DARWIN ON SPECIES DEFINITIONS AND SPECIATION:

Darwin 1859 On the Origin of Species by Natural Selection
or
The Preservation of Favoured Races in the Struggle for Life

(Prepared by James Mallet, 20 January 1995)

p. 44 Beginning of Ch II.  Variation under Nature
"No one definition (of species) has as yet satisfied all
naturalists; yet every naturalist knows vaguely what he means
when he speaks of a species.  Generally the term includes the
unknown element of a distinct act of creation.  The term
"variety" is almost equally difficult to define; but here
community of descent is almost universally implied, though it
can rarely be proved."

p. 47
Practically, when a naturalist can unite two forms together by
others having intermediate characters, he treats the one as a
variety of the other, ranking the most common, but sometimes
the one first described, as the species, and the other as the
variety.  But cases of great difficulty, which I will not here
enumerate, sometimes occur in deciding whether or not to rank
one form as a variety of another, even when they are closely
connected by intermediate links; nor will the commonly-assumed
hybrid nature of the intermediate links always remove the difficulty. ...

     Hence, in determining whether a form should be ranked as
a species or a variety, the opinion of naturalists having
sound judgement and wide experience seems the only guide to
follow.

Editorial note: The above sentence was used by Ernst Mayr
(1982, The Growth of Biological Thought) to illustrate that
Darwin didn't understand the true nature of species. 
The preceding discussion was omitted.


p. 49
Many of the cases of strongly-marked varieties or doubtful species
well deserve consideration; for several interesting lines of argument,
from geographical distribution, analogical variation, hybridism, etc.,
have been brought to bear on the attempt to determine their rank. I
will here give only a single instance,--the well-known one of the
primrose and cowslip, or Primula veris and elatior. These plants
differ considerably in appearance; they have a different flavour and
emit a different odour; they flower at slightly different periods;
they grow in somewhat different stations; they ascend mountains to
different heights; they have different geographical ranges; and
lastly, according to very numerous experiments made during several
years by that most careful observer Gartner, they can be crossed only
with much difficulty. We could hardly wish for better evidence of the
two forms being specifically distinct. On the other hand, they are
united by many intermediate links, and it is very doubtful whether
these links are hybrids; and there is, as it seems to me, an
overwhelming amount of experimental evidence, showing that they
descend from common parents, and consequently must be ranked as
varieties.

Close investigation, in most cases, will bring naturalists to an
agreement how to rank doubtful forms. Yet it must be confessed, that
it is in the best-known countries that we find the greatest number of
forms of doubtful value. I have been struck with the fact, that if any
animal or plant in a state of nature be highly useful to man, or from
any cause closely attract his attention, varieties of it will almost
universally be found recorded. These varieties, moreover, will be
often ranked by some authors as species. Look at the common oak, how
closely it has been studied; yet a German author makes more than a
dozen species out of forms, which are very generally considered as
varieties; and in this country the highest botanical authorities and
practical men can be quoted to show that the sessile and pedunculated
oaks are either good and distinct species or mere varieties.

p. 52 mid Ch II.
From these remarks it will be seen that I look at the term
species, as one arbitrarily given for the sake of convenience
to a set of individuals closely resembling each other, and
that it does not essentially differ from the term variety,
which is given to less distinct and more fluctuating forms.
The term variety, again, in comparison with mere individual
differences, is also applied arbitrarily, and for mere
convenience sake.

p. 58 end Ch. II.
Editorial note: Interpretations of this section have
been very influential in the debate about whether
Darwin's book was really about speciation, in spite
of its title.  In the 1930s and 1940s, biologists'
understanding of this section led to the attempt to
formulate a "new species concept", later known as
"the biological species concept".  

Mayr (1942, p. 147), for example: "It is thus quite
true, as several recent authors have indicated, that
Darwin's book was misnamed, because it is a book on
evolutionary changes in general and the factors that
control them (selection and so forth), but not a treatise
on the origin of species".

Mayr (1963, p.12)  "Darwin succeeded in convincing
the world of the occurrence of evolution and ... he
found (in natural selection) the mechanism that is
responsible for evolutionary change and adaptation. 
It is not nearly so widely recognized that Darwin failed
to solve the problem indicated by the title of his work."

Again, Mayr (1963, p.14): "In retrospect, it is
apparent that Darwin's failure, as well as that of the
antievolutionists, resulted to a large extent from a
misunderstanding of the true nature of species."

Mayr quotes the following section of the origin in
support of these arguments (see p. 267 in his 1982
Growth of Biological Thought). However, Mayr puts
a full stop directly after "distinguished from species",
omitting the whole section starting with  "... , - except
....".  In this way Mayr changes the whole sense of the
(admittedly rather long) sentence.

Finally, then, varieties have the same general characters as
species, for they cannot be distinguished from species, --
except, firstly, by the discovery of intermediate linking
forms, and the occurrence of such links cannot affect the
actual characters of the forms which they connect; and except,
secondly, by a certain amount of difference, for two forms, if
differing very little, are generally ranked as varieties,
notwithstanding that intermediate linking forms have not been
discovered; but the amount of difference considered necessary
to give to two forms the rank of species is quite indefinite.

p. 103 Ch. IV.  Natural selection.  In section "Circumstances
favourable and unfavourable to natural selection, namely,
intercrossing, isolation, number of individuals."
Intercrossing plays a very important part in nature in keeping
the individuals of the same species, or of the same variety,
true and uniform in character.   

Editorial note: In this section, Darwin discusses various
problems with assuming that geographic isolation was
important in speciation.  Evidently, this was a topic
discussed in his day.

p. 104-6
Isolation, also, is an important element in the process of
natural selection.  In a confined or isolated area ... natural
selection will tend to modify all the individuals of a varying
species throughout the area in the same manner in relation to
the same conditions.  Intercrosses also, with the individuals
of the same species, which otherwise would have inhabited the
surrounding and differently circumstanced districts, will be
prevented.  But isolation probably acts more efficiently in
checking the immigration of better adapted organisms, after
any physical change, such as of climate or elevation of the
land, &c; and thus new places in the natural economy of the
country are left open for the old inhabitants to struggle for,
and become adapted to, through modification in their structure
and constitution.  Lastly, isolation, by checking immigration
and consequently competition, will give time for any new
variety to be slowly improved; and this may sometimes be of
importance in the production of new species.  If, however, an
isolated area be very small, either from being surrounded by
barriers, or from having very peculiar physical conditions,
the total number of the individuals supported on it will
necessarily be very small; and fewness of individuals will
greatly retard the production of new species through natural
selection, by decreasing the chance of the appearance of
favourable mutations.

     If we ... look at any small isolated area, such as an
oceanic island, although the total number of the species
inhabiting it, will be found to be small, as we shall see in
our chapter on geographical distribution; yet of these species
a very large proportion are endemic...  Hence an oceanic
island at first sight seems to have been highly favourable for
the production of new species.  But we may thus greatly
deceive ourselves, for to ascertain whether a small isolated
area, or a large open area like a continent, has been most
favourable for the production of organic forms, we ought to
make the comparison of equal times; and this we are incapable
of doing.

     Although I do not doubt that isolation is of considerable
importance in the production of new species, on the whole I am
inclined to believe that largeness of area is of more
importance, more especially in the production of new species,
which will prove capable of enduring for a long period, and of
spreading widely.  Throughout, a great and open area, not only
will there be a better chance of favourable variations arising
from the large number of individuals of the same species there
supported, but the conditions of life are infinitely complex
from the large number of already existing species; and if some
of these many species become modified and improved, others
will have to be improved in a corresponding degree or they
will be exterminated. ...  the course of modification will
generally have been rapid on large areas; and what is more
important, that the new forms produced on large areas, which
already have been victorious over many competitors, will be
those that will spread most widely, will give rise to most
varieties and species, and will thus play an important part in
the changing history of the organic world.

     We can, perhaps, on these views, understand some facts
which will be again alluded to in our chapter on geographical
distribution; for instance, that the productions of the
smaller continent of Australia have formerly yielded, and
apparently now are yielding, before those of the larger
Europaeo-Asiatic area.  Thus, also, it is that continental
productions have everywhere become so largely naturalized on
islands.  

p. 107-8
     Hence, perhaps, it comes that the flora of Madeira,
according to Oswald Heer, resembles the extinct Tertiary flora
of Europe.  All fresh-water basins, taken together, make a
small area compared with that of the sea or of the land; and,
consequently, the competition between freshwater productions
will have been less severe than elsewhere; new forms will have
been slowly formed, and old forms more slowly exterminated.

...  I conclude, looking to the future, that for terrestrial
productions a large continental area, which will probably
undergo many oscillations of level, and which consequently
will exist for long periods in a broken condition, will be the
most favourable for the production of many new forms of life,
likely to endure long and to spread widely.  When converted by
subsidence into large separate islands, there will still exist
many individuals of the same species on each island:
intercrossing on the confines of each species will thus be
checked: after physical changes of any kind, immigration will
be prevented, so that new species in the polity of each island
will have to be filled up by modifications of the old
inhabitants; and time will be allowed for the varieties in
each to become well modified and perfected.  When, by renewed
elevation, the islands shall be re-converted into a
continental area, there will again be severe competition: the
most favoured or improved varieties will be enabled to spread:
there will be much extinction of the less improved forms, and
the relative proportional numbers of the various inhabitants
of the renewed continent will again be changed; and there will
be a fair field for natural selection to improve still further
the inhabitants, and thus produce a new species.

p. 171 Ch. VI.  Difficulties on Theory.

"Firstly, why, if species have descended from other species by
insensibly fine gradations, do we not everywhere see
innumerable transitional forms?  Why is not all nature in
confusion instead of species being, as we see them, well
defined?

     Secondly (organs of trifling importance such as the
giraffe's tail vs. organs of inimitable perfection, such as
the eye)


p. 172
     Thirdly, can instincts be acquired and modified by
natural selection?

     Fourthly, how can we account for species, when crossed,
being sterile and producing sterile offspring, whereas, when
varieties are crossed, their fertility is unimpaired?

p. 177
     To sum up, I believe that species come to be tolerably
well-defined objects, and do not at any one period present an
inextricable chaos of varying and intermediate links: firstly
because new varieties are very slowly formed...
p. 178
     Secondly, areas now continuous must often have existed
within the recent period in isolated portions, in which many
forms, more especially amongst the classes which unite for
each birth and wander much, may have separately been rendered
sufficiently distinct to rank as representative species. ...
p. 178-9
     Thirdly, when two or more varieties have been formed in
different portions of a strictly continuous area, intermediate
varieties will, it is probable, at first have been formed in
the intermediate zones, but they will generally have had a
short duration.  (Because intermediate zones are smaller and
will have lesser numbers of the species occupying them, and)
From this cause alone, the intermediate varieties will be
liable to accidental extermination; and during the process of
further modification through natural selection, they will
almost certainly be beaten and supplanted by the forms which
they connect; for these from existing in greater numbers will,
in the aggregate, present more variation, and thus be further
improved through natural selection and gain further
advantages.
p.179
     Lastly, looking not to any one time, but to all time, if
my theory be true, numberless intermediate varieties, linking
most closely all the species of the same group together, must
assuredly have existed; but the very process of natural
selection constantly tends .. to exterminate the parent-forms
and the intermediate links.

Editorial note: the following chapter has been much
misunderstood.  I interpret it as a clear refutation of a
simple "reproductive isolation" view or "biological
concept" of species, and at the same time a strong
argument for hybrid problems being a result of
pleiotropic effects of natural selection, rather than
being selected directly as "isolating mechanisms".
The section refers to sterility, rather than
inviability or any form of pre-mating isolation.

p.246 in Ch VIII. Hybridism
     Pure species have of course their organs of reproduction
in a perfect condition, yet when intercrossed they produce
either few or no offspring.  Hybrids, on the other hand have
their reproductive organs functionally impotent, ...

     The fertility of varieties, that is of the forms known or
believed to have descended from common parents, when
intercrossed, and likewise the fertility of their mongrel
offspring, is, on my theory, of equal importance with the
sterility of species; for it seems to make a broad and clear
distinction between varieties and species.

p. 246-7
Kölreuter makes the rule (of sterility of species)
universal; but then he cuts the knot, for in ten cases in
which he found two forms, considered by most authors as
distinct species, quite fertile together, he unhesitatingly
ranks them as varieties.  Gärtner, also, makes the rule
equally universal; and he disputes the entire fertility of
Kölreuter's ten cases.
p. 248
... for all practical purposes it is most difficult to say
where perfect fertility ends and sterility begins.  I  think
no better evidence of this can be required than that the two
most experienced observers who have ever lived, namely,
Kölreuter and Gärtner, should have arrived at
diametrically opposite conclusions in regard to the very same
species.
p.257
Now the fertility of first crosses between species, and of the
hybrids produced from them, is largely governed by their
systematic affinity.  This is clearly shown by hybrids never
having been raised between species ranked by systematists in
distinct families; and on the other hand by very closely
allied species generally uniting with facility.  But the
correspondence between systematic affinity and facility of
crossing is by no means strict. ... (for example) In the same
family there may be a genus, as Dianthus in which very many
species can most readily be crossed; and another genus, as
Silene, in which the most persevering efforts have failed to
produce between extremely close species a single hybrid.
p. 260-1
     Now do these complex and singular rules indicate that
species have been endowed with sterility simply to prevent
their becoming confounded in nature?  I think not.  For why
should sterility be so extremely different in degree, when
various species are crossed, all of which we must suppose it
would be equally important to keep from blending together?
Why should the degree of sterility be innately variable in the
individuals of the same species?  Why should some species
cross with facility, and yet produce very sterile hybrids; and
other species cross with extreme difficulty, and yet produce
fairly fertile hybrids?  Why, it may even be asked, has the
production of hybrids been permitted? to grant to species the
special power of producing hybrids, and then to stop their
further propagation by different degrees of sterility, not
strictly related to the facility of the first union between
their parents, seems to be a strange arrangement.

     The foregoing rules and facts, on the other hand, appear
to me clearly to indicate that the sterility, both of the
first crosses and of hybrids is simply incidental or dependent
on unknown differences, chiefly in the reproductive systems,
of the species which are crossed.  The differences being of so
peculiar and limited a nature, that in reciprocal crosses
between two species the male sexual element in one will often
freely act on the female sexual element of the other, but not
in reversed direction.

p. 267-8
Fertility of Varieties when crossed, and of their Mongrel
offspring {, Not Universal - added to heading in 6th
edition 1872}. -- It may be urged, as a most forceful argument,
that there must be some essential distinction between species
and varieties, and that there must be some error in all the
foregoing remarks, inasmuch as varieties, however much they
may differ from each other in external appearance, cross with
perfect facility.  I fully admit that this is almost
invariably the case.  But if we look to varieties produced
under nature, we are immediately involved in hopeless
difficulties; for if two hitherto reputed varieties be found
in any degree sterile together, they are at once ranked by
most naturalists as species.  For instance the blue and red
pimpernel, the primrose and the cowslip, which are considered
by many of our best botanists as varieties, are said by
Gärtner not to be quite fertile when crossed, and he
consequently ranks them as undoubted species.

p. 269
I have as yet spoken as if the varieties of the same species
were invariably fertile when intercrossed. But it seems to me
impossible to resist the evidence of a certain amount of sterility
in the few following cases, which I will briefly abstract.  The
evidence is at least as good as that from which we believe in the
sterility of a multitude of species.  The evidence is, also, derived
from hostile witnesses, who in all other cases consider fertility
and sterility as safe criterions of specific distinction.  

    Gärtner : dwarf maize yellow seeds x tall red seeds
        partial sterility/seed abortion, pre-mating isol?
    Girou de Buzareingues: 3 varieties of gourd
        reduced fertilization efficiency    
    So hostile a witness, as Gärtner: yellow x white varieties
        of Verbascum – less seed when intercrossed; but
        more seed when intercrossed with same variety
        of different species.
    Darwin: hollyhocks own obss. (dropped in 6th Ed.)
    Kölreuter: one variety of common tobacco was more
        fertile with another species, N. glutinosa, than
        others, yet all were varieties of the same species.

p. 271-2: From these facts; from the great difficulty of
ascertaining the infertility of varieties in a state of nature, for
a supposed variety if infertile in any degree would generally
be ranked as species; from man selecting only external
characters in the production of the most distinct domestic
varieties, and from not wishing or being able to produce
recondite and functional differences in the reproductive
system; from these several considerations and facts, I do
not think that the very general fertility of varieties can be
proved to be of universal occurrence, or to form a
fundamental distinction between varieties and species.  The
general fertility of varieties does not seem to me sufficient to
overthrow the view which I have taken with respect to the
very general, but not invariable, sterility of first crosses and
of hybrids, namely, that it is not a special endowment, but is
incidental on slowly acquired modifications, more especially
in the reproductive systems of the forms which are crossed.

p.278, in the final summary of the chapter on Hybridism:

Finally, then, the facts briefly given in this chapter do not
seem to me opposed to, but even rather to support the view,
that there is no fundamental distinction between species and
varieties.

p. 297 Ch IX.  On the imperfection of the Geological Record.
     It is all-important to remember that naturalists have no
golden rule by which to distinguish species and varieties;
they grant some little variability to each species, but when
they meet with a somewhat greater amount of difference between
any two forms, they rank both as species, unless they are
enabled to connect them together by close intermediate
gradations.

p. 484-5. Ch. XIV Recapitulation and conclusion.
     When the views entertained in this volume on the origin
of species, or when analogous views are generally admitted, we
can dimly foresee that there will be a considerable revolution
in natural history.  Systematists will be able to pursue their
labours as at present; but they will not be incessantly
haunted by the shadowy doubt whether this or that form be in
essence a species. .. The endless disputes whether or not some
fifty species of British brambles are true species will cease.
Systematists will have only to decide (not that this will be
easy) whether any form be sufficiently constant and distinct
from other forms, to be capable of definition; and if
definable, whether the differences be sufficiently important
to deserve a specific name. ... Hereafter, we shall be
compelled to acknowledge that the only distinction between
species and well-marked varieties is, that the latter are
known, or believed, to be connected at the present day by
intermediate gradations, whereas species were formerly thus
connected.  Hence, without quite rejecting the consideration
of the present existence of intermediate gradations between
two forms, we shall be led to weigh more carefully and to
value higher the actual amount of difference between them.  It
is quite possible that forms now generally acknowledged to be
merely varieties may hereafter be thought worthy of specific
names, as with the primrose and the cowslip; and in this case
scientific and common language will come into accordance.  In
short, we shall have to treat species in the same manner as
those naturalists treat genera, who admit that genera are
merely artificial combinations made for convenience.  This may
not be a cheering prospect; but we shall at least be freed
from the vain search for the undiscovered and undiscoverable
essence of the term species.

Darwin 1863 in his review of HW Bates' 1862 paper: 
" It is hardly an exaggeration to say, that whilst reading and
reflecting on the various facts given in this memoir, we feel to
be as near witnesses, as we can ever hope to be, of the
creation of new species on this earth"


Darwin, C 1871

The Descent of Man, and Selection in Relation to Sex.

pp. 214 - 215:
"Independently of blending from intercrossing, the complete
absence, in a well-investigated region, of varieties linking
together any two closely-allied forms, is probably the most
important of all the criterions of their specific
distinctness".