Chapter 1

Species Concepts and Their A p p l i c a t i o n

Ernst Mayr

Darwin's choice of title for his great e v o l u t i o n a r y classic, O n the O r i g i n of

Species, was n o accident. T h e o r i g i n of n e w "varieties" w i t h i n species had
been taken for granted since the time of the Greeks. Likewise the occur-
rence of gradations, of "scales of perfection" a m o n g " h i g h e r " and " l o w e r "
organisms, was a familiar concept, t h o u g h usually interpreted i n a strictly
static manner. T h e species remained the great fortress of stability, and this
stability was the crux of the anti-evolutionist argument. "Descent w i t h
modification," true b i o l o g i c a l e v o l u t i o n , c o u l d be p r o v e d o n l y b y d e m o n -
strating that one species c o u l d originate f r o m another. It is a familiar and
often-told story h o w D a r w i n succeeded i n c o n v i n c i n g the w o r l d of the
occurrence of e v o l u t i o n and h o w — i n natural s e l e c t i o n — h e f o u n d the
mechanism that is responsible for e v o l u t i o n a r y change and adaptation. It is
not nearly so w i d e l y recognized that D a r w i n failed to solve the p r o b l e m
indicated b y the title of his w o r k . A l t h o u g h he demonstrated the m o -
dification of species i n the time dimension, he never seriously attempted a
rigorous analysis of the p r o b l e m of the multiplication of species, of the
splitting of one species into t w o . I have examined the reasons for this
failure ( M a y r 1959a) and f o u n d that foremost a m o n g them was D a r w i n ' s
uncertainty about the nature of species. The same can be said of those
authors w h o attempted to solve the p r o b l e m of speciation b y saltation or
other heterodox hypotheses. T h e y all failed to find solutions that are
workable i n the light of the m o d e r n appreciation of the p o p u l a t i o n struc-
ture of species. A n understanding of the nature of species, then, is an indis-
pensable prerequisite for the understanding of the e v o l u t i o n a r y process.

Species Concepts

The term species is frequently used to designate a class of similar things

to which a name has been attached. M o s t often this term is applied to
living organisms, such as birds, fishes, flowers, or trees, but it has also been
used for inanimate objects and even for human artifacts. M i n e r a l o g i s t s
speak of species of minerals, physicists of nuclear species; interior decora-
tors consider tables and chairs species of furniture. The application of the

L
16 E. M a y r

same term b o t h to organisms and to inanimate objects has led to much

confusion and an almost endless number of species definitions ( M a y r 1963,
1969); these, h o w e v e r , can be reduced to three basic species concepts. T h e
first t w o , m a i n l y applicable to inanimate objects, have considerable histori-
cal significance, because their advocacy was the cause of much past confu-
sion. T h e third is the species concept n o w prevailing i n b i o l o g y .

T h e T y p o l o g i c a l Species C o n c e p t
T h e t y p o l o g i c a l species concept, g o i n g back to the philosophies of Plato
and A r i s t o t l e (and thus sometimes called the essentialist concept), was the
species concept of Linnaeus and his followers (Cain 1958). A c c o r d i n g to
this concept, the observed d i v e r s i t y of the universe reflects the existence of
a limited number of u n d e r l y i n g "universals" or types (eidos of Plato). In-
dividuals d o not stand i n any special relation to one another, b e i n g merely
expressions of the same type. V a r i a t i o n is the result of imperfect manifesta-
tions of the idea implicit in each species. T h e presence of the same underly-
ing essence is inferred f r o m similarity, and m o r p h o l o g i c a l similarity is,
therefore, the species criterion for the essentialist. T h i s is the so-called
m o r p h o l o g i c a l species concept. M o r p h o l o g i c a l characteristics do p r o v i d e
valuable clues for the determination of species status. H o w e v e r , using
degree of m o r p h o l o g i c a l difference as the primary criterion for species
status is completely different f r o m utilizing m o r p h o l o g i c a l evidence to-
gether w i t h various other kinds of evidence i n order to determine whether
or not a p o p u l a t i o n deserves species rank under the b i o l o g i c a l species
concept. Degree of m o r p h o l o g i c a l difference is not the decisive criterion in
the ranking of taxa as species. T h i s is quite apparent f r o m the difficulties
into w h i c h a m o r p h o l o g i c a l - t y p o l o g i c a l species concept leads i n taxonomic
practice. Indeed, its o w n adherents abandon the t y p o l o g i c a l species con-
cept w h e n e v e r they discover that they have named as a separate species
s o m e t h i n g that is merely an individual variant.

T h e N o m i n a l i s t i c Species C o n c e p t
T h e nominalists ( O c c a m and his followers) deny the existence of " r e a l "
universals. F o r them o n l y individuals exist; species are man-made abstrac-
tions. ( W h e n they have to deal w i t h a species, they treat it as an individual
o n a higher plane.) T h e nominalistic species concept was popular i n France
in the eighteenth century and still has adherents today. Bessey (1908)
expressed this v i e w p o i n t particularly w e l l : " N a t u r e produces individuals
and n o t h i n g more . . . species have n o actual existence i n nature. T h e y are
mental concepts and n o t h i n g more . . . species have been invented i n order
that w e m a y refer to great numbers of individuals c o l l e c t i v e l y . "
A n y naturalist, whether a primitive native or a trained population
geneticist, k n o w s that this is simply not true. Species o f a n i m a l s are not
 Species Concepts, Application 17

human constructs, nor are they types i n the sense of Plato and A r i s t o t l e ;
but they are something for w h i c h there is n o equivalent i n the realm of
inanimate objects.
From the middle of the eighteenth century o n , the inapplicability of
these t w o medieval species concepts to b i o l o g i c a l species became increas-
ingly apparent. A n entirely n e w concept, applicable o n l y to species of
organisms, began to emerge i n the later w r i t i n g s of Buffon and of m a n y
other naturalists and taxonomists of the nineteenth century ( M a y r 1968).

The Biological Species Concept

This concept stresses the fact that species consist of populations and that
species have reality and an internal genetic cohesion o w i n g to the histori-
cally evolved genetic p r o g r a m that is shared b y all members of the species.
According to this concept, then, the members of a species constitute (1) a
reproductive c o m m u n i t y . T h e individuals of a species of animals respond to
one another as potential mates and seek one another for the purpose of
reproduction. A multitude of devices ensures intraspecific r e p r o d u c t i o n i n
all organisms. T h e species is also (2) a n e c o l o g i c a l u n i t that, regardless of the
individuals c o m p o s i n g it, interacts as a unit w i t h other species w i t h w h i c h
it shares the environment. T h e species, finally, is (3) a g e n e t i c u n i t consisting
of a large intercommunicating gene p o o l , whereas an i n d i v i d u a l is m e r e l y
a temporary vessel h o l d i n g a small p o r t i o n of the contents of the gene p o o l
for a short period of time. These three properties raise the species above
the typological interpretation of a "class of objects" ( M a y r 1963, 21). T h e
species definition that results f r o m this theoretical species concept is: Species
are groups of i n t e r b r e e d i n g n a t u r a l p o p u l a t i o n s that a r e r e p r o d u c i i v e l y isolated
from other such g r o u p s .
The development of the b i o l o g i c a l concept of the species is one of the
earliest manifestations of the emancipation of b i o l o g y f r o m an inappropri-
ate philosophy based o n the phenomena of inanimate nature. T h e species
concept is called b i o l o g i c a l not because it deals w i t h b i o l o g i c a l taxa, but
because the definition is biological. It utilizes criteria that are meaningless
as far as the inanimate w o r l d is concerned.
W h e n difficulties are encountered, it is important to focus o n the basic
biological meaning of the species: A species is a protected gene p o o l .
It is a M e n d e l i a n p o p u l a t i o n that has its o w n devices (called isolating
mechanisms) to protect it f r o m harmful gene f l o w f r o m other gene pools.
Genes of the same gene p o o l f o r m harmonious combinations because
they have become coadapted b y natural selection. M i x i n g the genes of t w o
different species leads to a h i g h frequency of disharmonious gene c o m b i n a -
tions; mechanisms that prevent this are therefore favored b y selection.
Thus it is quite clear that the w o r d "species" i n b i o l o g y is a relational term.
18 E. M a y r

A is a species i n relation to B or C because it is reproductively isolated from

them. T h e b i o l o g i c a l species concept has its primary significance w i t h
respect to sympatric and synchronic populations (existing at a single locali-
ty and at the same time), and t h e s e — t h e "nondimensional species"—are
precisely the ones where the application of the concept faces the fewest
difficulties. The more distant t w o populations are i n space and time, the
more difficult it becomes to test their species status in relation to each
other, but also the more irrelevant b i o l o g i c a l l y this becomes.
The b i o l o g i c a l species concept also solves the paradox caused b y the
conflict between the fixity of the species of the naturalist and the fluidity of
the species of the evolutionist. It was this conflict that made Linnaeus deny
e v o l u t i o n and D a r w i n the reality of species ( M a y r 1957). The biological
species combines the discreteness of the local species at a g i v e n time w i t h
an e v o l u t i o n a r y potential for continuing change.

T h e Species C a t e g o r y a n d Species Taxa

The a d v o c a c y of three different species concepts has been one of the t w o

major reasons for the "species p r o b l e m . " The second is that many authors
have failed to make a distinction between the definition of the species
category and the delimitation of species taxa (for fuller discussion see M a y r
1969).
A c a t e g o r y designates a g i v e n rank or level i n a hierarchic classification.
Such terms as "species," "genus," "family," and "order" designate categories.
A category, thus, is an abstract term, a class name, w h i l e the organisms
placed i n these categories are concrete z o o l o g i c a l objects.
O r g a n i s m s , i n turn, are classified not as individuals, but as groups of
organisms. W o r d s like "bluebirds," "thrushes," "songbirds," or "vertebrates"
refer to such groups. These are the concrete objects of classification. A n y
such g r o u p of populations is called a t a x o n if it is considered sufficiently
distinct to be w o r t h y of being formally assigned to a definite category i n
the hierarchic classification . A t a x o n is a t a x o n o m i c g r o u p of a n y r a n k that is
sufficiently distinct to be w o r t h y of b e i n g assigned to a definite category.
T w o aspects of the t a x o n must be stressed. A taxon always refers to
specified organisms. Thus the species is not a taxon, but any g i v e n species,
such as the r o b i n ( T a r d u s m i g r a t o r i u s ) is. Second, the t a x o n must be formally
recognized as such, b y being described under a designated name.
Categories, w h i c h designate a rank i n a hierarchy, and taxa, w h i c h
designate named g r o u p i n g s of organisms, are thus t w o v e r y different kinds
of phenomena. A somewhat analogous situation exists i n our human affairs.
Fred Smith is a concrete person, but "captain" or "professor" is his rank
in a hierarchy of levels.
 Species Concepts, Application 19

The A s s i g n m e n ^ o f T a x a to t h e Species Category

Much of the task of the taxonomist consists of assigning taxa to the

appropriate categorical rank. In this procedure there is a drastic difference
between the species taxon and the higher taxa. H i g h e r taxa are defined
by intrinsic characteristics. Birds is the class of feathered vertebrates. A n y
and all species that satisfy the definition of "feathered vertebrates" b e l o n g
to the class of birds. A n essentialist (typological) definition is satisfactory
and sufficient at the level of the higher taxa. It is, h o w e v e r , irrelevant and
misleading to define species i n an essentialistic w a y because the species is
not defined b y intrinsic, but b y r e l a t i o n a l properties.
Let me explain this. There are certain w o r d s that indicate a relational
property, like the w o r d "brother." Being a brother is not an inherent
property of an individual, as hardness is a p r o p e r t y of a stone. A n i n d i v i -
dual is a brother o n l y w i t h respect to someone else. The w o r d "species"
likewise designates such a relational property. A p o p u l a t i o n is a species
with respect to all other populations w i t h w h i c h it exhibits the relationship
of reproductive i s o l a t i o n — n o n i n t e r b r e e d i n g . If o n l y a single p o p u l a t i o n
existed i n the entire w o r l d , it w o u l d be meaningless to call it a species.
Noninterbreeding between populations is manifested b y a gap. It is this
gap between populations that coexist (are sympatric) at a single locality
at a g i v e n time w h i c h delimits the species recognized b y the local natural-
ist. Whether one studies birds, mammals, butterflies, or snails near one's
home t o w n , one finds each species clearly delimited and sharply separated
from all other species. This demarcation is sometimes referred to as the
species delimitation in a n o n d i m e n s i o n a l s y s t e m (a system w i t h o u t the
dimensions of space and time).
A n y o n e can test the reality of these discontinuities for himself, even
where the m o r p h o l o g i c a l differences are slight. In eastern N o r t h A m e r i c a ,
for instance, there are four similar species of the thrush genus C a t h a r u s
(Table 1.1), the veery (C. fuscescens), the hermit thrush (C. g u t t a t u s ) , the
olive-backed or Swainson's thrush (C. u s t u l a t u s ) , and the gray-cheeked
thrush (C. m i n i m u s ) . These four species are sufficiently similar visually
to confuse not o n l y the human observer, but also silent males of the
other species. The species-specific songs and call notes, h o w e v e r , permit
easy species discrimination, as observationally substantiated b y D i l g e r
(1956). Rarely d o more than t w o species breed i n the same area, and the
overlapping species, / + g, g + u, and u + m , usually differ considerably
in their foraging habits and niche preference, so that c o m p e t i t i o n is m i n i -
mized w i t h each other and w i t h t w o other thrushes, the r o b i n ( T u r d u s
migratorius) and the w o o d thrush ( H y l o c i c h l a m u s t e l i n a ) , w i t h w h i c h they
share their geographic range and m a n y ecological requirements. In connec-
tion w i t h their different foraging and m i g r a t o r y habits the four species
20 E. M a y r

Table 1.1

Characteristics of four eastern N o r t h American species of C a t h a r u s (from Dilger 1956)

Characteristic
compared C. fuscescens C. g u t t a t u s C. ustulatus C. minimus

Breeding
range Southernmost M o r e northerly Boreal Arctic

Wintering No. South So. United C. America No. South

area America States to Argentina America

Breeding Bottomland Coniferous M i x e d or Stunted

habitat woods with woods mixed pure tall northern fir
lush under- with deciduous coniferous and spruce
growth forests forests

Foraging G r o u n d and G r o u n d (inner Largely Ground

arboreal (forest forest edges) arboreal (forest
interior) (forest interior)
interior)

Nest Ground Ground Trees Trees

Spotting on
eggs Rare Rare Always Always

Relative wing
length Medium Short V e r y long Medium

Hostile call veer chuck peep beer

pheu seeeep chuck-burr

Song V e r y distinct V e r y distinct V e r y distinct V e r y distinct

Flight song Absent Absent Absent Present

differ f r o m one another (and f r o m other thrushes) i n the relative length of

w i n g and leg elements and i n the shape of the bill. There are thus many
small differences between these at first sight v e r y similar species. M o s t
important, n o hybrids or intermediates a m o n g these four species have ever
been found. Each is a separate genetic, behavioral, and ecological system,
separated f r o m the others b y a complete biological discontinuity, a gap.

Difficulties in the A p p l i c a t i o n of the Biological Species Concept

T h e practicing taxonomist often has difficulties w h e n he endeavors to

assign populations to the correct rank. Sometimes the difficulty is caused
by a lack of information concerning the degree of variability of the species
w i t h w h i c h he is dealing. H e l p f u l hints o n the solution of such practical
difficulties are g i v e n i n the technical taxonomic literature ( M a y r 1969).
M o r e interesting to the evolutionist are the difficulties that are intro-
duced w h e n the dimensions of time and space are added. M o s t species
taxa d o not consist merely of a single local population but are an aggregate
of numerous local populations that exchange genes w i m each other to
 Species Concepts, Application 21

a greater or lejger degree. T h e more distant that t w o populations are f r o m

each other, the more likely they are to differ i n a number of characteristics. I
show elsewhere ( M a y r 1963, ch. 10 and 11 ) that some of these p o p u l a -
tions are incipient species, h a v i n g acquired some but not all characteristics
of species. O n e or another of the three most characteristic properties of
species t a x a — r e p r o d u c t i v e isolation, ecological difference, and m o r p h o -
logical d i s t i n g u i s h a b i l i t y — i s i n such cases o n l y incompletely d e v e l o p e d .
The application of the species concept to such incompletely speciated
populations raises considerable difficulties. There are six w h o l l y different
situations that may cause difficulties.

1. Evolutionary c o n t i n u i t y in space a n d t i m e W i d e s p r e a d species m a y have

terminal populations that behave t o w a r d each other as distinct species
even though they are connected b y a chain of interbreeding populations.
Cases of reproductive isolation a m o n g geographically distant populations
of a single species are discussed i n M a y r 1963, ch. 16.

2. A c q u i s i t i o n of r e p r o d u c t i v e isolation w i t h o u t corresponding morphological

change W h e n the reconstruction of the g e n o t y p e i n an isolated p o p u l a -
Hon has resulted i n the acquisition of reproductive isolation, such a p o p u l a -
Hon must be considered a b i o l o g i c a l species. If the correlated m o r p h o l o g i -
cal change is v e r y slight or unnoticeable, such a species is called a sibling
species ( M a y r 1963, ch. 3).

3. M o r p h o l o g i c a l differentiation w i t h o u t a c q u i s i t i o n of r e p r o d u c t i v e isolation
Isolated populations sometimes acquire a degree of m o r p h o l o g i c a l diver-
gence one w o u l d ordinarily expect o n l y i n a different species. Y e t some
such populations, although as different m o r p h o l o g i c a l l y as g o o d species,
interbreed indiscriminately where they come i n contact. T h e W e s t Indian
snail genus C e r i o n illustrates this situation particularly w e l l (fig. 1.1).

4. Reproductive isolation d e p e n d e n t on habitat isolation N u m e r o u s cases have

been described i n the literature i n w h i c h natural populations acted t o w a r d
each other like g o o d species (in areas of contact) as l o n g as their habitats
were undisturbed. Y e t the reproductive isolation broke d o w n as soon as
the characteristics of these habitats were changed, usually b y the interfer-
ence of man. Such cases of secondary b r e a k d o w n of isolation are discussed
in M a y r 1963, ch. 6.

5. Incompleteness of isolating mechanisms V e r y few isolating mechanisms

are all-or-none devices (see M a y r 1963, ch. 5). T h e y are built up step b y
step, and most isolating mechanisms of an incipient species are imperfect
and incomplete. Species level is reached w h e n the process of speciation has
22 E. M a y r

Figure 1.1
T h e distribution pattern of populations of the halophilous land snail C e r i o n on the Banes
Peninsula in eastern Cuba. Numbers refer to distinctive races or "species." Where two
populations come in contact (with one exception) they hybridize ( H ) , regardless of degree
of difference. In other cases contact is prevented b y a barrier ( B ) . I n = isolated inland
population.
 Species Concepts, Application 23

become irreversible, even if some of the secondary isolating mechanisms

have not yet reached perfection (see M a y r 1963, ch. 17).

6. A t t a i n m e n t of different levels of s p e c i a t i o n in different local p o p u l a t i o n s The

perfecting of isolating mechanisms m a y proceed i n different populations of
a polytypic species (one h a v i n g several subspecies) at different rates. T w o
widely overlapping species may, as a consequence, be completely distinct
at certain localities but m a y freely h y b r i d i z e at others. M a n y cases of
sympatric h y b r i d i z a t i o n discussed i n M a y r 1963, ch. 6, fit this characteriza-
tion (see M a y r 1969, for advice o n handling such situations).
These six types of phenomena are consequences of the gradual nature
of the ordinary process of speciation (excluding p o l y p l o i d y ; see M a y r
1963, 254). Determination of species status of a g i v e n p o p u l a t i o n is difficult
or arbitrary i n m a n y of these cases.

Difficulties Posed by U n i p a r e n t a l R e p r o d u c t i o n

The task of assembling individuals into populations and species taxa is

very difficult i n most cases i n v o l v i n g uniparental (asexual) reproduction.
Self-fertilization, parthenogenesis, pseudogamy, and vegetative reproduc-
tion are forms of uniparental reproduction. T h e b i o l o g i c a l species concept,
which is based o n the presence or absence of interbreeding between natural
populations, cannot be applied to groups w i t h o b l i g a t o r y asexual repro-
duction because interbreeding of populations is nonexistent i n these groups.
The nature of this dilemma is discussed i n more detail elsewhere ( M a y r
1963, 1969). Fortunately, there seem to be rather well-defined discontinu-
ities among most kinds of uniparentally reproducing organisms. These
discontinuities are apparently produced b y natural selection f r o m the v a r i -
ous mutations that occur i n the asexual lines (clones). It is customary to
utilize the existence of such discontinuities and the amount of m o r p h o l o g i -
cal difference between them to delimit species a m o n g uniparentally repro-
ducing types.

The I m p o r t a n c e of a N o n a r b i t r a r y Definition of Species

The clarification of the species concept has led to a clarification of m a n y

evolutionary problems as w e l l as, often, to a simplification of practical
problems i n t a x o n o m y . The correct classification of the m a n y different
kinds of varieties (phena), of p o l y m o r p h i s m ( M a y r 1963, ch. 7), of p o l y -
typic species (ibid. ch. 12), and of b i o l o g i c a l races (ibid. ch. 15) w o u l d
be impossible w i t h o u t the arranging of natural populations and phenotypes
into biological species. It was impossible to solve, indeed e v e n to state
precisely, the problem of the multiplication of species until the b i o l o g i c a l
24 E. M a y r

species concept had been d e v e l o p e d . T h e genetics of speciation, the role of

species in large-scale e v o l u t i o n a r y trends, and other major evolutionary
problems c o u l d not be discussed profitably until the species p r o b l e m was
settled. It is evident then that the species p r o b l e m is of great importance
in e v o l u t i o n a r y b i o l o g y and that the g r o w i n g agreement o n the concept of
the b i o l o g i c a l species has resulted in a u n i f o r m i t y of standards and a
precision that is beneficial for practical as w e l l as theoretical reasons.

T h e B i o l o g i c a l M e a n i n g of Species

The fact that the organic w o r l d is o r g a n i z e d into species seems so funda-

mental that one usually forgets to ask w h y there are species, what their
m e a n i n g is in the scheme of things. There is n o better w a y of answering
these questions than to try to conceive of a w o r l d w i t h o u t species. Let us
think, for instance, of a w o r l d in w h i c h there are o n l y individuals, all
b e l o n g i n g to a single interbreeding c o m m u n i t y . Each i n d i v i d u a l is in v a r y -
i n g degrees different f r o m e v e r y other one, and each i n d i v i d u a l is capable
of m a t i n g w i t h those others that are most similar to it. In such a w o r l d , each
i n d i v i d u a l w o u l d be, so to speak, the center of a series of concentric rings
of increasingly more different individuals. A n y t w o mates w o u l d be o n the
average rather different f r o m each other and w o u l d produce a vast array of
genetically different types a m o n g their offspring. N o w let us assume that
one of these recombinations is particularly w e l l adapted for one of the
available niches. It is prosperous in this niche, but w h e n the time for m a t i n g
comes, this superior g e n o t y p e w i l l inevitably be b r o k e n up. There is n o
m e c h a n i s m that w o u l d prevent such a destruction of superior gene c o m b i -
nations, and there is, therefore, n o possibility of the gradual i m p r o v e m e n t
of gene combinations. T h e significance of the species n o w becomes e v i -
dent. T h e r e p r o d u c t i v e isolation of a species is a protective device that
guards against the breaking up of its well-integrated, coadapted gene
system. O r g a n i z i n g organic d i v e r s i t y into species creates a system that
permits genetic diversification and the accumulation of favorable genes and
gene c o m b i n a t i o n s w i t h o u t the danger of destruction of the basic gene
c o m p l e x . T h e r e are definite limits to the amount of genetic variability that
can be a c c o m m o d a t e d in a single gene p o o l w i t h o u t p r o d u c i n g t o o h i g h a
proportion of inviable recombinants. O r g a n i z i n g genetic diversity into
protected gene p o o l s — t h a t is, species—guarantees that these limits are
not o v e r s t e p p e d . T h i s is the b i o l o g i c a l m e a n i n g of species.

References

Bessey, C . E 1 9 0 8 ‫״‬ , T h e taxonomic aspect of the species. A m e r i c a n Naturalist 42:218-

224. ^
Cain, A . J., 1958, Logic and memory in Linnaeus's system of taxonomy. P r o c . L i n n . Soc,
London, 169:144-163.
 Species Concepts, Application 25

Mayr, E., 1957,^pecies concepts and definitions. A m e r . A s s o c . A d v . S c i . , Publ. N o . 5 0 : 1 - 2 2 ,

Washington, D . C .
Mayr, E., 1959a, D a r w i n and the evolutionary theory in b i o l o g y . In E v o l u t i o n a n d Anthropol-
o g y : A C e n t e n n i a l A p p r o a c h , Anthropological Society of America, Washington, D . C .
Mayr, E 1 9 6 3 ‫״‬ , A n i m a l S p e c i e s a n d E v o l u t i o n , Cambridge: Harvard University Press.
Mayr, E., 1968, Illiger and the biological species concept. ] . H i s t . B i o l . 1 : 1 6 3 - 1 7 8 .
Mayr, E 1 9 6 9 ‫״‬ , P r i n c i p l e s o f S y s t e m a t i c Z o o l o g y . N e w York: M c G r a w - H i l l .
Mayr, E., 1976, Is the Species a Class or an Individual? S y s t e m a t i c Z o o l o g y 2 5 : 1 9 2 .
Mayr, E., 1987, T h e O n t o l o g i c a l Status of Species: Scientific Progress and Philosophical
Terminology. B i o l o g y a n d P h i l o s o p h y 2 : 1 4 5 - 1 6 6 .
Mayr, E., 1987, Answers to These Comments. B i o l o g y a n d P h i l o s o p h y 2 : 2 1 2 - 2 2 0 .
Mayr, E., 1988, T h e Species Category. In E. M a y r , Toward a New Philosophy o f Biology.
Cambridge, Harvard University Press, 3 1 5 - 3 3 4 .
Mayr, E., 1988, T h e W h y and H o w of Species. B i o l o g y a n d P h i l o s o p h y 3 : 4 3 1 - 4 4 2 .
Mayr, E., and L. Short, 1970, Species Taxa of North American Birds. Cambridge: Nuttall
Ornithological Club, Publication N u m b e r 9.