far more rapidly and comprehensively
than they can do if the sole cause of adaptation to them is the more
frequent survival of individuals who happen to have varied in favourable
ways.

I will add only that, considering the width and depth of the effects
which acceptance of one or other of these hypotheses must have on our
views of Life, Mind, Morals, and Politics, the question—Which of them is
true? demands, beyond all other questions whatever, the attention of
scientific men.

 _Brighton, January, 1887._




                   THE FACTORS OF ORGANIC EVOLUTION.


                        [_April and May, 1886_]




                                   I.


Within the recollection of men now in middle life, opinion concerning
the derivation of animals and plants was in a chaotic state. Among the
unthinking there was tacit belief in creation by miracle, which formed
an essential part of the creed of Christendom; and among the thinking
there were two parties, each of which held an indefensible hypothesis.
Immensely the larger of these parties, including nearly all whose
scientific culture gave weight to their judgments, though not accepting
literally the theologically-orthodox doctrine, made a compromise between
that doctrine and the doctrines which geologists had established; while
opposed to them were some, mostly having no authority in science, who
held a doctrine which was heterodox both theologically and
scientifically. Professor Huxley, in his lecture on “The Coming of Age
of the Origin of Species,” remarks concerning the first of these parties
as follows:—

  “One-and-twenty years ago, in spite of the work commenced by Hutton
  and continued with rare skill and patience by Lyell, the dominant
  view of the past history of the earth was catastrophic. Great and
  sudden physical revolutions, wholesale creations and extinctions of
  living beings, were the ordinary machinery of the geological epic
  brought into fashion by the misapplied genius of Cuvier. It was
  gravely maintained and taught that the end of every geological epoch
  was signalised by a cataclysm, by which every living being on the
  globe was swept away, to be replaced by a brand-new creation when
  the world returned to quiescence. A scheme of nature which appeared
  to be modelled on the likeness of a succession of rubbers of whist,
  at the end of each of which the players upset the table and called
  for a new pack, did not seem to shock anybody.

  I may be wrong, but I doubt if, at the present time, there is a
  single responsible representative of these opinions left. The
  progress of scientific geology has elevated the fundamental
  principle of uniformitarianism, that the explanation of the past is
  to be sought in the study of the present, into the position of an
  axiom; and the wild speculations of the catastrophists, to which we
  all listened with respect a quarter of a century ago, would hardly
  find a single patient hearer at the present day.”

Of the party above referred to as not satisfied with this conception
described by Professor Huxley, there were two classes. The great
majority were admirers of the _Vestiges of the Natural History of
Creation_—a work which, while it sought to show that organic evolution
has taken place, contended that the cause of organic evolution, is “an
impulse” supernaturally “imparted to the forms of life, advancing them,
... through grades of organization.” Being nearly all very inadequately
acquainted with the facts, those who accepted the view set forth in the
_Vestiges_ were ridiculed by the well-instructed for being satisfied
with evidence, much of which was either invalid or easily cancelled by
counter-evidence, and at the same time they exposed themselves to the
ridicule of the more philosophical for being content with a supposed
explanation which was in reality no explanation: the alleged “impulse”
to advance giving us no more help in understanding the facts than does
Nature's alleged “abhorrence of a vacuum” help us to understand the
ascent of water in a pump. The remnant, forming the second of these
classes, was very small. While rejecting this mere verbal solution,
which both Dr. Erasmus Darwin and Lamarck had shadowed forth in other
language, there were some few who, rejecting also the hypothesis
indicated by both Dr. Darwin and Lamarck, that the promptings of desires
or wants produced growths of the parts subserving them, accepted the
single _vera causa_ assigned by these writers—the modification of
structures resulting from modification of functions. They recognized as
the sole process in organic development, the adaptation of parts and
powers consequent on the effects of use and disuse—that continual
moulding and re-moulding of organisms to suit their circumstances, which
is brought about by direct converse with such circumstances.

But while this cause accepted by these few is a true cause, since
unquestionably during the life of the individual organism changes of
function produce changes of structure; and while it is a tenable
hypothesis that changes of structure so produced are inheritable; yet it
was manifest to those not prepossessed, that this cause cannot with
reason be assigned for the greater part of the facts. Though in plants
there are some characters which may not irrationally be ascribed to the
direct effects of modified functions consequent on modified
circumstances, yet the majority of the traits presented by plants are
not to be thus explained. It is impossible that the thorns by which a
briar is in large measure defended against browsing animals, can have
been developed and moulded by the continuous exercise of their
protective actions; for in the first place, the great majority of the
thorns are never touched at all, and, in the second place, we have no
ground whatever for supposing that those which are touched are thereby
made to grow, and to take those shapes which render them efficient.
Plants which are rendered uneatable by the thick woolly coatings of
their leaves, cannot have had these coatings produced by any process of
reaction against the action of enemies; for there is no imaginable
reason why, if one part of a plant is eaten, the rest should thereafter
begin to develop the hairs on its surface. By what direct effect of
function on structure, can the shell of a nut have been evolved? Or how
can those seeds which contain essential oils, rendering them unpalatable
to birds, have been made to secrete such essential oils by these actions
of birds which they restrain? Or how can the delicate plumes borne by
some seeds, and giving the wind power to waft them to new stations, be
due to any immediate influences of surrounding conditions? Clearly in
these and in countless other cases, change of structure cannot have been
directly caused by change of function. So is it with animals to a large
extent, if not to the same extent. Though we have proof that by rough
usage the dermal layer may be so excited as to produce a greatly
thickened epidermal layer, sometimes quite horny; and though it is a
feasible hypothesis that an effect of this kind persistently produced
may be inherited; yet no such cause can explain the carapace of the
turtle, the armour of the armadillo, or the imbricated covering of the
manis. The skins of these animals are no more exposed to habitual hard
usage than are those of animals covered by hair. The strange
excrescences which distinguish the heads of the hornbills, cannot
possibly have arisen from any reaction against the action of surrounding
forces; for even were they clearly protective, there is no reason to
suppose that the heads of these birds need protection more than the
heads of other birds. If, led by the evidence that in animals the amount
of covering is in some cases affected by the degree of exposure, it were
admitted as imaginable that the development of feathers from preceding
dermal growths had resulted from that extra nutrition caused by extra
superficial circulation, we should still be without explanation of the
structure of a feather. Nor should we have any clue to the specialities
of feathers—the crests of various birds, the tails sometimes so
enormous, the curiously placed plumes of the bird of paradise, &c., &c.
Still more obviously impossible is it to explain as due to use or disuse
the colours of animals. No direct adaptation to function could have
produced the blue protuberances on a mandril's face, or the striped hide
of a tiger, or the gorgeous plumage of a kingfisher, or the eyes in a
peacock's tail, or the multitudinous patterns of insects' wings. One
single case, that of a deer's horns, might alone have sufficed to show
how insufficient was the assigned cause. During their growth, a deer's
horns are not used at all; and when, having been cleared of the dead
skin and dried-up blood-vessels covering them, they are ready for use,
they are nerveless and non-vascular, and hence are incapable of
undergoing any changes of structure consequent on changes of function.

Of these few then, who rejected the belief described by Professor
Huxley, and who, espousing the belief in a continuous evolution, had to
account for this evolution, it must be said that though the cause
assigned was a true cause, yet, even admitting that it operated through
successive generations, it left unexplained the greater part of the
facts. Having been myself one of these few, I look back with surprise at
the way in which the facts which were congruous with the espoused view
monopolized consciousness and kept out the facts which were incongruous
with it—conspicuous though many of them were. The misjudgment was not
unnatural. Finding it impossible to accept any doctrine which implied a
breach in the uniform course of natural causation, and, by implication,
accepting as unquestionable the origin and development of all organic
forms by accumulated modifications naturally caused, that which appeared
to explain certain classes of these modifications, was supposed to be
capable of explaining the rest: the tendency being to assume that these
would eventually be similarly accounted for, though it was not clear
how.

Returning from this parenthetic remark, we are concerned here chiefly to
remember that, as said at the outset, there existed thirty years ago, no
tenable theory about the genesis of living things. Of the two
alternative beliefs, neither would bear critical examination.

                  *       *       *       *       *

Out of this dead lock we were released—in large measure, though not I
believe entirely—by the _Origin of Species_. That work brought into view
a further factor; or rather, such factor, recognized as in operation by
here and there an observer (as pointed out by Mr. Darwin in his
introduction to the second edition), was by him for the first time seen
to have played so immense a part in the genesis of plants and animals.

Though laying myself open to the charge of telling a thrice-told tale, I
feel obliged here to indicate briefly the several great classes of facts
which Mr. Darwin's hypothesis explains; because otherwise that which
follows would scarcely be understood. And I feel the less hesitation in
doing this because the hypothesis which it replaced, not very widely
known at any time, has of late so completely dropped into the
background, that the majority of readers are scarcely aware of its
existence, and do not therefore understand the relation between Mr.
Darwin's successful interpretation and the preceding unsuccessful
attempt at interpretation. Of these classes of facts, four chief ones
may be here distinguished.

In the first place, such adjustments as those exemplified above are made
comprehensible. Though it is inconceivable that a structure like that of
the pitcher-plant could have been produced by accumulated effects of
function on structure; yet it is conceivable that successive selections
of favourable variations might have produced it; and the like holds of
the no less remarkable appliance of the Venus's Fly-trap, or the still
more astonishing one of that water-plant by which infant-fish are
captured. Though it is impossible to imagine how, by direct influence of
increased use, such dermal appendages as a porcupine's quills could have
been developed; yet, profiting as the members of a species otherwise
defenceless might do by the stiffness of their hairs, rendering them
unpleasant morsels to eat, it is a feasible supposition that from
successive survivals of individuals thus defended in the greatest
degrees, and the consequent growth in successive generations of hairs
into bristles, bristles into spines, spines into quills (for all these
are homologous), this change could have arisen. In like manner, the odd
inflatable bag of the bladder-nosed seal, the curious fishing-rod with
its worm-like appendage carried on the head of the _lophius_ or angler,
the spurs on the wings of certain birds, the weapons of the sword-fish
and saw-fish, the wattles of fowls, and numberless such peculiar
structures, though by no possibility explicable as due to effects of use
or disuse, are explicable as resulting from natural selection operating
in one or other way.

In the second place, while showing us how there have arisen countless
modifications in the forms, structures, and colours of each part, Mr.
Darwin has shown us how, by the establishment of favourable variations,
there may arise new parts. Though the first step in the production of
horns on the heads of various herbivorous animals, may have been the
growth of callosities consequent on the habit of butting—such
callosities thus functionally initiated being afterwards developed in
the most advantageous ways by selection; yet no explanation can be thus
given of the sudden appearance of a duplicate set of horns, as
occasionally happens in sheep: an addition which, where it proved
beneficial, might readily be made a permanent trait by natural
selection. Again, the modifications which follow use and disuse can by
no possibility account for changes in the numbers of vertebræ; but after
recognizing spontaneous, or rather fortuitous, variation as a factor, we
can see that where an additional vertebra hence resulting (as in some
pigeons) proves beneficial, survival of the fittest may make it a
constant character; and there may, by further like additions, be
produced extremely long strings of vertebræ, such as snakes show us.
Similarly with the mammary glands. It is not an unreasonable supposition
that by the effects of greater or less function, inherited through
successive generations, these may be enlarged or diminished in size; but
it is out of the question to allege such a cause for changes in their
numbers. There is no imaginable explanation of these save the
establishment by inheritance of spontaneous variations, such as are
known to occur in the human race.

So too, in the third place, with certain alterations in the connections
of parts. According to the greater or smaller demands made on this or
that limb, the muscles moving it may be augmented or diminished in bulk;
and, if there is inheritance of changes so wrought, the limb may, in
course of generations, be rendered larger or smaller. But changes in the
arrangements or attachments of muscles cannot be thus accounted for. It
is found, especially at the extremities, that the relations of tendons
to bones and to one another are not always the same. Variations in their
modes of connection may occasionally prove advantageous, and may thus
become established. Here again, then, we have a class of structural
changes to which Mr. Darwin's hypothesis gives us the key, and to which
there is no other key.

Once more there are the phenomena of mimicry. Perhaps in a more striking
way than any others, these show how traits which seem inexplicable are
explicable as due to the more frequent survival of individuals that have
varied in favorable ways. We are enabled to understand such marvelous
simulations as those of the leaf-insect, those of beetles which
“resemble glittering dew-drops upon the leaves;” those of caterpillars
which, when asleep, stretch themselves out so as to look like twigs. And
we are shown how there have arisen still more astonishing
imitations—those of one insect by another. As Mr. Bates has proved,
there are cases in which a species of butterfly, rendered so unpalatable
to insectivorous birds by its disagreeable taste that they will not
catch it, is simulated in its colors and markings by a species which is
structurally quite different—so simulated that even a practiced
entomologist is liable to be deceived: the explanation being that an
original slight resemblance, leading to occasional mistakes on the part
of birds, was increased generation after generation by the more frequent
escape of the most-like individuals, until the likeness became thus
great.

But now, recognizing in full this process brought into clear view by Mr.
Darwin, and traced out by him with so much care and skill, can we
conclude that, taken alone, it accounts for organic evolution? Has the
natural selection of favourable variations been the sole factor? On
critically examining the evidence, we shall find reason to think that it
by no means explains all that has to be explained. Omitting for the
present any consideration of a factor which may be distinguished as
primordial, it may be contended that the above-named factor alleged by
Dr. Erasmus Darwin and by Lamarck, must be recognized as a co-operator.
Utterly inadequate to explain the major part of the facts as is the
hypothesis of the inheritance of functionally-produced modifications,
yet there is a minor part of the facts, very extensive though less,
which must be ascribed to this cause.

                  *       *       *       *       *

When discussing the question more than twenty years ago (_Principles of
Biology_, § 166), I instanced the decreased size of the jaws in the
civilized races of mankind, as a change not accounted for by the natural
selection of favourable variations; since no one of the decrements by
which, in thousands of years, this reduction has been effected, could
have given to an individual in which it occurred, such advantage as
would cause his survival, either through diminished cost of local
nutrition or diminished weight to be carried. I did not then exclude, as
I might have done, two other imaginable causes. It may be said that
there is some organic correlation between increased size of brain and
decreased size of jaw: Camper's doctrine of the facial angle being
referred to in proof. But this argument may be met by pointing to the
many examples of small-jawed people who are also small-brained, and by
citing not infrequent cases of individuals remarkable for their mental
powers, and at the same time distinguished by jaws not less than the
average but greater. Again, if sexual selection be named as a possible
cause, there is the reply that, even supposing such slight diminution of
jaw as took place in a single generation to have been an attraction, yet
the other incentives to choice on the part of men have been too many and
great to allow this one to weigh in an adequate degree; while, during
the greater portion of the period, choice on the part of women has
scarcely operated: in earlier times they were stolen or bought, and in
later times mostly coerced by parents. Thus, reconsideration of the
facts does not show me the invalidity of the conclusion drawn, that this
decrease in size of jaw can have had no other cause than continued
inheritance of those diminutions consequent on diminutions of function,
implied by the use of selected and well-prepared food. Here, however, my
chief purpose is to add an instance showing, even more clearly, the
connexion between change of function and change of structure. This
instance, allied in nature to the other, is presented by those
varieties, or rather sub-varieties, of dogs, which, having been
household pets, and habitually fed on soft food, have not been called on
to use their jaws in tearing and crunching, and have been but rarely
allowed to use them in catching prey and in fighting. No inference can
be drawn from the sizes of the jaws themselves, which, in these dogs,
have probably been shortened mainly by selection. To get direct proof of
the decrease of the muscles concerned in closing the jaws or biting,
would require a series of observations very difficult to make. But it is
not difficult to get indirect proof of this decrease by looking at the
bony structures with which these muscles are connected. Examination of
the skulls of sundry indoor dogs contained in the Museum of the College
of Surgeons, proves the relative smallness of such parts. The only
pug-dog's skull is that of an individual not perfectly adult; and though
its traits are quite to the point they cannot with safety be taken as
evidence. The skull of a toy-terrier has much restricted areas of
insertion for the temporal muscles; has weak zygomatic arches; and has
extremely small attachments for the masseter muscles. Still more
significant is the evidence furnished by the skull of a King Charles's
spaniel, which, if we allow three years to a generation, and bear in
mind that the variety must have existed before Charles the Second's
reign, we may assume belongs to something approaching to the hundredth
generation of these household pets. The relative breadth between the
outer surfaces of the zygomatic arches is conspicuously small; the
narrowness of the temporal fossæ is also striking; the zygomata are very
slender; the temporal muscles have left no marks whatever, either by
limiting lines or by the character of the surfaces covered; and the
places of attachment for the masseter muscles are very feebly developed.
At the Museum of Natural History, among skulls of dogs there is one
which, though unnamed, is shown by its small size and by its teeth, to
have belonged to one variety or other of lap-dogs, and which has the
same traits in an equal degree with the skull just described. Here,
then, we have two if not three kinds of dogs which, similarly leading
protected and pampered lives, show that in the course of generations the
parts concerned in clenching the jaws have dwindled. To what cause must
this decrease be ascribed? Certainly not to artificial selection; for
most of the modifications named make no appreciable external signs: the
width across the zygomata could alone be perceived. Neither can natural
selection have had anything to do with it; for even were there any
struggle for existence among such dogs, it cannot be contended that any
advantage in the struggle could be gained by an individual in which a
decrease took place. Economy of nutrition, too, is excluded. Abundantly
fed as such dogs are, the constitutional tendency is to find places
where excess of absorbed nutriment may be conveniently deposited, rather
than to find places where some cutting down of the supplies is
practicable. Nor again can there be alleged a possible correlation
between these diminutions and that shortening of the jaws which has
probably resulted from selection; for in the bull-dog, which has also
relatively short jaws, these structures concerned in closing them are
unusually large. Thus there remains as the only conceivable cause, the
diminution of size which results from diminished use. The dwindling of a
little-exercised part has, by inheritance, been made more and more
marked in successive generations.

                  *       *       *       *       *

Difficulties of another class may next be exemplified—those which
present themselves when we ask how there can be effected by the
selection of favourable variations, such changes of structure as adapt
an organism to some useful action in which many different parts
co-operate. None can fail to see how a simple part may, in course of
generations, be greatly enlarged, if each enlargement furthers, in some
decided way, maintenance of the species. It is easy to understand, too,
how a complex part, as an entire limb, may be increased as a whole by
the simultaneous due increase of its co-operative parts; since if, while
it is growing, the channels of supply bring to the limb an unusual
quantity of blood, there will naturally result a proportionately greater
size of all its components—bones, muscles, arteries, veins, &c. But
though in cases like this, the co-operative parts forming some large
complex part may be expected to vary together, nothing implies that they
necessarily do so; and we have proof that in various cases, even when
closely united, they do not do so. An example is furnished by those
blind crabs named in the _Origin of Species_ which inhabit certain dark
caves of Kentucky, and which, though they have lost their eyes, have not
lost the foot-stalks which carried their eyes. In describing the
varieties which have been produced by pigeon-fanciers, Mr. Darwin notes
the fact that along with changes in length of beak produced by
selection, there have not gone proportionate changes in length of
tongue. Take again the case of teeth and jaws. In mankind these have not
varied together. During civilization the jaws have decreased, but the
teeth have not decreased in proportion; and hence that prevalent
crowding of them, often remedied in childhood by extraction of some, and
in other cases causing that imperfect development which is followed by
early decay. But the absence of proportionate variation in co-operative
parts that are close together, and are even bound up in the same mass,
is best seen in those varieties of dogs named above as illustrating the
inherited effects of disuse. We see in them, as we see in the human
race, that diminution in the jaws has not been accompanied by
corresponding diminution in the teeth. In the catalogue of the College
of Surgeons Museum, there is appended to the entry which identifies a
Blenheim Spaniel's skull, the words—“the teeth are closely crowded
together,” and to the entry concerning the skull of a King Charles's
Spaniel the words—“the teeth are closely packed, p. 3, is placed quite
transversely to the axis of the skull.” It is further noteworthy that in
a case where there is no diminished use of the jaws, but where they have
been shortened by selection, a like want of concomitant variation is
manifested: the case being that of the bull-dog, in the upper jaw of
which also, “the premolars ... are excessively crowded, and placed
obliquely or even transversely to the long axis of the skull.”[1]

If, then, in cases where we can test it, we find no concomitant
variation in co-operative parts that are near together—if we do not find
it in parts which, though belonging to different tissues, are so closely
united as teeth and jaws—if we do not find it even when the co-operative
parts are not only closely united, but are formed out of the same
tissue, like the crab's eye and its peduncle; what shall we say of
co-operative parts which, besides being composed of different tissues,
are remote from one another? Not only are we forbidden to assume that
they vary together, but we are warranted in asserting that they can have
no tendency to vary together. And what are the implications in cases
where increase of a structure can be of no service unless there is
concomitant increase in many distant structures, which have to join it
in performing the action for which it is useful?

As far back as 1864 (_Principles of Biology_, § 166) I named in
illustration an animal carrying heavy horns—the extinct Irish elk; and
indicated the many changes in bones, muscles, blood-vessels, nerves,
composing the fore-part of the body, which would be required to make an
increment of size in such horns advantageous. Here let me take another
instance—that of the giraffe: an instance which I take partly because,
in the sixth edition of the _Origin of Species_, issued in 1872, Mr.
Darwin has referred to this animal when effectually disposing of certain
arguments urged against his hypothesis. He there says:—

  “In order that an animal should acquire some structure specially and
  largely developed, it is almost indispensable that several other
  parts should be modified and co-adapted. Although every part of the
  body varies slightly, it does not follow that the necessary parts
  should always vary in the right direction and to the right degree”
  (p. 179).

And in the summary of the chapter, he remarks concerning the adjustments
in the same quadruped, that “the prolonged use of all the parts together
with inheritance will have aided in an important manner in their
co-ordination” (p. 199): a remark probably having reference chiefly to
the increased massiveness of the lower part of the neck; the increased
size and strength of the thorax required to bear the additional burden;
and the increased strength of the fore-legs required to carry the
greater weight of both. But now I think that further consideration
suggests the belief that the entailed modifications are much more
numerous and remote than at first appears; and that the greater part of
these are such as cannot be ascribed in any degree to the selection of
favourable variations, but must be ascribed exclusively to the inherited
effects of changed functions. Whoever has seen a giraffe gallop will
long remember the sight as a ludicrous one. The reason for the
strangeness of the motions is obvious. Though the fore limbs and the
hind limbs differ so much in length, yet in galloping they have to keep
pace—must take equal strides. The result is that at each stride, the
angle which the hind limbs describe round their centre of motion is much
larger than the angle described by the fore limbs. And beyond this, as
an aid in equalizing the strides, the hind part of the back is at each
stride bent very much downwards and forwards. Hence the hind-quarters
appear to be doing nearly all the work. Now a moment's observation shows
that the bones and muscles composing the hind-quarters of the giraffe,
perform actions differing in one or other way and degree, from the
actions performed by the homologous bones and muscles in a mammal of
ordinary proportions, and from those in the ancestral mammal which gave
origin to the giraffe. Each further stage of that growth which produced
the large fore-quarters and neck, entailed some adapted change in sundry
of the numerous parts composing the hind-quarters; since any failure in
the adjustment of their respective strengths would entail some defect in
speed and consequent loss of life when chased. It needs but to remember
how, when continuing to walk with a blistered foot, the taking of steps
in such a modified way as to diminish pressure on the sore point, soon
produces aching of muscles which are called into unusual action, to see
that over-straining of any one of the muscles of the giraffe's
hind-quarters might quickly incapacitate the animal when putting out all
its powers to escape; and to be a few yards behind others would cause
death. Hence if we are debarred from assuming that co-operative parts
vary together even when adjacent and closely united—if we are still more
debarred from assuming that with increased length of fore-legs or of
neck, there will go an appropriate change in any one muscle or bone in
the hind-quarters; how entirely out of the question it is to assume that
there will simultaneously take place the appropriate changes in _all_
those many components of the hind-quarters which severally require
re-adjustment. It is useless to reply that an increment of length in the
fore-legs or neck might be retained and transmitted to posterity,
waiting an appropriate variation in a particular bone or muscle in the
hind-quarters, which, being made, would allow of a further increment.
For besides the fact that until this secondary variation occurred the
primary variation would be a disadvantage often fatal; and besides the
fact that before such an appropriate secondary variation might be
expected in the course of generations to occur, the primary variation
would have died out; there is the fact that the appropriate variation of
one bone or muscle in the hind-quarters would be useless without
appropriate variations of all the rest—some in this way and some in
that—a number of appropriate variations which it is impossible to
suppose.

Nor is this all. Far more numerous appropriate variations would be
indirectly necessitated. The immense change in the ratio of
fore-quarters to hind-quarters would make requisite a corresponding
change of ratio in the appliances carrying on the nutrition of the two.
The entire vascular system, arterial and veinous, would have to undergo
successive unbuildings and rebuildings to make its channels everywhere
adequate to the local requirements; since any want of adjustment in the
blood-supply in this or that set of muscles, would entail incapacity,
failure of speed, and loss of life. Moreover the nerves supplying the
various sets of muscles would have to be proportionately changed; as
well as the central nervous tracts from which they issued. Can we
suppose that all these appropriate changes, too, would be step by step
simultaneously made by fortunate spontaneous variations, occurring along
with all the other fortunate spontaneous variations? Considering how
immense must be the number of these required changes, added to the
changes above enumerated, the chances against any adequate
re-adjustments fortuitously arising must be infinity to one.

If the effects of use and disuse of parts are inheritable, then any
change in the fore parts of the giraffe which affects the action of the
hind limbs and back, will simultaneously cause, by the greater or less
exercise of it, a re-moulding of each component in the hind limbs and
back in a way adapted to the new demands; and generation after
generation the entire structure of the hind-quarters will be
progressively fitted to the changed structure of the fore-quarters: all
the appliances for nutrition and innervation being at the same time
progressively fitted to both. But in the absence of this inheritance of
functionally-produced modifications, there is no seeing how the required
re-adjustments can be made.

                  *       *       *       *       *

Yet a third class of difficulties stands in the way of the belief that
the natural selection of useful variations is the sole factor of organic
evolution. This class of difficulties, already pointed out in § 166 of
the _Principles of Biology_, I cannot more clearly set forth than in the
words there used. Hence I may perhaps be excused for here quoting them.

  “Where the life is comparatively simple, or where surrounding
  circumstances render some one function supremely important, the
  survival of the fittest may readily bring about the appropriate
  structural change, without any aid from the transmission of
  functionally-acquired modifications. But in proportion as the life
  grows complex—in proportion as a healthy existence cannot be secured
  by a large endowment of some one power, but demands many powers; in
  the same proportion do there arise obstacles to the increase of any
  particular power, by “the preservation of favoured races in the
  struggle for life.” As fast as the faculties are multiplied, so fast
  does it become possible for the several members of a species to have
  various kinds of superiorities over one another. While one saves its
  life by higher speed, another does the like by clearer vision,
  another by keener scent, another by quicker hearing, another by
  greater strength, another by unusual power of enduring cold or
  hunger, another by special sagacity, another by special timidity,
  another by special courage; and others by other bodily and mental
  attributes. Now it is unquestionably true that, other things equal,
  each of these attributes, giving its possessor an extra chance of
  life, is likely to be transmitted to posterity. But there seems no
  reason to suppose that it will be increased in subsequent
  generations by natural selection. That it may be thus increased, the
  individuals not possessing more than average endowments of it, must
  be more frequently killed off than individuals highly endowed with
  it; and this can happen only when the attribute is one of greater
  importance, for the time being, than most of the other attributes.
  If those members of the species which have but ordinary shares of
  it, nevertheless survive by virtue of other superiorities which they
  severally possess; then it is not easy to see how this particular
  attribute can be developed by natural selection in subsequent
  generations. The probability seems rather to be, that by
  gamogenesis, this extra endowment will, on the average, be
  diminished in posterity—just serving in the long run to compensate
  the deficient endowments of other individuals, whose special powers
  lie in other directions; and so to keep up the normal structure of
  the species. The working out of the process is here somewhat
  difficult to follow; but it appears to me that as fast as the number
  of bodily and mental faculties increases, and as fast as the
  maintenance of life comes to depend less on the amount of any one,
  and more on the combined action of all; so fast does the production
  of specialities of character by natural selection alone, become
  difficult. Particularly does this seem to be so with a species so
  multitudinous in its powers as mankind; and above all does it seem
  to be so with such of the human powers as have but minor shares in
  aiding the struggle for life—the æsthetic faculties, for example.”

Dwelling for a moment on this last illustration of the class of
difficulties described, let us ask how we are to interpret the
development of the musical faculty. I will not enlarge on the family
antecedents of the great composers. I will merely suggest the inquiry
whether the greater powers possessed by Beethoven and Mozart, by Weber
and Rossini, than by their fathers, were not due in larger measure to
the inherited effects of daily exercise of the musical faculty by their
fathers, than to inheritance, with increase, of spontaneous variations;
and whether the diffused musical powers of the Bach clan, culminating in
those of Johann Sebastian, did not result in part from constant
practice; but I will raise the more general question—How came there that
endowment of musical faculty which characterizes modern Europeans at
large, as compared with their remote ancestors. The monotonous chants of
low savages cannot be said to show any melodic inspiration; and it is
not evident that an individual savage who had a little more musical
perception than the rest, would derive any such advantage in the
maintenance of life as would secure the spread of his superiority by
inheritance of the variation. And then what are we to say of harmony? We
cannot suppose that the appreciation of this, which is relatively
modern, can have arisen by descent from the men in whom successive
variations increased the appreciation of it—the composers and musical
performers; for on the whole, these have been men whose worldly
prosperity was not such as enabled them to rear many children inheriting
their special traits. Even if we count the illegitimate ones, the
survivors of these added to the survivors of the legitimate ones, can
hardly be held to have yielded more than average numbers of descendants;
and those who inherited their special traits have not often been thereby
so aided in the struggle for existence as to further the spread of such
traits. Rather the tendency seems to have been the reverse.

Since the above passage was written, I have found in the second volume
of _Animals and Plants under Domestication_, a remark made by Mr.
Darwin, practically implying that among creatures which depend for their
lives on the efficiency of numerous powers, the increase of any one by
the natural selection of a variation is necessarily difficult. Here it
is.

  “Finally, as indefinite and almost illimitable variability is the
  usual result of domestication and cultivation, with the same part or
  organ varying in different individuals in different or even in
  directly opposite ways; and as the same variation, if strongly
  pronounced, usually recurs only after long intervals of time, any
  particular variation would generally be lost by crossing, reversion,
  and the accidental destruction of the varying individuals, unless
  carefully preserved by man.”—Vol. ii, 292.

Remembering that mankind, subject as they are to this domestication and
cultivation, are not, like domesticated animals, under an agency which
picks out and preserves particular variations; it results that there
must usually be among them, under the influence of natural selection
alone, a continual disappearance of any useful variations of particular
faculties which may arise. Only in cases of variations which are
specially preservative, as for example, great cunning during a
relatively barbarous state, can we expect increase from natural
selection alone. We cannot suppose that minor traits, exemplified among
others by the æsthetic perceptions, can have been evolved by natural
selection. But if there is inheritance of functionally-produced
modifications of structure, evolution of such minor traits is no longer
inexplicable.

                  *       *       *       *       *

Two remarks made by Mr. Darwin have implications from which the same
general conclusion must, I think, be drawn. Speaking of the variability
of animals and plants under domestication, he says:—

  “Changes of any kind in the conditions of life, even extremely
  slight changes, often suffice to cause variability.... Animals and
  plants continue to be variable for an immense period after their
  first domestication; ... In the course of time they can be
  habituated to certain changes, so as to become less variable; ...
  There is good evidence that the power of changed conditions
  accumulates; so that two, three, or more generations must be exposed
  to new conditions before any effect is visible.... Some variations
  are induced by the direct action of the surrounding conditions on
  the whole organization, or on certain parts alone, and other
  variations are induced indirectly through the reproductive system
  being affected in the same manner as is so common with organic
  beings when removed from their natural conditions.”—(_Animals and
  Plants under Domestication_, vol. ii, 270.)

There are to be recognized two modes of this effect produced by changed
conditions on the reproductive system, and consequently on offspring.
Simple arrest of development is one. But beyond the variations of
offspring arising from imperfectly developed reproductive systems in
parents—variations which must be ordinarily in the nature of
imperfections—there are others due to a changed balance of functions
caused by changed conditions. The fact noted by Mr. Darwin in the above
passage, “that the power of changed conditions accumulates; so that two,
three, or more generations must be exposed to new conditions before any
effect is visible,” implies that during these generations there is going
on some change of constitution consequent on the changed proportions and
relations of the functions. I will not dwell on the implication, which
seems tolerably clear, that this change must consist of such
modifications of organs as adapt them to their changed functions; and
that if the influence of changed conditions “accumulates,” it must be
through the inheritance of such modifications. Nor will I press the
question—What is the nature of the effect registered in the reproductive
elements, and which is subsequently manifested by variations?—Is it an
effect entirely irrelevant to the new requirements of the variety?—Or is
it an effect which makes the variety less fit for the new
requirements?—Or is it an effect which makes it more fit for the new
requirements? But not pressing these questions, it suffices to point out
the necessary implication that changed functions of organs _do_, in some
way or other, register themselves in changed proclivities of the
reproductive elements. In face of these facts it cannot be denied that
the modified action of a part produces an inheritable effect—be the
nature of that effect what it may.

The second of the remarks above adverted to as made by Mr. Darwin, is
contained in his sections dealing with correlated variations. In the
_Origin of Species_, p. 114, he says—

  “The whole organization is so tied together during its growth and
  development, that when slight variations in any one part occur, and
  are accumulated through natural selection, other parts become
  modified.”

And a parallel statement contained in _Animals and Plants under
Domestication_, vol. ii, p. 320, runs thus—

  “Correlated variation is an important subject for us; for when one
  part is modified through continued selection, either by man or under
  nature, other parts of the organization will be unavoidably
  modified. From this correlation it apparently follows that, with our
  domesticated animals and plants, varieties rarely or never differ
  from each other by some single character alone.”

By what process does a changed part modify other parts? By modifying
their functions in some way or degree, seems the necessary answer. It is
indeed, imaginable, that where the part changed is some dermal appendage
which, becoming larger, has abstracted more of the needful material from
the general stock, the effect may consist simply in diminishing the
amount of this material available for other dermal appendages, leading
to diminution of some or all of them, and may fail to affect in
appreciable ways the rest of the organism: save perhaps the
blood-vessels near the enlarged appendage. But where the part is an
active one—a limb, or viscus, or any organ which constantly demands
blood, produces waste matter, secretes, or absorbs—then all the other
active organs become implicated in the change. The functions performed
by them have to constitute a moving equilibrium; and the function of one
cannot, by alteration of the structure performing it, be modified in
degree or kind, without modifying the functions of the rest—some
appreciably and others inappreciably, according to the directness or
indirectness of their relations. Of such inter-dependent changes, the
normal ones are naturally inconspicuous; but those which are partially
or completely abnormal, sufficiently carry home the general truth. Thus,
unusual cerebral excitement affects the excretion through the kidneys in
quantity or quality or both. Strong emotions of disagreeable kinds check
or arrest the flow of bile. A considerable obstacle to the circulation
offered by some important structure in a diseased or disordered state,
throwing more strain upon the heart, causes hypertrophy of its muscular
walls; and this change which is, so far as concerns the primary evil, a
remedial one, often entails mischiefs in other organs. “Apoplexy and
palsy, in a scarcely credible number of cases, are directly dependent on
hypertrophic enlargement of the heart.” And in other cases, asthma,
dropsy, and epilepsy are caused. Now if a result of this
inter-dependence as seen in the individual organism, is that a local
modification of one part produces, by changing their functions,
correlative modifications of other parts, then the question here to be
put is—Are these correlative modifications, when of a kind falling
within normal limits, inheritable or not. If they are inheritable, then
the fact stated by Mr. Darwin that “when one part is modified through
continued selection,” “other parts of the organization will be
unavoidably modified” is perfectly intelligible: these entailed
secondary modifications are transmitted _pari passu_ with the successive
modifications produced by selection. But what if they are not
inheritable? Then these secondary modifications caused in the
individual, not being transmitted to descendants, the descendants must
commence life with organizations out of balance, and with each increment
of change in the part affected by selection, their organizations must
get more out of balance—must have a larger and larger amounts of
re-organization to be made during their lives. Hence the constitution of
the variety must become more and more unworkable.

The only imaginable alternative is that the re-adjustments are effected
in course of time by natural selection. But, in the first place, as we
find no proof of concomitant variation among directly co-operative parts
which are closely united, there cannot be assumed any concomitant
variation among parts which are both indirectly co-operative and far
from one another. And, in the second place, before all the many required
re-adjustments could be made, the variety would die out from defective
constitution. Even were there no such difficulty, we should still have
to entertain a strange group of propositions, which would stand as
follows:—1. Change in one part entails, by reaction on the organism,
changes, in other parts, the functions of which are necessarily changed.
2. Such changes worked in the individual, affect, in some way, the
reproductive elements: these being found to evolve unusual structures
when the constitutional balance has been continuously disturbed. 3. But
the changes in the reproductive elements thus caused, are not such as
represent these functionally-produced changes: the modifications
conveyed to offspring are irrelevant to these various modifications
functionally produced in the organs of the parents. 4. Nevertheless,
while the balance of functions cannot be re-established through
inheritance of the effects of disturbed functions on structures, wrought
throughout the individual organism; it can be re-established by the
inheritance of fortuitous variations which occur in all the affected
organs without reference to these changes of function.

Now without saying that acceptance of this group of propositions is
impossible, we may certainly say that it is not easy.

                  *       *       *       *       *

“But where are the direct proofs that inheritance of
functionally-produced modifications takes place?” is a question which
will be put by those who have committed themselves to the current
exclusive interpretation. “Grant that there are difficulties; still,
before the transmitted effects of use and disuse can be legitimately
assigned in explanation of them, we must have good evidence that the
effects of use and disuse _are_ transmitted.”

Before dealing directly with this demurrer, let me deal with it
indirectly, by pointing out that the lack of recognized evidence may be
accounted for without assuming that there is not plenty of it.
Inattention and reluctant attention lead to the ignoring of facts which
really exist in abundance; as is well illustrated in the case of
pre-historic implements. Biassed by the current belief that no traces of
man were to be found on the Earth's surface, save in certain superficial
formations of very recent date, geologists and anthropologists not only
neglected to seek such traces, but for a long time continued to
pooh-pooh those who said they had found them. When M. Boucher de Perthes
at length succeeded in drawing the eyes of scientific men to the flint
implements discovered by him in the quarternary deposits of the Somme
valley; and when geologists and anthropologists had thus been convinced
that evidences of human existence were to be found in formations of
considerable age, and thereafter began to search for them; they found
plenty of them all over the world. Or again, to take an instance closely
germane to the matter, we may recall the fact that the contemptuous
attitude towards the hypothesis of organic evolution which naturalists
in general maintained before the publication of Mr. Darwin's work,
prevented them from seeing the multitudinous facts by which it is
supported. Similarly, it is very possible that their alienation from the
belief that there is a transmission of those changes of structure which
are produced by changes of action, makes naturalists slight the evidence
which supports that belief and refuse to occupy themselves in seeking
further evidence.

If it be asked how it happens that there have been recorded
multitudinous instances of variations fortuitously arising and
re-appearing in offspring, while there have not been recorded instances
of the transmission of changes functionally produced, there are three
replies. The first is that changes of the one class are many of them
conspicuous, while those of the other class are nearly all
inconspicuous. If a child is born with six fingers, the anomaly is not
simply obvious but so startling as to attract much notice; and if this
child, growing up, has six-fingered descendants, everybody in the
locality hears of it. A pigeon with specially-coloured feathers, or one
distinguished by a broadened and upraised tail, or by a protuberance of
the neck, draws attention by its oddness; and if in its young the trait
is repeated, occasionally with increase, the fact is remarked, and there
follows the thought of establishing the peculiarity by selection. A lamb
disabled from leaping by the shortness of its legs, could not fail to be
observed; and the fact that its offspring were similarly short-legged,
and had a consequent inability to get over fences, would inevitably
become widely known. Similarly with plants. That this flower had an
extra number of petals, that that was unusually symmetrical, and that
another differed considerably in colour from the average of its kind,
would be easily seen by an observant gardener; and the suspicion that
such anomalies are inheritable having arisen, experiments leading to
further proofs that they are so, would frequently be made. But it is not
thus with functionally-produced modifications. The seats of these are in
nearly all cases the muscular, osseous, and nervous systems, and the
viscera—parts which are either entirely hidden or greatly obscured.
Modification in a nervous centre is inaccessible to vision; bones may be
considerably altered in size or shape without attention being drawn to
them; and, covered with thick coats as are most of the animals open to
continuous observation, the increases or decreases in muscles must be
great before they become externally perceptible.

A further important difference between the two inquiries is that to
ascertain whether a fortuitous variation is inheritable, needs merely a
little attention to the selection of individuals and the observation of
offspring; while to ascertain whether there is inheritance of a
functionally-produced modification, it is requisite to make arrangements
which demand the greater or smaller exercise of some part or parts; and
it is difficult in many cases to find such arrangements, troublesome to
maintain them even for one generation, and still more through successive
generations.

Nor is this all. There exist stimuli to inquiry in the one case which do
not exist in the other. The money-interest and the interest of the
fancier, acting now separately and now together, have prompted
multitudinous individuals to make experiments which have brought out
clear evidence that fortuitous variations are inherited. The
cattle-breeders who profit by producing certain shapes and qualities;
the keepers of pet animals who take pride in the perfections of those
they have bred; the florists, professional and amateur, who obtain new
varieties and take prizes; form a body of men who furnish naturalists
with countless of the required proofs. But there is no such body of men,
led either by pecuniary interest or the interest of a hobby, to
ascertain by experiments whether the effects of use and disuse are
inheritable.

Thus, then, there are amply sufficient reasons why there is a great deal
of direct evidence in the one case and but little in the other: such
little being that which comes out incidentally. Let us look at what
there is of it.

                  *       *       *       *       *

Considerable weight attaches to a fact which Brown-Séquard discovered,
quite by accident, in the course of his researches. He found that
certain artificially-produced lesions of the nervous system, so small
even as a section of the sciatic nerve, left, after healing, an
increasing excitability which ended in liability to epilepsy; and there
afterwards came out the unlooked-for result that the offspring of
guinea-pigs which had thus acquired an epileptic habit such that a pinch
on the neck would produce a fit, inherited an epileptic habit of like
kind. It has, indeed, been since alleged that guinea pigs tend to
epilepsy, and that phenomena of the kind described, occur where there
have been no antecedents like those in Brown-Séquard's case. But
considering the improbability that the phenomena observed by him
happened to be nothing more than phenomena which occasionally arise
naturally, we may, until there is good proof to the contrary, assign
some value to his results.

Evidence not of this directly experimental kind, but nevertheless of
considerable weight, is furnished by other nervous disorders. There is
proof enough that insanity admits of being induced by circumstances
which, in one or other way, derange the nervous functions—excesses of
this or that kind; and no one questions the accepted belief that
insanity is inheritable. Is it alleged that the insanity which is
inheritable is that which spontaneously arises, and that the insanity
which follows some chronic perversion of functions is not inheritable?
This does not seem a very reasonable allegation; and until some warrant
for it is forthcoming, we may fairly assume that there is here a further
support for belief in the transmission of functionally-produced changes.

Moreover, I find among physicians the belief that nervous disorders of a
less severe kind are inheritable. Men who have prostrated their nervous
systems by prolonged overwork or in some other way, have children more
or less prone to nervousness. It matters not what may be the form of
inheritance—whether it be of a brain in some way imperfect, or of a
deficient blood-supply; it is in any case the inheritance of
functionally-modified structures.

Verification of the reasons above given for the paucity of this direct
evidence, is yielded by contemplation of it; for it is observable that
the cases named are cases which, from one or other cause, have thrust
themselves on observation. They justify the suspicion that it is not
because such cases are rare that many of them cannot be cited; but
simply because they are mostly unobtrusive, and to be found only by that
deliberate search which nobody makes. I say nobody, but I am wrong.
Successful search has been made by one whose competence as an observer
is beyond question, and whose testimony is less liable than that of all
others to any bias towards the conclusion that such inheritance takes
place. I refer to the author of the _Origin of Species_.

                  *       *       *       *       *

Now-a-days most naturalists are more Darwinian than Mr. Darwin himself.
I do not mean that their beliefs in organic evolution are more decided;
though I shall be supposed to mean this by the mass of readers, who
identify Mr. Darwin's great contribution to the theory of organic
evolution, with the theory of organic evolution itself, and even with
the theory of evolution at large. But I mean that the particular factor
which he first recognized as having played so immense a part in organic
evolution, has come to be regarded by his followers as the sole factor,
though it was not so regarded by him. It is true that he apparently
rejected altogether the causal agencies alleged by earlier inquirers. In
the Historical Sketch prefixed to the later editions of his _Origin of
Species_ (p. xiv, note), he writes:—“It is curious how largely my
grandfather, Dr. Erasmus Darwin, anticipated the views and erroneous
grounds of opinion of Lamarck in his 'Zoonomia' (vol. i, pp. 500-510),
published in 1794.” And since, among the views thus referred to, was the
view that changes of structure in organisms arise by the inheritance of
functionally-produced changes, Mr. Darwin seems, by the above sentence,
to have implied his disbelief in such inheritance. But he did not mean
to imply this; for his belief in it as a cause of evolution, if not an
important cause, is proved by many passages in his works. In the first
chapter of the _Origin of Species_ (p. 11 of the first edition), he says
respecting the inherited effects of habit, that “with animals the
increased use or disuse of parts has had a marked influence;” and he
gives as instances the changed relative weights of the wing bones and
leg bones of the wild duck and the domestic duck, “the great and
inherited development of the udders in cows and goats,” and the drooping
ears of various domestic animals. Here are other passages taken from the
latest edition of the work.

  “I think there can be no doubt that use in our domestic animals has
  strengthened and enlarged certain parts, and disuse diminished them;
  and that such modifications are inherited” (p. 108). [And on the
  following pages he gives five further examples of such effects.]
  “Habit in producing constitutional peculiarities and use in
  strengthening and disuse in weakening and diminishing organs, appear
  in many cases to have been potent in their effects” (p. 131). “When
  discussing special cases, Mr. Mivart passes over the effects of the
  increased use and disuse of parts, which I have always maintained to
  be highly important, and have treated in my 'Variation under
  Domestication' at greater length than, as I believe, any other
  writer” (p. 176). “Disuse, on the other hand, will account for the
  less developed condition of the whole inferior half of the body,
  including the lateral fins” (p. 188). “I may give another instance
  of a structure which apparently owes its origin exclusively to use
  or habit” (p. 188). “It appears probable that disuse has been the
  main agent in rendering organs rudimentary” (pp. 400-401). “On the
  whole, we may conclude that habit, or use and disuse, have, in some
  cases, played a considerable part in the modification of the
  constitution and structure; but that the effects have often been
  largely combined with, and sometimes overmastered by, the natural
  selection of innate variations” (p. 114).

In his subsequent work, _The Variation of Animals and Plants under
Domestication_, where he goes into full detail, Mr. Darwin gives more
numerous illustrations of the inherited effects of use and disuse. The
following are some of the cases, quoted from volume i of the first
edition.

  Treating of domesticated rabbits, he says:—“the want of exercise has
  apparently modified the proportional length of the limbs in
  comparison with the body” (p. 116). “We thus see that the most
  important and complicated organ [the brain] in the whole
  organization is subject to the law of decrease in size from disuse”
  (p. 129). He remarks that in birds of the oceanic islands “not
  persecuted by any enemies, the reduction of their wings has probably
  been caused by gradual disuse.” After comparing one of these, the
  water-hen of Tristan d'Acunha, with the European water-hen, and
  showing that all the bones concerned in flight are smaller, he
  adds—“Hence in the skeleton of this natural species nearly the same
  changes have occurred, only carried a little further, as with our
  domestic ducks, and in this latter case I presume no one will
  dispute that they have resulted from the lessened use of the wings
  and the increased use of the legs” (pp. 286-7). “As with other
  long-domesticated animals, the instincts of the silk-moth have
  suffered. The caterpillars, when placed on a mulberry-tree, often
  commit the strange mistake of devouring the base of the leaf on
  which they are feeding, and consequently fall down; but they are
  capable, according to M. Robinet, of again crawling up the trunk.
  Even this capacity sometimes fails, for M. Martins placed some
  caterpillars on a tree, and those which fell were not able to
  remount and perished of hunger; they were even incapable of passing
  from leaf to leaf” (p. 304).

Here are some instances of like meaning from volume ii.

  “In many cases there is reason to believe that the lessened use of
  various organs has affected the corresponding parts in the
  offspring. But there is no good evidence that this ever follows in
  the course of a single generation.... Our domestic fowls, ducks, and
  geese have almost lost, not only in the individual but in the race,
  their power of flight; for we do not see a chicken, when frightened,
  take flight like a young pheasant.... With domestic pigeons, the
  length of the sternum, the prominence of its crest, the length of
  the scapulæ and furcula, the length of the wings as measured from
  tip to tip of the radius, are all reduced relatively to the same
  parts in the wild pigeon.” [After detailing kindred diminutions in
  fowls and ducks, Mr. Darwin adds] “The decreased weight and size of
  the bones, in the foregoing cases, is probably the indirect result
  of the reaction of the weakened muscles on the bones” (pp. 297-8).
  “Nathusius has shown that, with the improved races of the pig, the
  shortened legs and snout, the form of the articular condyles of the
  occiput, and the position of the jaws with the upper canine teeth
  projecting in a most anomalous manner in front of the lower canines,
  may be attributed to these parts not having been fully exercised....
  These modifications of structure, which are all strictly inherited,
  characterise several improved breeds, so that they cannot have been
  derived from any single domestic or wild stock. With respect to
  cattle, Professor Tanner has remarked that the lungs and liver in
  the improved breeds 'are found to be considerably reduced in size
  when compared with those possessed by animals having perfect
  liberty....' The cause of the reduced lungs in highly-bred animals
  which take little exercise is obvious” (pp. 299-300). [And on pp.
  301, 302 and 303, he gives facts showing the effects of use and
  disuse in changing, among domestic animals, the characters of the
  ears, the lengths of the intestines, and, in various ways, the
  natures of the instincts.]

But Mr. Darwin's admission, or rather his assertion, that the
inheritance of functionally-produced modifications has been a factor in
organic evolution, is made clear not by these passages alone and by
kindred ones. It is made clearer still by a passage in the preface to
the second edition of his _Descent of Man_. He there protests against
that current version of his views in which this factor makes no
appearance. The passage is as follows.

  “I may take this opportunity of remarking that my critics frequently
  assume that I attribute all changes of corporeal structure and
  mental power exclusively to the natural selection of such variations
  as are often called spontaneous; whereas, even in the first edition
  of the 'Origin of Species,' I distinctly stated that great weight
  must be attributed to the inherited effects of use and disuse, with
  respect both to the body and mind.”

Nor is this all. There is evidence that Mr. Darwin's belief in the
efficiency of this factor, became stronger as he grew older and
accumulated more evidence. The first of the extracts above given, taken
from the sixth edition of the _Origin of Species_, runs thus:—

  “I think there can be no doubt that use in our domestic animals has
  strengthened and enlarged certain parts, and disuse diminished them;
  and that such modifications are inherited.”

Now on turning to the first edition, p. 134, it will be found that
instead of the words—“I think there can be no doubt,” the words
originally used were—“I think there can be _little_ doubt.” That this
deliberate erasure of a qualifying word and substitution of a word
implying unqualified belief, was due to a more decided recognition of a
factor originally under-estimated, is clearly implied by the wording of
the above-quoted passage from the preface to the _Descent of Man_; where
he says that “_even_ in the first edition of the 'Origin of Species,'”
&c.: the implication being that much more in subsequent editions, and
subsequent works, had he insisted on this factor. The change thus
indicated is especially significant as having occurred at a time of life
when the natural tendency is towards fixity of opinion.

During that earlier period when he was discovering the multitudinous
cases in which his own hypothesis afforded solutions, and simultaneously
observing how utterly futile in these multitudinous cases was the
hypothesis propounded by his grandfather and Lamarck, Mr. Darwin was,
not unnaturally, almost betrayed into the belief that the one is
all-sufficient and the other inoperative. But in the mind of one so
candid and ever open to more evidence, there naturally came a reaction.
The inheritance of functionally-produced modifications, which, judging
by the passage quoted above concerning the views of these earlier
enquirers, would seem to have been at one time denied, but which as we
have seen was always to some extent recognized, came to be recognized
more and more, and deliberately included as a factor of importance.

                  *       *       *       *       *

Of this reaction displayed in the later writings of Mr. Darwin, let us
now ask—Has it not to be carried further? Was the share in organic
evolution which Mr. Darwin latterly assigned to the transmission of
modifications caused by use and disuse, its due share? Consideration of
the groups of evidences given above, will, I think, lead us to believe
that its share has been much larger than he supposed even in his later
days.

There is first the implication yielded by extensive classes of phenomena
which remain inexplicable in the absence of this factor. If, as we see,
co-operative parts do not vary together, even when few and close
together, and may not therefore be assumed to do so when many and
remote, we cannot account for those innumerable changes in organization
which are implied when, for advantageous use of some modified part, many
other parts which join it in action have to be modified.

Further, as increasing complexity of structure, accompanying increasing
complexity of life, implies increasing number of faculties, of which
each one conduces to preservation of self or descendants; and as the
various individuals of a species, severally requiring something like the
normal amounts of all these, may individually profit, here by an unusual
amount of one, and there by an unusual amount of another; it follows
that as the number of faculties becomes greater, it becomes more
difficult for any one to be further developed by natural selection. Only
where increase of some one is _predominantly_ advantageous does the
means seem adequate to the end. Especially in the case of powers which
do not subserve self-preservation in appreciable degrees, does
development by natural selection appear impracticable.

It is a fact recognized by Mr. Darwin, that where, by selection through
successive generations, a part has been increased or decreased, its
reaction upon other parts entails changes in them. This reaction is
effected through the changes of function involved. If the changes of
structure produced by such changes of function, are inheritable, then
the re-adjustment of parts throughout the organism, taking place
generation after generation, maintains an approximate balance; but if
not, then generation after generation the organism must get more and
more out of gear, and tend to become unworkable.

Further, as it is proved that change in the balance of functions
registers its effects on the reproductive elements, we have to choose
between the alternatives that the registered effects are irrelevant to
the particular modifications which the organism has undergone, or that
they are such as tend to produce repetitions of these modifications. The
last of these alternatives makes the facts comprehensible; but the first
of them not only leaves us with several unsolved problems, but is
incongruous with the general truth that by reproduction, ancestral
traits, down to minute details, are transmitted.

Though, in the absence of pecuniary interests and the interests in
hobbies, no such special experiments as those which have established the
inheritance of fortuitous variations have been made to ascertain whether
functionally-produced modifications are inherited; yet certain apparent
instances of such inheritance have forced themselves on observation
without being sought for. In addition to other indications of a less
conspicuous kind, is the one I have given above—the fact that the
apparatus for tearing and mastication has decreased with decrease of its
function, alike in civilized man and in some varieties of dogs which
lead protected and pampered lives. Of the numerous cases named by Mr.
Darwin, it is observable that they are yielded not by one class of parts
only, but by most if not all classes—by the dermal system, the muscular
system, the osseous system, the nervous system, the viscera; and that
among parts liable to be functionally modified, the most numerous
observed cases of inheritance are furnished by those which admit of
preservation and easy comparison—the bones: these cases, moreover, being
specially significant as showing how, in sundry unallied species,
parallel changes of structure have occurred along with parallel changes
of habit.

What, then, shall we say of the general implication? Are we to stop
short with the admission that inheritance of functionally-produced
modifications takes place only in cases in which there is evidence of
it? May we properly assume that these many instances of changes of
structure caused by changes of function, occurring in various tissues
and various organs, are merely special and exceptional instances having
no general significance? Shall we suppose that though the evidence which
already exists has come to light without aid from a body of inquirers,
there would be no great increase were due attention devoted to the
collection of evidence? This is, I think, not a reasonable supposition.
To me the _ensemble_ of the facts suggests the belief, scarcely to be
resisted, that the inheritance of functionally-produced modifications
takes place universally. Looking at physiological phenomena as
conforming to physical principles, it is difficult to conceive that a
changed play of organic forces which in many cases of different kinds
produces an inherited change of structure, does not do this in all
cases. The implication, very strong I think, is that the action of every
organ produces on it a reaction which, usually not altering its rate of
nutrition, sometimes leaves it with diminished nutrition consequent on
diminished action, and at other times increases its nutrition in
proportion to its increased action; that while generating a modified
_consensus_ of functions and of structures, the activities are at the
same time impressing this modified _consensus_ on the sperm-cells and
germ-cells whence future individuals are to be produced; and that in
ways mostly too small to be identified, but occasionally in more
conspicuous ways and in the course of generations, the resulting
modifications of one or other kind show themselves. Further, it seems to
me that as there are certain extensive classes of phenomena which are
inexplicable if we assume the inheritance of fortuitous variations to be
the sole factor, but which become at once explicable if we admit the
inheritance of functionally-produced changes, we are justified in
concluding that this inheritance of functionally-produced changes has
been not simply a co-operating factor in organic evolution, but has been
a co-operating factor without which organic evolution, in its higher
forms at any rate, could never have taken place.

Be this or be it not a warrantable conclusion, there is, I think, good
reason for a provisional acceptance of the hypothesis that the effects
of use and disuse are inheritable; and for a methodic pursuit of
inquiries with the view of either establishing it or disproving it. It
seems scarcely reasonable to accept without clear demonstration, the
belief that while a trivial difference of structure arising
spontaneously is transmissible, a massive difference of structure,
maintained generation after generation by change of function, leaves no
trace in posterity. Considering that unquestionably the modification of
structure by function is a _vera causa_, in so far as concerns the
individual; and considering the number of facts which so competent an
observer as Mr. Darwin regarded as evidence that transmission of such
modifications takes place in particular cases; the hypothesis that such
transmission takes place in conformity with a general law, holding of
all active structures, should, I think, be regarded as at least a good
working hypothesis.

                  *       *       *       *       *

But now supposing the broad conclusion above drawn to be
granted—supposing all to agree that from the beginning, along with
inheritance of useful variations fortuitously arising, there has been
inheritance of effects produced by use and disuse; do there remain no
classes of organic phenomena unaccounted for? To this question I think
it must be replied that there do remain classes of organic phenomena
unaccounted for. It may, I believe, be shown that certain cardinal
traits of animals and plants at large are still unexplained; and that a
further factor must be recognized. To show this, however, will require
another paper.

-----

Footnote 1:

  It is probable that this shortening has resulted not directly but
  indirectly, from the selection of individuals which were noted for
  tenacity of hold; for the bull-dog's peculiarity in this respect seems
  due to relative shortness of the upper jaw, giving the underhung
  structure which, involving retreat of the nostrils, enables the dog to
  continue breathing while holding.




                                  II.


Ask a plumber who is repairing your pump, how the water is raised in it,
and he replies—“By suction.” Recalling the ability which he has to suck
up water into his mouth through a tube, he is certain that he
understands the pump's action. To inquire what he means by suction,
seems to him absurd. He says you know as well as he does, what he means;
and he cannot see that there is any need for asking how it happens that
the water rises in the tube when he strains his mouth in a particular
way. To the question why the pump, acting by suction, will not make the
water rise above 32 feet, and practically not so much, he can give no
answer; but this does not shake his confidence in his explanation.

On the other hand an inquirer who insists on knowing what suction is,
may obtain from the physicist answers which give him clear ideas, not
only about it but about many other things. He learns that on ourselves
and all things around, there is an atmospheric pressure amounting to
about 15 pounds on the square inch: 15 pounds being the average weight
of a column of air having a square inch for its base and extending
upwards from the sea-level to the limit of the Earth's atmosphere. He is
made to observe that when he puts one end of a tube into water and the
other end into his mouth, and then draws back his tongue, so leaving a
vacant space, two things happen. One is that the pressure of air outside
his cheeks, no longer balanced by an equal pressure of air inside,
thrusts his cheeks inwards; and the other is that the pressure of air on
the surface of the water, no longer balanced by an equal pressure of air
within the tube and his mouth (into which part of the air from the tube
has gone) the water is forced up the tube in consequence of the unequal
pressure. Once understanding thus the nature of the so-called suction,
he sees how it happens that when the plunger of the pump is raised and
relieves from atmospheric pressure the water below it, the atmospheric
pressure on the water in the well, not being balanced by that on the
water in the tube, forces the water higher up the tube, so that it
follows the plunger. And now he sees why the water cannot be raised
beyond the theoretic limit of 32 feet: a limit made much lower in
practice by imperfections in the apparatus. For if, simplifying the
conception, he supposes the tube of the pump to be a square inch in
section, then the atmospheric pressure of 15 pounds per square inch on
the water in the well, can raise the water in the tube to such height
only that the entire column of it weighs 15 pounds. Having been thus
enlightened about the pump's action, the action of a barometer becomes
intelligible. He perceives how, under the conditions established, the
weight of the column of mercury balances that of an atmospheric column
of equal diameter; and how, as the weight of the atmospheric column
varies, there is a corresponding variation in the weight of the
mercurial column,—shown by change of height. Moreover, having previously
supposed that he understood the ascent of a balloon when he ascribed it
to relative lightness, he now sees that he did not truly understand it.
For he did not recognize it as a result of that upward pressure caused
by the difference between the weight of the mass formed by the gas in
the balloon _plus_ the cylindrical column of air extending above it to
the limit of the atmosphere, and the weight of a similar cylindrical
column of air extending down to the under surface of the balloon: this
difference of weight causing an equivalent upward pressure on the under
surface.

Why do I introduce these familiar truths so entirely irrelevant to my
subject? I do it to show, in the first place, the contrast between a
vague conception of a cause and a distinct conception of it; or rather,
the contrast between that conception of a cause which results when it is
simply classed with some other or others which familiarity makes us
think we understand, and that conception of a cause which results when
it is represented in terms of definite physical forces admitting of
measurement. And I do it to show, in the second place, that when we
insist on resolving a verbally-intelligible cause into its actual
factors, we get not only a clear solution of the problem before us, but
we find that the way is opened to solutions of sundry other problems.
While we rest satisfied with unanalyzed causes, we may be sure both that
we do not rightly comprehend the production of the particular effects
ascribed to them, and that we overlook other effects which would be
revealed to us by contemplation of the causes as analyzed. Especially
must this be so where the causation is complex. Hence we may infer that
the phenomena presented by the development of species, are not likely to
be truly conceived unless we keep in view the concrete agencies at work.
Let us look closely at the facts to be dealt with.

                  *       *       *       *       *

The growth of a thing is effected by the joint operation of certain
forces on certain materials; and when it dwindles, there is either a
lack of some materials, or the forces co-operate in a way different from
that which produces growth. If a structure has varied, the implication
is that the processes which built it up were made unlike the parallel
processes in other cases, by the greater or less amount of some one or
more of the matters or actions concerned. Where there is unusual
fertility, the play of vital activities is thereby shown to have
deviated from the ordinary play of vital activities; and conversely, if
there is infertility. If the germs, or ova, or seed, or offspring
partially developed, survive more or survive less, it is either because
their molar or molecular structures are unlike the average ones, or
because they are affected in unlike ways by surrounding agencies. When
life is prolonged, the fact implies that the combination of actions,
visible and invisible, constituting life, retains its equilibrium longer
than usual in presence of environing forces which tend to destroy its
equilibrium. That is to say, growth, variation, survival, death, if they
are to be reduced to the forms in which physical science can recognize
them, must be expressed as effects of agencies definitely
conceived—mechanical forces, light, heat, chemical affinity, &c.

This general conclusion brings with it the thought that the phrases
employed in discussing organic evolution, though convenient and indeed
needful, are liable to mislead us by veiling the actual agencies. That
which really goes on in every organism is the working together of
component parts in ways conducing to the continuance of their combined
actions, in presence of things and actions outside; some of which tend
to subserve, and others to destroy, the combination. The matters and
forces in these two groups, are the sole causes properly so called. The
words “natural selection,” do not express a cause in the physical sense.
They express a mode of co-operation among causes—or rather, to speak
strictly, they express an effect of this mode of co-operation. The idea
they convey seems perfectly intelligible. Natural selection having been
compared with artificial selection, and the analogy pointed out, there
apparently remains no indefiniteness: the inconvenience being, however,
that the definiteness is of a wrong kind. The tacitly implied Nature
which selects, is not an embodied agency analogous to the man who
selects artificially; and the selection is not the picking out of an
individual fixed on, but the overthrowing of many individuals by
agencies which one successfully resists, and hence continues to live and
multiply. Mr. Darwin was conscious of these misleading implications. In
the introduction to his _Animals and Plants under Domestication_ (p. 6)
he says:—

  “For brevity sake I sometimes speak of natural selection as an
  intelligent power; ... I have, also, often personified the word
  Nature; for I have found it difficult to avoid this ambiguity; but I
  mean by nature only the aggregate action and product of many natural
  laws,—and by laws only the ascertained sequence of events.”

But while he thus clearly saw, and distinctly asserted, that the factors
of organic evolution are the concrete actions, inner and outer, to which
every organism is subject, Mr. Darwin, by habitually using the
convenient figure of speech, was, I think, prevented from recognizing so
fully as he would otherwise have done, certain fundamental consequences
of these actions.

Though it does not personalize the cause, and does not assimilate its
mode of working to a human mode of working, kindred objections may be
urged against the expression to which I was led when seeking to present
the phenomena in literal terms rather than metaphorical terms—the
survival of the fittest;[2] for in a vague way the first word, and in a
clear way the second word, calls up an anthropocentric idea. The thought
of survival inevitably suggests the human view of certain sets of
phenomena, rather than that character which they have simply as groups
of changes. If, asking what we really know of a plant, we exclude all
the ideas associated with the words life and death, we find that the
sole facts known to us are that there go on in the plant certain
inter-dependent processes, in presence of certain aiding and hindering
influences outside of it; and that in some cases a difference of
structure or a favourable set of circumstances, allows these
inter-dependent processes to go on for longer periods than in other
cases. Again, in the working together of those many actions, internal
and external, which determine the lives or deaths of organisms, we see
nothing to which the words fitness and unfitness are applicable in the
physical sense. If a key fits a lock, or a glove a hand, the relation of
the things to one another is presentable to the perceptions. No approach
to fitness of this kind is made by an organism which continues to live
under certain conditions. Neither the organic structures themselves, nor
their individual movements, nor those combined movements of certain
among them which constitute conduct, are related in any analogous way to
the things and actions in the environment. Evidently the word fittest,
as thus used, is a figure of speech; suggesting the fact that amid
surrounding actions, an organism characterized by the word has either a
greater ability than others of its kind to maintain the equilibrium of
its vital activities, or else has so much greater a power of
multiplication that though not longer lived than they, it continues to
live in posterity more persistently. And indeed, as we here see, the
word fittest has to cover cases in which there may be less ability than
usual to survive individually, but in which the defect is more than made
good by higher degrees of fertility.

I have elaborated this criticism with the intention of emphasizing the
need for studying the changes which have gone on, and are ever going on,
in organic bodies, from an exclusively physical point of view. On
contemplating the facts from this point of view, we become aware that,
besides those special effects of the co-operating forces which eventuate
in the longer survival of one individual than of others, and in the
consequent increase through generations, of some trait which furthered
its survival, many other effects are being wrought on each and all of
the individuals. Bodies of every class and quality, inorganic as well as
organic, are from instant to instant subject to the influences in their
environments; are from instant to instant being changed by these in ways
that are mostly inconspicuous; and are in course of time changed by them
in conspicuous ways. Living things in common with dead things, are, I
say, being thus perpetually acted upon and modified; and the changes
hence resulting, constitute an all-important part of those undergone in
the course of organic evolution. I do not mean to imply that changes of
this class pass entirely unrecognized; for, as we shall see, Mr. Darwin
takes cognizance of certain secondary and special ones. But the effects
which are not taken into account, are those primary and universal
effects which give certain fundamental characters to all organisms.
Contemplation of an analogy will best prepare the way for appreciation
of them, and of the relation they bear to those which at present
monopolize attention.

An observant rambler along shores, will, here and there, note places
where the sea has deposited things more or less similar, and separated
them from dissimilar things—will see shingle parted from sand; larger
stones sorted from smaller stones; and will occasionally discover
deposits of shells more or less worn by being rolled about. Sometimes
the pebbles or boulders composing the shingle at one end of a bay, he
will find much larger than those at the other: intermediate sizes,
having small average differences, occupying the space between the
extremes. An example occurs, if I remember rightly, some mile or two to
the west of Tenby; but the most remarkable and well-known example is
that afforded by the Chesil bank. Here, along a shore some sixteen miles
long, there is a gradual increase in the sizes of the stones; which,
being at one end but mere pebbles, are at the other end immense
boulders. In this case, then, the breakers and the undertow have
effected a selection—have at each place left behind those stones which
were too large to be readily moved, while taking away others small
enough to be moved easily. But now, if we contemplate exclusively this
selective action of the sea, we overlook certain important effects which
the sea simultaneously works. While the stones have been differently
acted upon in so far that some have been left here and some carried
there; they have been similarly acted upon in two allied, but
distinguishable, ways. By perpetually rolling them about and knocking
them one against another, the waves have so broken off their most
prominent parts as to produce in all of them more or less rounded forms;
and then, further, the mutual friction of the stones simultaneously
caused, has smoothed their surfaces. That is to say in general terms,
the actions of environing agencies, so far as they have operated
indiscriminately, have produced in the stones a certain unity of
character; at the same time that they have, by their differential
effects, separated them: the larger ones having withstood certain
violent actions which the smaller ones could not withstand.

Similarly with other assemblages of objects which are alike in their
primary traits but unlike in their secondary traits. When simultaneously
exposed to the same set of actions, some of these actions, rising to a
certain intensity, may be expected to work on particular members of the
assemblage changes which they cannot work in those which are markedly
unlike; while others of the actions will work in all of them similar
changes, because of the uniform relations between these actions and
certain attributes common to all members of the assemblage. Hence it is
inferable that on living organisms, which form an assemblage of this
kind, and are unceasingly exposed in common to the agencies composing
their inorganic environments, there must be wrought two such sets of
effects. There will result a universal likeness among them consequent on
the likeness of their respective relations to the matters and forces
around; and there will result, in some cases, the differences due to the
differential effects of these matters and forces, and in other cases,
the changes which, being life-sustaining or life-destroying, eventuate
in certain natural selections.

I have, above, made a passing reference to the fact that Mr. Darwin did
not fail to take account of some among these effects directly produced
on organisms by surrounding inorganic agencies. Here are extracts from
the sixth edition of the _Origin of Species_ showing this.

  “It is very difficult to decide how far changed conditions, such as
  of climate, food, &c., have acted in a definite manner. There is
  reason to believe that in the course of time the effects have been
  greater than can be proved by clear evidence.... Mr. Gould believes
  that birds of the same species are more brightly coloured under a
  clear atmosphere, than when living near the coast or on islands; and
  Wollaston is convinced that residence near the sea affects the
  colours of insects. Moquin-Tandon gives a list of plants which, when
  growing near the sea-shore, have their leaves in some degree fleshy,
  though not elsewhere fleshy” (pp. 106-7). “Some observers are
  convinced that a damp climate affects the growth of the hair, and
  that with the hair the horns are correlated” (p. 159).

In his subsequent work, _Animals and Plants under Domestication_, Mr.
Darwin still more clearly recognizes these causes of change in
organization. A chapter is devoted to the subject. After premising that
“the direct action of the conditions of life, whether leading to
definite or indefinite results, is a totally distinct consideration from
the effects of natural selection;” he goes on to say that changed
conditions of life “have acted so definitely and powerfully on the
organisation of our domesticated productions, that they have sufficed to
form new sub-varieties or races, without the aid of selection by man or
of natural selection.” Of his examples here are two.

  “I have given in detail in the ninth chapter the most remarkable
  case known to me, namely, that in Germany several varieties of maize
  brought from the hotter parts of America were transformed in the
  course of only two or three generations.” (Vol. ii, p. 277.) [And in
  this ninth chapter concerning these and other such instances he says
  “some of the foregoing differences would certainly be considered of
  specific value with plants in a state of nature.” (Vol. i, p. 321.)]
  “Mr. Meehan, in a remarkable paper, compares twenty-nine kinds of
  American trees, belonging to various orders, with their nearest
  European allies, all grown in close proximity in the same garden and
  under as nearly as possible the same conditions.” And then
  enumerating six traits in which the American forms all of them
  differ in like ways from their allied European forms, Mr. Darwin
  thinks there is no choice but to conclude that these “have been
  definitely caused by the long-continued action of the different
  climate of the two continents on the trees.” (Vol. ii, pp. 281-2.)

But the fact we have to note is that while Mr. Darwin thus took account
of special effects due to special amounts and combinations of agencies
in the environment, he did not take account of the far more important
effects due to the general and constant operation of these agencies.[3]
If a difference between the quantities of a force which acts on two
organisms, otherwise alike and otherwise similarly conditioned, produces
some difference between them; then, by implication, this force produces
in both of them effects which they show in common. The inequality
between two things cannot have a value unless the things themselves have
values. Similarly if, in two cases, some unlikeness of proportion among
the surrounding inorganic agencies to which two plants or two animals
are exposed, is followed by some unlikeness in the changes wrought on
them; then it follows that these several agencies taken separately, work
changes in both of them. Hence we must infer that organisms have certain
structural characters in common, which are consequent on the action of
the medium in which they exist: using the word medium in a comprehensive
sense, as including all physical forces falling upon them as well as
matters bathing them. And we may conclude that from the primary
characters thus produced there must result secondary characters.

Before going on to observe those general traits of organisms due to the
general action of the inorganic environment upon them, I feel tempted to
enlarge on the effects produced by each of the several matters and
forces constituting the environment. I should like to do this not only
to give a clear preliminary conception of the ways in which all
organisms are affected by these universally-present agents, but also to
show that, in the first place, these agents modify inorganic bodies as
well as organic bodies, and that, in the second place, the organic are
far more modifiable by them than the inorganic. But to avoid undue
suspension of the argument, I content myself with saying that when the
respective effects of gravitation, heat, light, &c, are studied, as well
as the respective effects, physical and chemical, of the matters forming
the media, water and air, it will be found that while more or less
operative on all bodies, each modifies organic bodies to an extent
immensely greater than the extent to which it modifies inorganic bodies.

                  *       *       *       *       *

Here, not discriminating among the special effects which these various
forces and matters in the environment produce on both classes of bodies,
let us consider their combined effects, and ask—What is the most general
trait of such effects?

Obviously the most general trait is the greater amount of change wrought
on the outer surface than on the inner mass. In so far as the matters of
which the medium is composed come into play, the unavoidable implication
is that they act more on the parts directly exposed to them than on the
parts sheltered from them. And in so far as the forces pervading the
medium come into play, it is manifest that, excluding gravity, which
affects outer and inner parts indiscriminately, the outer parts have to
bear larger shares of their actions. If it is a question of heat, then
the exterior must lose it or gain it faster than the interior; and in a
medium which is now warmer and now colder, the two must habitually
differ in temperature to some extent—at least where the size is
considerable. If it is a question of light, then in all but absolutely
transparent masses, the outer parts must undergo more of any change
producible by it than the inner parts—supposing other things equal; by
which I mean, supposing the case is not complicated by any such
convexities of the outer surface as produce internal concentrations of
rays. Hence then, speaking generally, the necessity is that the primary
and almost universal effect of the converse between the body and its
medium, is to differentiate its outside from its inside. I say almost
universal, because where the body is both mechanically and chemically
stable, like, for instance, a quartz crystal, the medium may fail to
work either inner or outer change.

Of illustrations among inorganic bodies, a convenient one is supplied by
an old cannon-ball that has been long lying exposed. A coating of rust,
formed of flakes within flakes, incloses it; and this thickens year by
year, until, perhaps, it reaches a stage at which its exterior loses as
much by rain and wind as its interior gains by further oxidation of the
iron. Most mineral masses—pebbles, boulders, rocks—if they show any
effect of the environment at all, show it only by that disintegration of
surface which follows the freezing of absorbed water: an effect which,
though mechanical rather than chemical, equally illustrates the general
truth. Occasionally a “rocking-stone” is thus produced. There are formed
successive layers relatively friable in texture, each of which, thickest
at the most exposed parts, and being presently lost by weathering,
leaves the contained mass in a shape more rounded than before; until,
resting on its convex under-surface, it is easily moved. But of all
instances perhaps the most remarkable is one to be seen on the west bank
of the Nile at Philæ, where a ridge of granite 100 feet high, has had
its outer parts reduced in course of time to a collection of
boulder-shaped masses, varying from say a yard in diameter to six or
eight feet, each one of which shows in progress an exfoliation of
successively-formed shells of decomposed granite: most of the masses
having portions of such shells partially detached.

If, now, inorganic masses, relatively so stable in composition, thus
have their outer parts differentiated from their inner parts, what must
we say of organic masses, characterized by such extreme chemical
instability?—instability so great that their essential material is named
protein, to indicate the readiness with which it passes from one
isomeric form to another. Clearly the necessary inference is that this
effect of the medium must be wrought inevitably and promptly, wherever
the relation of outer and inner has become settled: a qualification for
which the need will be seen hereafter.

                  *       *       *       *       *

Beginning with the earliest and most minute kinds of living things, we
necessarily encounter difficulties in getting direct evidence; since, of
the countless species now existing, all have been subject during
millions upon millions of years to the evolutionary process, and have
had their primary traits complicated and obscured by those endless
secondary traits which the natural selection of favourable variations
has produced. Among protophytes it needs but to think of the
multitudinous varieties of diatoms and desmids, with their
elaborately-constructed coverings; or of the definite methods of growth
and multiplication among such simple _Algæ_ as the _Conjugatæ_; to see
that most of their distinctive characters are due to inherited
constitutions, which have been slowly moulded by survival of the fittest
to this or that mode of life. To disentangle such parts of their
developmental changes as are due to the action of the medium, is
therefore hardly possible. We can hope only to get a general conception
of it by contemplating the totality of the facts.

The first cardinal fact is that all protophytes are cellular—all show us
this contrast between outside and inside. Supposing the multitudinous
specialities of the envelope in different orders and genera of
protophytes to be set against one another, and mutually cancelled, there
remains as a trait common to them—an envelope unlike that which it
envelopes. The second cardinal fact is that this simple trait is the
earliest trait displayed in germs, or spores, or other parts from which
new individuals are to arise; and that, consequently, this trait must be
regarded as having been primordial. For it is an established truth of
organic evolution that embryos show us, in general ways, the forms of
remote ancestors; and that the first changes undergone, indicate, more
or less clearly, the first changes which took place in the series of
forms through which the existing form has been reached. Describing, in
successive groups of plants, the early transformations of these
primitive units, Sachs[4] says of the lowest Algæ that “the conjugated
protoplasmic body clothes itself with a cell-wall” (p. 10); that in “the
spores of Mosses and Vascular Cryptogams” and in “the pollen of
Phanerogams” ... “the protoplasmic body of the mother-cell breaks up
into four lumps, which quickly round themselves off and contract, and
become enveloped by a cell-membrane only after complete separation” (p.
13); that in the _Equisetaceæ_ “the young spores, when first separated,
are still naked, but they soon become surrounded by a cell-membrane” (p.
14); and that in higher plants, as in the pollen of many Dicotyledons,
“the contracting daughter-cells secrete cellulose even during their
separation” (p. 14). Here, then, in whatever way we interpret it, the
fact is that there quickly arises an outer layer different from the
contained matter. But the most significant evidence is furnished by “the
masses of protoplasm that escape into water from the injured sacs of
_Vaucheria_, which often instantly become rounded into globular bodies,”
and of which the “hyaline protoplasm envelopes the whole as a skin” (p.
41) which “is denser than the inner and more watery substance” (p. 42).
As in this case the protoplasm is but a fragment, and as it is removed
from the influence of the parent-cell, this differentiating process can
scarcely be regarded as anything more than the effect of
physico-chemical actions: a conclusion which is supported by the
statement of Sachs that “not only every vacuole in a solid protoplasmic
body, but also every thread of protoplasm which penetrates the
sap-cavity, and finally the inner side of the protoplasm-sac which
encloses the sap-cavity, is also bounded by a skin” (p. 42). If then
“every portion of a protoplasmic body immediately surrounds itself, when
it becomes isolated, with such a skin,” which is shown in all cases to
arise at the surface of contact with sap or water, this primary
differentiation of outer from inner must be ascribed to the direct
action of the medium. Whether the coating thus initiated is secreted by
the protoplasm, or whether, as seems more likely, it results from
transformation of it, matters not to the argument. Either way the action
of the medium causes its formation; and either way the many varied and
complex differentiations which developed cell-walls display, must be
considered as originating from those variations of this
physically-generated covering which natural selection has taken
advantage of.

The contained protoplasm of a vegetal cell, which has self-mobility and
when liberated sometimes performs amœba-like motions for a time, may be
regarded as an imprisoned amœba; and when we pass from it to a free
amœba, which is one of the simplest types of first animals, or
_Protozoa_, we naturally meet with kindred phenomena. The general trait
which here concerns us, is that while its plastic or semi-fluid sarcode
goes on protruding, in irregular ways, now this and now that part of its
periphery, and again withdrawing into its interior first one and then
another of these temporary processes, perhaps with some small portion of
food attached, there is but an indistinct differentiation of outer from
inner (a fact shown by the frequent coalescence of the pseudopodia in
Rhizopods); but that when it eventually becomes quiescent, the surface
becomes differentiated from the contents: the passing into an encysted
state, doubtless in large measure due to inherited proclivity, being
furthered, and having probably been once initiated, by the action of the
medium. The connexion between constancy of relative position among the
parts of the sarcode, and the rise of a contrast between superficial and
central parts, is perhaps best shown in the minutest and simplest
_Infusoria_, the _Monadinæ_. The genus _Monas_ is described by Kent as
“plastic and unstable in form, possessing no distinct cuticular
investment; ... the food-substances incepted at all parts of the
periphery”;[5] and the genus _Scytomonas_ he says “differs from _Monas_
only in its persistent shape and accompanying greater rigidity of the
peripheral or ectoplasmic layer.”[6] Describing generally such low
forms, some of which are said to have neither nucleus nor vacuole, he
remarks that in types somewhat higher “the outer or peripheral border of
the protoplasmic mass, while not assuming the character of a distinct
cell-wall or so-called cuticle, presents, as compared with the inner
substance of that mass, a slightly more solid type of composition.”[7]
And it is added that these forms having so slightly differentiated an
exterior, “while usually exhibiting a more or less characteristic normal
outline, can revert at will to a pseud-amœboid and repent state.”[8]
Here, then, we have several indications of the truth that the permanent
externality of a certain part of the substance, is followed by
transformation of it into a coating unlike the substance it contains.
Indefinite and structureless in the simplest of these forms, as instance
again the _Gregarina_,[9] the limiting membrane becomes, in higher
_Infusoria_, definite and often complex: showing that the selection of
favourable variations has had largely to do with its formation. In such
types as the _Foraminifera_, which, almost structureless internally
though they are, secrete calcareous shells, it is clear that the nature
of this outer layer is determined by inherited constitution. But
recognition of this consists with the belief that the action of the
medium initiated the outer layer, specialized though it now is; and that
even still, contact with the medium excites secretion of it.

A remarkable analogy remains to be named. When we study the action of
the medium in an inorganic mass, we are led to see that between the
outer changed layer and the inner unchanged mass, comes a surface where
active change is going on. Here we have to note that, alike in the
plant-cell and in the animal-cell, there is a similar relation of parts.
Immediately inside the envelope comes the primordial utricle in the one
case, and in the other case the layer of active sarcode. In either case
the living protoplasm, placed in the position of a lining to the cuticle
of the cell, is shielded from the direct action of the medium, and yet
is not beyond the reach of its influences.

                  *       *       *       *       *

Limited, as thus far drawn, to a certain common trait of those minute
organisms which are mostly below the reach of unaided vision, the
foregoing conclusion appears trivial enough. But it ceases to appear
trivial on passing into a wider field, and observing the implications,
direct and indirect, as they concern plants and animals of sensible
sizes.

Popular expositions of science have so far familiarized many readers
with a certain fundamental trait of living things around, that they have
ceased to perceive how marvellous a trait it is, and, until interpreted
by the Theory of Evolution, how utterly mysterious. In past times, the
conception of an ordinary plant or animal which prevailed, not
throughout the world at large only but among the most instructed, was
that it is a single continuous entity. One of these living things was
unhesitatingly regarded as being in all respects a unit. Parts it might
have, various in their sizes, forms, and compositions; but these were
components of a whole which had been from the beginning in its original
nature a whole. Even to naturalists fifty years ago, the assertion that
a cabbage or a cow, though in one sense a whole, is in another sense a
vast society of minute individuals, severally living in greater or less
degrees, and some of them maintaining their independent lives
unrestrained, would have seemed an absurdity. But this truth which, like
so many of the truths established by science, is contrary to that common
sense in which most people have so much confidence, has been gradually
growing clear since the days when Leeuwenhoek and his contemporaries
began to examine through lenses the minute structures of common plants
and animals. Each improvement in the microscope, while it has widened
our knowledge of those minute forms of life described above, has
revealed further evidence of the fact that all the larger forms of life
consist of units severally allied in their fundamental traits to these
minute forms of life. Though, as formulated by Schwann and Schleiden,
the cell-doctrine has undergone qualifications of statement; yet the
qualifications have not been such as to militate against the general
proposition that organisms visible to the naked eye, are severally
compounded of invisible organisms—using that word in its most
comprehensive sense. And then, when the development of any animal is
traced, it is found that having been primarily a nucleated cell, and
having afterwards become by spontaneous fission a cluster of nucleated
cells, it goes on through successive stages to form out of such cells,
ever multiplying and modifying in various ways, the several tissues and
organs composing the adult.

On the hypothesis of evolution this universal trait has to be accepted
not as a fact that is strange but unmeaning. It has to be accepted as
evidence that all the visible forms of life have arisen by union of the
invisible forms; which, instead of flying apart when they divided,
remained together. Various intermediate stages are known. Among plants,
those of the _Volvox_ type show us the component protophytes so feebly
combined that they severally carry on their lives with no appreciable
subordination to the life of the group. And among animals, a parallel
relation between the lives of the units and the life of the group is
shown us in _Uroglena_ and _Syncrypta_. From these first stages upwards,
may be traced through successively higher types, an increasing
subordination of the units to the aggregate; though still a
subordination leaving to them conspicuous amounts of individual
activity. Joining which facts with the phenomena presented by the
cell-multiplication and aggregation of every unfolding germ, naturalists
are now accepting the conclusion that by this process of composition
from _Protozoa_, were formed all classes of the _Metazoa_[10]—(as
animals formed by this compounding are now called); and that in a
similar way from _Protophyta_, were formed all classes of what I suppose
will be called _Metaphyta_, though the word does not yet seem to have
become current.

And now what is the general meaning of these truths, taken in connexion
with the conclusion reached in the last section. It is that this
universal trait of the _Metazoa_ and _Metaphyta_, must be ascribed to
the primitive action and re-action between the organism and its medium.
The operation of those forces which produced the primary differentiation
of outer from inner in early minute masses of protoplasm, pre-determined
this universal cell-structure of all embryos, plant and animal, and the
consequent cell-composition of adult forms arising from them. How
unavoidable is this implication, will be seen on carrying further an
illustration already used—that of the shingle-covered shore, the pebbles
on which, while being in some cases selected, have been in all cases
rounded and smoothed. Suppose a bed of such shingle to be, as we often
see it, solidified, along with interfused material, into a conglomerate.
What in such case must be considered as the chief trait of such
conglomerate; or rather—what must we regard as the chief cause of its
distinctive characters? Evidently the action of the sea. Without the
breakers, no pebbles; without the pebbles, no conglomerate. Similarly
then, in the absence of that action of the medium by which was effected
the differentiation of outer from inner in those microscopic portions of
protoplasm constituting the earliest and simplest animals and plants,
there could not have existed this cardinal trait of composition which
all the higher animals and plants show us.

So that, active as has been the part played by natural selection,
alike in modifying and moulding the original units—largely as
survival of the fittest has been instrumental in furthering and
controlling the combination of these units into visible organisms,
and eventually into large ones; yet we must ascribe to the direct
effect of the medium on the first forms of life, that character of
which this everywhere-operative factor has taken advantage.

                  *       *       *       *       *

Let us turn now to another and more obvious attribute of higher
organisms, for which also there is this same general cause. Let us
observe how, on a higher platform, there recurs this differentiation of
outer from inner—how this primary trait in the living units with which
life commences, re-appears as a primary trait in those aggregates of
such units which constitute visible organisms.

In its simplest and most unmistakable form, we see this in the early
changes of an unfolding ovum of primitive type. The original fertilized
single cell, having by spontaneous fission multiplied into a cluster of
such cells, there begins to show itself a contrast between periphery and
centre; and presently there is formed a sphere consisting of a
superficial layer unlike its contents. The first change, then, is the
rise of a difference between that outer part which holds direct converse
with the surrounding medium, and that inclosed part which does not. This
primary differentiation in these compound embryos of higher animals,
parallels the primary differentiation undergone by the simplest living
things.

Leaving, for the present, succeeding changes of the compound embryo, the
significance of which we shall have to consider by-and-by, let us pass
now to the adult forms of visible plants and animals. In them we find
cardinal traits which, after what we have seen above, will further
impress us with the importance of the effects wrought on the organism by
its medium.

From the thallus of a sea-weed up to the leaf of a highly developed
phænogam, we find, at all stages, a contrast between the inner and outer
parts of these flattened masses of tissue. In the higher _Algæ_ “the
outermost layers consist of smaller and firmer cells, while the inner
cells are often very large, and sometimes extremely long;”[11] and in
the leaves of trees the epidermal layer, besides differing in the sizes
and shapes of its component cells from the parenchyma forming the inner
substance of the leaf, is itself differentiated by having a continuous
cuticle, and by having the outer walls of its cells unlike the inner
walls.[12] Especially significant is the structure of such intermediate
types as the Liverworts. Beyond the differentiation of the covering
cells from the contained cells, and the contrast between upper surface
and under surface, the frond of _Marchantia polymorpha_ clearly shows us
the direct effect of incident forces; and shows us, too, how it is
involved with the effect of inherited proclivities. The frond grows from
a flat disc-shaped gemma, the two sides of which are alike. Either side
may fall uppermost; and then of the developing shoot, the side exposed
to the light “is under all circumstances the upper side which forms
stomata, the dark side becomes the under side which produces root-hairs
and leafy processes.”[13] So that while we have undeniable proof that
the contrasted influences of the medium on the two sides, initiate the
differentiation, we have also proof that the completion of it is
determined by the transmitted structure of the type; since it is
impossible to ascribe the development of stomata to the direct action of
air and light. On turning from foliar expansions, to stems and roots,
facts of like meaning meet us. Speaking generally of epidermal tissue
and inner tissue, Sachs remarks that “the contrast of the two is the
plainer the more the part of the plant concerned is exposed to air and
light.”[14] Elsewhere, in correspondence with this, it is said that in
roots the cells of the epidermis, though distinguished by bearing hairs,
“are otherwise similar to those of the fundamental tissue” which they
clothe,[15] while the cuticular covering is relatively thin; whereas in
stems the epidermis (often further differentiated) is composed of layers
of cells which are smaller and thicker-walled: a stronger contrast of
structure corresponding to a stronger contrast of conditions. By way of
meeting the suggestion that these respective differences are wholly due
to the natural selection of favourable variations, it will suffice if I
draw attention to the unlikeness between imbedded roots and exposed
roots. While in darkness, and surrounded by moist earth, the outermost
protective coats, even of large roots, are comparatively thin; but when
the accidents of growth entail permanent exposure to light and air,
roots acquire coverings allied in character to the coverings of
branches. That the action of the medium causes these and converse
changes, cannot be doubted when we find, on the one hand, that “roots
can become directly transformed into leaf-bearing shoots,” and, on the
other hand, that in some plants certain “apparent roots are only
underground shoots,” and that nevertheless “they are similar to true
roots in function and in the formation of tissue, but have no root-cap,
and, when they come to the light above ground, continue to grow in the
manner of ordinary leaf-shoots.”[16] If, then, in highly developed
plants inheriting pronounced structures, this differentiating influence
of the medium is so marked, it must have been all-important at the
outset while types were undetermined.

As with plants so with animals, we find good reason for inferring that
while the specialities of the tegumentary parts must be ascribed to the
natural selection of favourable variations, their most general traits
are due to the direct action of surrounding agencies. Here we come upon
the border of those changes which are ascribable to use and disuse. But
from this class of changes we may fitly exclude those in which the parts
concerned are wholly or mainly passive. A corn and a blister will
conveniently serve to illustrate the way in which certain outer actions
initiate in the superficial tissues, effects of very marked kinds, which
are related neither to the needs of the organism nor to its normal
structure. They are neither adaptive changes nor changes towards
completion of the type. After noting them we may pass to allied, but
still more instructive, changes. Continuous pressure on any portion of
the surface causes absorption, while intermittent pressure causes
growth: the one impeding circulation and the passage of plasma from the
capillaries into the tissues, and the other aiding both. There are yet
further mechanically-produced effects. That the general character of the
ribbed skin on the under surfaces of the feet and insides of the hands
is directly due to friction and intermittent pressure, we have the
proofs:—first, that the tracts most exposed to rough usage are the most
ribbed; second, that the insides of hands subject to unusual amounts of
rough usage, as those of sailors, are strongly ribbed all over; and
third, that in hands which are very little used, the parts commonly
ribbed become quite smooth. These several kinds of evidence, however,
full of meaning as they are, I give simply to prepare the way for
evidence of a much more conclusive kind.

Where a wide ulcer has eaten away the deep-seated layer out of which the
epidermis grows, or where this layer has been destroyed by an extensive
burn, the process of healing is very significant. From the subjacent
tissues, which in the normal order have no concern with outward growth,
there is produced a new skin, or rather a pro-skin; for this substituted
outward-growing layer contains no hair-follicles or other specialities
of the original one. Nevertheless, it is like the original one in so far
that it is a continually renewed protective covering. Doubtless it may
be contended that this make-shift skin results from the inherited
proclivity of the type—the tendency to complete afresh the structure of
the species when injured. We cannot, however, ignore the immediate
influence of the medium, on recalling the facts above named, or on
remembering the further fact that an inflamed surface of skin, when not
sheltered from the air, will throw out a film of coagulable lymph. But
that the direct action of the medium is a chief factor we are clearly
shown by another case. Accident or disease occasionally causes permanent
eversion, or protrusion, of mucous membrane. After a period of
irritability, great at first but decreasing as the change advances, this
membrane assumes the general character of ordinary skin. Nor is this
all: its microscopic structure changes. Where it is a mucous membrane of
the kind covered by cylinder-epithelium, the cylinders gradually
shorten, becoming finally flat, and there results a squamous epithelium:
there is a near approach in minute composition to epidermis. Here a
tendency towards completion of the type cannot be alleged; for there is,
contrariwise, divergence from the type. The effect of the medium is so
great that, in a short time, it overcomes the inherited proclivity and
produces a structure of opposite kind to the normal one.

With but little break we come here upon a significant analogy, parallel
to an analogy already described. As was pointed out, an inorganic body
that is modifiable by its medium, acquires, after a time, an outer coat
which has already undergone such change as surrounding agencies can
effect; has a contained mass which is as yet unchanged, because
unreached; and has a surface between the two where change is going on—a
region of activity. And we saw that alike in the vegetal cell and the
animal cell there exist analogous distributions: of course with the
difference that the innermost part is not inert. Now we have to note
that in those aggregates of cells constituting the _Metaphyta_ and
_Metazoa_, analogous distributions also exist. In plants they are of
course not to be looked for in leaves and other deciduous portions, but
only in portions of long duration—stems and branches. Naturally, too, we
need not expect them in plants having modes of growth which early
produce an outer practically dead part, that effectually shields the
inner actively living part of the stem from the influence of the
medium—long-lived acrogens such as tree-ferns and long-lived endogens
such as palms. But in the highest plants, exogens, which have the
actively living part of their stems within reach of environing agencies,
we find this part,—the cambium layer,—is one from which there is a
growth inwards forming wood, and a growth outwards forming bark: there
is an increasingly thick covering (where it does not scale off) of
tissue changed by the medium, and inside this a film of highest
vitality. In so far as concerns the present argument, it is the same
with the _Metazoa_, or at least all of them which have developed
organizations. The outer skin grows up from a limiting plane, or layer,
a little distance below the surface—a place of predominant vital
activity. Here perpetually arise new cells, which, as they develop, are
thrust outwards and form the epidermis: flattening and drying up as they
approach the surface, whence, having for a time served to shield the
parts below, they finally scale off and leave younger ones to take their
places. This still undifferentiated tissue forming the base of the
epidermis, and existing also as a source of renewal in internal organs,
is the essentially living substance; and facts above given imply that it
was the action of the medium on this essentially living substance,
which, during early stages in the organization of the _Metazoa_,
initiated that protective envelope which presently became an inherited
structure—a structure which, though now mainly inherited, still
continues to be modifiable by its initiator.

Fully to perceive the way in which these evidences compel us to
recognize the influence of the medium as a primordial factor, we need
but conceive them as interpreted without it. Suppose, for instance, we
say that the structure of the epidermis is wholly determined by the
natural selection of favourable variations; what must be the position
taken in presence of the fact above named, that when mucous membrane is
exposed to the air its cell-structure changes into the cell-structure of
skin? The position taken must be this:—Though mucous membrane in a
highly-evolved individual organism, thus shows the powerful effect of
the medium on its surface; yet we must not suppose that the medium had
the effect of producing such a cell-structure on the surfaces of
primitive forms, undifferentiated though they were; or, if we suppose
that such an effect was produced on them, we must not suppose that it
was inheritable. Contrariwise, we must suppose that such effect of the
medium either was not wrought at all, or that it was evanescent: though
repeated through millions upon millions of generations it left no
traces. And we must conclude that this skin-structure arose only in
consequence of spontaneous variations not physically initiated (though
like those physically initiated) which natural selection laid hold of
and increased. Does any one think this a tenable position?

                  *       *       *       *       *

And now we approach the last and chief series of morphological phenomena
which must be ascribed to the direct action of environing matters and
forces. These are presented to us when we study the early stages in the
development of the embryos of the _Metazoa_ in general.

We will set out with the fact already noted in passing, that after
repeated spontaneous fissions have changed the original fertilized
germ-cell into that cluster of cells which forms a gemmule or a
primitive ovum, the first contrast which arises is between the
peripheral parts and the central parts. Where, as with lower creatures
which do not lay up large stores of nutriment with the germs of their
offspring, the inner mass is inconsiderable, the outer layer of cells,
which are presently made quite small by repeated subdivisions, forms a
membrane extending over the whole surface—the blastoderm. The next stage
of development, which ends in this covering layer becoming double, is
reached in two ways—by invagination and by delamination; but which is
the original way and which the abridged way, is not quite certain. Of
invagination, multitudinously exemplified in the lowest types, Mr.
Balfour says:—“On purely _à priori_ grounds there is in my opinion more
to be said for invagination than for any other view”;[17] and, for
present purposes, it will suffice if we limit ourselves to this: making
its nature clear to the general reader by a simple illustration.

Take a small india-rubber ball—not of the inflated kind, nor of the
solid kind, but of the kind about an inch or so in diameter with a small
hole through which, under pressure, the air escapes. Suppose that
instead of consisting of india-rubber its wall consists of small cells
made polyhedral in form by mutual pressure, and united together. This
will represent the blastoderm. Now with the finger, thrust in one side
of the ball until it touches the other: so making a cup. This action
will stand for the process of invagination. Imagine that by continuance
of it, the hemispherical cup becomes very much deepened and the opening
narrowed, until the cup becomes a sac, of which the introverted wall is
everywhere in contact with the outer wall. This will represent the
two-layered “gastrula”—the simplest ancestral form of the _Metazoa_: a
form which is permanently represented in some of the lowest types; for
it needs but tentacles round the mouth of the sac, to produce a common
hydra. Here the fact which it chiefly concerns us to remark, is that of
these two layers the outer, called in embryological language the
epiblast, continues to carry on direct converse with the forces and
matters in the environment; while the inner, called the hypoblast, comes
in contact with such only of these matters as are put into the
food-cavity which it lines. We have further to note that in the embryos
of _Metazoa_ at all advanced in organization, there arises between these
two layers a third—the mesoblast. The origin of this is seen in types
where the developmental process is not obscured by the presence of a
large food-yolk. While the above-described introversion is taking place,
and before the inner surfaces of the resulting epiblast and hypoblast
have come into contact, cells, or amœboid units equivalent to them, are
budded off from one or both of these inner surfaces, or some part of one
or other; and these form a layer which eventually lies between the other
two—a layer which, as this mode of formation implies, never has any
converse with the surrounding medium and its contents, or with the
nutritive bodies taken in from it. The striking facts to which this
description is a necessary introduction, may now be stated. From the
outer layer, or epiblast, are developed the permanent epidermis and its
out-growths, the nervous system, and the organs of sense. From the
introverted layer, or hypoblast, are developed the alimentary canal and
those parts of its appended organs, liver, pancreas, &c., which are
concerned in delivering their secretions into the alimentary canal, as
well as the linings of those ramifying tubes in the lungs which convey
air to the places where gaseous exchange is effected. And from the
mesoblast originate the bones, the muscles, the heart and blood-vessels,
and the lymphatics, together with such parts of various internal organs
as are most remotely concerned with the outer world. Minor
qualifications being admitted, there remain the broad general facts,
that out of that part of the external layer which remains permanently
external, are developed all the structures which carry on intercourse
with the medium and its contents, active and passive; out of the
introverted part of this external layer, are developed the structures
which carry on intercourse with the quasi-external substances that are
taken into the interior—solid food, water, and air; while out of the
mesoblast are developed structures which have never had, from first to
last, any intercourse with the environment. Let us contemplate these
general facts.

Who would have imagined that the nervous system is a modified portion of
the primitive epidermis? In the absence of proofs furnished by the
concurrent testimony of embryologists during the last thirty or forty
years, who would have believed that the brain arises from an unfolded
tract of the outer skin, which, sinking down beneath the surface,
becomes imbedded in other tissues and eventually surrounded by a bony
case? Yet the human nervous system in common with the nervous systems of
lower animals is thus originated. In the words of Mr. Balfour, early
embryological changes imply that—

  “the functions of the central nervous system, which were originally
  taken by the whole skin, became gradually concentrated in a special
  part of the skin which was step by step removed from the surface,
  and has finally become in the higher types a well-defined organ
  imbedded in the subdermal tissues.... The embryological evidence
  shows that the ganglion-cells of the central part of the nervous
  system are originally derived from the simple undifferentiated
  epithelial cells of the surface of the body.”[18]

Less startling perhaps, though still startling enough, is the fact that
the eye is evolved out of a portion of the skin; and that while the
crystalline lens and its surroundings thus originate, the “percipient
portions of the organs of special sense, especially of optic organs, are
often formed from the same part of the primitive epidermis” which forms
the central nervous system.[19] Similarly is it with the organs for
smelling and hearing. These, too, begin as sacs formed by in-foldings of
the epidermis; and while their parts are developing they are joined from
within by nervous structures which were themselves epidermic in origin.
How are we to interpret these strange transformations? Observing, as we
pass, how absurd from the point of view of the special-creationist,
would appear such a filiation of structures, and such a round-about mode
of embryonic development, we have here to remark that the process is not
one to have been anticipated as a result of natural selection. After
numbers of spontaneous variations had occurred, as the hypothesis
implies, in useless ways, the variation which primarily initiated a
nervous centre might reasonably have been expected to occur in some
internal part where it would be fitly located. Its initiation in a
dangerous place and subsequent migration to a safe place, would be
incomprehensible. Not so if we bear in mind the cardinal truth above set
forth, that the structures for holding converse with the medium and its
contents, arise in that completely superficial part which is directly
affected by the medium and its contents; and if we draw the inference
that the external actions themselves initiate the structures. These once
commenced, and furthered by natural selection where favourable to life,
would form the first term of a series ending in developed sense organs
and a developed nervous system.[20]

Though it would enforce the argument, I must, for brevity's sake, pass
over the analogous evolution of that introverted layer, or hypoblast,
out of which the alimentary canal and attached organs arise. It will
suffice to emphasize the fact that having been originally external, this
layer continues in its developed form to have a quasi-externality, alike
in its digesting part and in its respiratory part; since it continues to
deal with matters alien to the organism. I must also refrain from
dwelling at length on the fact already adverted to, that the
intermediate derived layer, or mesoblast, which was at the outset
completely internal, originates those structures which ever remain
completely internal, and have no communication with the environment save
through the structures developed from the other two: an antithesis which
has great significance.

Here, instead of dwelling on these details, it will be better to draw
attention to the most general aspect of the facts. Whatever may be the
course of subsequent changes, the first change is the formation of a
superficial layer or blastoderm; and by whatever series of
transformations the adult structure is reached, it is from the
blastoderm that all the organs forming the adult originate. Why this
marvellous fact?

Meaning is given to it if we go back to the first stage in which
_Protozoa_, having by repeated fissions formed a cluster, then arranged
themselves into a hollow sphere, as do the protophytes forming a
_Volvox_. Originally alike all over its surface, the hollow sphere of
ciliated units thus formed, would, if not quite spherical, assume a
constant attitude when moving through the water; and hence one part of
the spheroid would more frequently than the rest come in contact with
nutritive matters to be taken in. A division of labour resulting from
such a variation being advantageous, and tending therefore to increase
in descendants, would end in a differentiation like that shown in the
gemmules of various low types of _Metazoa_, which, ovate in shape, are
ciliated over one part of the surface only. There would arise a form in
which the cilium-bearing units effected locomotion and aeration; while
on the others, assuming an amœba-like character, devolved the function
of absorbing food: a primordial specialization variously indicated by
evidence.[21] Just noting that an ancestral origin of this kind is
implied by the fact that in low types of _Metazoa_ a hollow sphere of
cells is the form first assumed by the unfolding embryo, I draw
attention to the point here of chief interest; namely that the primary
differentiation of this hollow sphere is in such case determined by a
difference in the converse of its parts with the medium and its
contents; and that the subsequent invagination arises by a continuance
of this differential converse.

Even neglecting this first stage and commencing with the next, in which
a “gastrula” has been produced by the permanent introversion of one
portion of the surface of the hollow sphere, it will suffice if we
consider what must thereafter have happened. That which continued to be
the outer surface was the part which from time to time touched quiescent
masses and occasionally received the collisions consequent on its own
motions or the motions of other things. It was the part to receive the
sound-vibrations occasionally propagated through the water; the part to
be affected more strongly than any other by those variations in the
amounts of light caused by the passing of small bodies close to it; and
the part which met those diffused molecules constituting odours. That is
to say, from the beginning the surface was the part on which there fell
the various influences pervading the environment, the part by which
there was received those impressions from the environment serving for
the guidance of actions, and the part which had to bear the mechanical
re-actions consequent upon such actions. Necessarily, therefore, the
surface was the part in which were initiated the various
instrumentalities for carrying on intercourse with the environment. To
suppose otherwise is to suppose that such instrumentalities arose
internally where they could neither be operated on by surrounding
agencies nor operate on them,—where the differentiating forces did not
come into play, and the differentiated structures had nothing to do; and
it is to suppose that meanwhile the parts directly exposed to the
differentiating forces remained unchanged. Clearly, then, organization
could not but begin on the surface; and having thus begun, its
subsequent course could not but be determined by its superficial origin.
And hence these remarkable facts showing us that individual evolution is
accomplished by successive in-foldings and in-growings. Doubtless
natural selection soon came into action, as, for example, in the removal
of the rudimentary nervous centres from the surface; since an individual
in which they were a little more deeply seated would be less likely to
be incapacitated by injury of them. And so in multitudinous other ways.
But nevertheless, as we here see, natural selection could operate only
under subjection. It could do no more than take advantage of those
structural changes which the medium and its contents initiated.

See, then, how large has been the part played by this primordial factor.
Had it done no more than give to _Protozoa_ and _Protophyta_ that
cell-form which characterizes them—had it done no more than entail the
cellular composition which is so remarkable a trait of _Metazoa_ and
_Metaphyta_—had it done no more than cause the repetition in all visible
animals and plants of that primary differentiation of outer from inner
which it first wrought in animals and plants invisible to the naked eye;
it would have done much towards giving to organisms of all kinds certain
leading traits. But it has done more than this. By causing the first
differentiations of those clusters of units out of which visible animals
in general arose, it fixed the starting place for organization, and
therefore determined the course of organization; and, doing this, gave
indelible traits to embryonic transformations and to adult structures.

                  *       *       *       *       *

Though mainly carried on after the inductive method, the argument at the
close of the foregoing section has passed into the deductive. Here let
us follow for a space the deductive method pure and simple. Doubtless in
biology _à priori_ reasoning is dangerous; but there can be no danger in
considering whether its results coincide with those reached by reasoning
_à posteriori_.

Biologists in general agree that in the present state of the world, no
such thing happens as the rise of a living creature out of non-living
matter. They do not deny, however, that at a remote period in the past,
when the temperature of the Earth's surface was much higher than at
present, and other physical conditions were unlike those we know,
inorganic matter, through successive complications, gave origin to
organic matter. So many substances once supposed to belong exclusively
to living bodies, have now been formed artificially, that men of science
scarcely question the conclusion that there are conditions under which,
by yet another step of composition, quaternary compounds of lower types
pass into those of highest types. That there once took place gradual
divergence of the organic from the inorganic, is, indeed, a necessary
implication of the hypothesis of Evolution, taken as a whole; and if we
accept it as a whole, we must put to ourselves the question—What were
the early stages of progress which followed, after the most complex form
of matter had arisen out of forms of matter a degree less complex?

At first, protoplasm could have had no proclivities to one or other
arrangement of parts; unless, indeed, a purely mechanical proclivity
towards a spherical form when suspended in a liquid. At the outset it
must have been passive. In respect of its passivity, primitive organic
matter must have been like inorganic matter. No such thing as
spontaneous variation could have occurred in it; for variation implies
some habitual course of change from which it is a divergence, and is
therefore excluded where there is no habitual course of change. In the
absence of that cyclical series of metamorphoses which even the simplest
living thing now shows us, as a result of its inherited constitution,
there could be no _point d'appui_ for natural selection. How, then, did
organic evolution begin?

If a primitive mass of organic matter was like a mass of inorganic
matter in respect of its passivity, and differed only in respect of its
greater changeableness; then we must infer that its first changes
conformed to the same general law as do the changes of an inorganic
mass. The instability of the homogeneous is a universal principle. In
all cases the homogeneous tends to pass into the heterogeneous, and the
less heterogeneous into the more heterogeneous. In the primordial units
of protoplasm, then, the step with which evolution commenced must have
been the passage from a state of complete likeness throughout the mass
to a state in which there existed some unlikeness. Further, the cause of
this step in one of these portions of organic matter, as in any portion
of inorganic matter, must have been the different exposure of its parts
to incident forces. What incident forces? Those of its medium or
environment. Which were the parts thus differently exposed? Necessarily
the outside and the inside. Inevitably, then, alike in the organic
aggregate and the inorganic aggregate (supposing it to have coherence
enough to maintain constant relative positions among its parts), the
first fall from homogeneity to heterogeneity must always have been the
differentiation of the external surface from the internal contents. No
matter whether the modification was physical or chemical, one of
composition or of decomposition, it comes within the same
generalization. The direct action of the medium was the primordial
factor of organic evolution.

                  *       *       *       *       *

And now, finally, let us look at the factors in their _ensemble_, and
consider the respective parts they play: observing, especially, the ways
in which, at successive stages, they severally give place one to another
in degree of importance.

Acting alone, the primordial factor must have initiated the primary
differentiation in all units of protoplasm alike. I say alike, but I
must forthwith qualify the word. For since surrounding influences,
physical and chemical, could not be absolutely the same in all places,
especially when the first rudiments of living things had spread over a
considerable area, there necessarily arose small contrasts between the
degrees and kinds of superficial differentiation effected. As soon as
these became decided, natural selection came into play; for inevitably
the unlikenesses produced among the units had effects on their lives:
there was survival of some among the modified forms rather than others.
Utterly in the dark though we are respecting the causes which set up
that process of fission everywhere occurring among the minutest forms of
life, we must infer that, when established, it furthered the spread of
those which were most favourably differentiated by the medium. Though
natural selection must have become increasingly active when once it had
got a start; yet the differentiating action of the medium never ceased
to be a co-operator in the development of these first animals and
plants. Again taking the lead as there arose the composite forms of
animals and plants, and again losing the lead with that advancing
differentiation of these higher types which gave more scope to natural
selection, it nevertheless continued, and must ever continue, to be a
cause, both direct and indirect, of modifications in structure.

Along with that remarkable process which, beginning in minute forms with
what is called conjugation, developed into sexual generation, there came
into play causes of frequent and marked fortuitous variations. The
mixtures of constitutional proclivities made more or less unlike by
unlikenesses of physical conditions, inevitably led to occasional
concurrences of forces producing deviations of structure. These were of
course mostly suppressed, but sometimes increased, by survival of the
fittest. When, along with the growing multiplication in forms of life,
conflict and competition became continually more active, fortuitous
variations of structure of no account in the converse with the medium,
became of much account in the struggle with enemies and competitors; and
natural selection of such variations became the predominant factor.
Especially throughout the plant-world its action appears to have been
immensely the most important; and throughout that large part of the
animal world characterized by relative inactivity, the survival of
individuals that had varied in favourable ways, must all along have been
the chief cause of the divergence of species and the occasional
production of higher ones.

But gradually with that increase of activity which we see on ascending
to successively higher grades of animals, and especially with that
increased complexity of life which we also see, there came more and more
into play as a factor, the inheritance of those modifications of
structure caused by modifications of function. Eventually, among
creatures of high organization, this factor became an important one; and
I think there is reason to conclude that, in the case of the highest of
creatures, civilized men, among whom the kinds of variation which affect
survival are too multitudinous to permit easy selection of any one, and
among whom survival of the fittest is greatly interfered with, it has
become the chief factor: such aid as survival of the fittest gives,
being usually limited to the preservation of those in whom the totality
of the faculties has been most favourably moulded by functional changes.

Of course this sketch of the relations among the factors must be taken
as in large measure a speculation. We are now too far removed from the
beginnings of life to obtain data for anything more than tentative
conclusions respecting its earliest stages; especially in the absence of
any clue to the mode in which multiplication, first agamogenetic and
then gamogenetic, was initiated. But it has seemed to me not amiss to
present this general conception, by way of showing how the deductive
interpretation harmonizes with the several inferences reached by
induction.

                  *       *       *       *       *

In his article on Evolution in the _Encyclopædia Britannica_, Professor
Huxley writes as follows:—

  “How far 'natural selection' suffices for the production of species
  remains to be seen. Few can doubt that, if not the whole cause, it
  is a very important factor in that operation.... On the evidence of
  palaeontology, the evolution of many existing forms of animal life
  from their predecessors is no longer an hypothesis, but an
  historical fact; it is only the nature of the physiological factors
  to which that evolution is due which is still open to discussion.”

With these passages I may fitly join a remark made in the admirable
address Prof. Huxley delivered before unveiling the statue of Mr. Darwin
in the Museum at South Kensington. Deprecating the supposition that an
authoritative sanction was given by the ceremony to the current ideas
concerning organic evolution, he said that “science commits suicide when
it adopts a creed.”

Along with larger motives, one motive which has joined in prompting the
foregoing articles, has been the desire to point out that already among
biologists, the beliefs concerning the origin of species have assumed
too much the character of a creed; and that while becoming settled they
have been narrowed. So far from further broadening that broader view
which Mr. Darwin reached as he grew older, his followers appear to have
retrograded towards a more restricted view than he ever expressed. Thus
there seems occasion for recognizing the warning uttered by Prof.
Huxley, as not uncalled for.

Whatever may be thought of the arguments and conclusions set forth in
this article and the preceding one, they will perhaps serve to show that
it is as yet far too soon to close the inquiry concerning the causes of
organic evolution.

-----

Footnote 2:

  Though Mr. Darwin approved of this expression and occasionally
  employed it, he did not adopt it for general use; contending, very
  truly, that the expression Natural Selection is in some cases more
  convenient. See _Animals and Plants under Domestication_ (first
  edition) Vol. i, p. 6; and _Origin of Species_ (sixth edition) p. 49.

Footnote 3:

  It is true that while not deliberately admitted by Mr. Darwin, these
  effects are not denied by him. In his _Animals and Plants under
  Domestication_ (vol. ii, 281), he refers to certain chapters in the
  _Principles of Biology_, in which I have discussed this general
  inter-action of the medium and the organism, and ascribed certain most
  general traits to it. But though, by his expressions, he implies a
  sympathetic attention to the argument, he does not in such way adopt
  the conclusion as to assign to this factor any share in the genesis of
  organic structures—much less that large share which I believe it has
  had. I did not myself at that time, nor indeed until quite recently,
  see how extensive and profound have been the influences on
  organization which, as we shall presently see, are traceable to the
  early results of this fundamental relation between organism and
  medium. I may add that it is in an essay on “Transcendental
  Physiology,” first published in 1857, that the line of thought here
  followed out in its wider bearings, was first entered upon.

Footnote 4:

  _Text-Book of Botany_, &c. by Julius Sachs. Translated by A. W.
  Bennett and W. T. T. Dyer.

Footnote 5:

  _A Manual of the Infusoria_, by W. Saville Kent. Vol. i, p. 232.

Footnote 6:

  _Ib._ Vol. i, p. 241.

Footnote 7:

  Kent, Vol. i, p. 56.

Footnote 8:

  _Ib._ Vol. i, p. 57.

Footnote 9:

  _The Elements of Comparative Anatomy_, by T. H. Huxley, pp. 7-9.

Footnote 10:

  _A Treatise on Comparative Embryology_, by F. M. Balfour, Vol. ii,
  chap. xiii.

Footnote 11:

  Sachs, p. 210.

Footnote 12:

  _Ibid._ pp. 83-4.

Footnote 13:

  _Ibid._ p. 185.

Footnote 14:

  _Ibid._ p. 80.

Footnote 15:

  Sachs, p. 83.

Footnote 16:

  _Ibid._ p. 147.

Footnote 17:

  _A Treatise on Comparative Embryology._ By Francis M. Balfour, LL.D.,
  F.R.S. Vol. ii, p. 343 (second edition).

Footnote 18:

  Balfour, l.c. Vol. ii, 400-1.

Footnote 19:

  Balfour, l.c. Vol. ii, p. 401.

Footnote 20:

  For a general delineation of the changes by which the development is
  effected, see Balfour, l.c. Vol. ii, pp. 401-4.

Footnote 21:

  _See_ Balfour, Vol. i, 149 and Vol. ii, 313-4.




                                 NOTE.


After the above articles were published, I received from Dr. Downes a
copy of a paper “On the Influence of Light on Protoplasm,” written by
himself and Mr. T. P. Blunt, M.A., which was communicated to the Royal
Society in 1878. It was a continuation of a preceding paper which,
referring chiefly to _Bacteria_, contended that—

  “Light is inimical to, and under favourable conditions may wholly
  prevent, the development of these organisms.”

This supplementary paper goes on to show that the injurious effect of
light upon protoplasm results only in presence of oxygen. Taking first a
comparatively simple type of molecule which enters into the composition
of organic matter, the authors say, after detailing experiments:—

  “It was evident, therefore, that oxygen was the agent of destruction
  under the influence of sunlight.”

And accounts of experiments upon minute organisms are followed by the
sentence—

  “It seemed, therefore, that in absence of an atmosphere, light
  failed entirely to produce any effect on such organisms as were able
  to appear.”

They sum up the results of their experiments in the paragraph—

  “We conclude, therefore, both from analogy and from direct
  experiment, that the observed action on these organisms is not
  dependent on light per se, but that the presence of free oxygen is
  necessary; light and oxygen together accomplishing what neither can
  do alone: and the inference seems irresistible that the effect
  produced is a gradual oxidation of the constituent protoplasm of
  these organisms, and that, in this respect, protoplasm, although
  living, is not exempt from laws which appear to govern the relations
  of light and oxygen to forms of matter less highly endowed. A force
  which is indirectly absolutely essential to life as we know it, and
  matter in the absence of which life has not yet been proved to
  exist, here unite for its destruction.”

What is the obvious implication? If oxygen in presence of light destroys
one of these minutest portions of protoplasm, what will be its effect on
a larger portion of protoplasm? It will work an effect on the surface
instead of on the whole mass. Not like the minutest mass made inert all
through, the larger mass will be made inert only on its outside; and,
indeed, the like will happen with the minutest mass if the light or the
oxygen is very small in quantity. Hence there will result an envelope of
changed matter, inclosing and protecting the unchanged protoplasm—there
will result a rudimentary cell-wall.




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