Charles Darwin.

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one soil or climate to another, and back again. During the convalescence of
animals, great benefit is derived from almost any change in their habits of
life. Again, both with plants and animals, there is the clearest evidence that
a cross between individuals of the same species, which differ to a certain
extent, gives vigor and fertility to the offspring; and that close interbreeding
continued during several generations between the nearest relations, if these
be kept under the same conditions of life, almost always leads to decreased
size, weakness, or sterility.

Hence it seems, that, on the one hand, slight changes in the conditions of


life benefit all organic beings, and on the other hand, that slight crosses,
that is, crosses between the males and females of the same species, which
have been subjected to slightly different conditions, or which have slightly
varied, give vigor and fertility to the offspring. But, as we have seen, organic
beings long habituated to certain uniform conditions under a state of nature,
when subjected, as under confinement, to a considerable change in their con-
ditions, very frequently are rendered more or less sterile; and we know that
a cross between two forms that have become widely or specifically different,
produce hybrids which are almost always in some degree sterile. I am fully
persuaded that this double parallelism is by no means an accident or an
illusion. He who is able to explain why the elephant, and a multitude of
other animals, are incapable of breeding when kept under only partial con-
finement in their native country, will be able to explain the primary cause of
hybrids being so generally sterile. He will at the same time be able to explain
how it is that the races of some of our domesticated animals, which have
often been subjected to new and not uniform conditions, are quite fertile
together, although they are descended from distinct species, which would
probably have been sterile if aboriginally crossed. The above two parallel
series of facts seem to be connected together by some common but unknown
bond, which is essentially related to the principle of life; this principle, ac-
cording to Mr. Herbert Spencer, being that life depends on, or consists in,
the incessant action and reaction of various forces which, as throughout
nature, are always tending toward an equilibrium; and when this tendency
is slightly disturbed by any change, the vital forces gain in power.


This subject may be here briefly discussed, and will be found to throw
some light on hybridism. Several plants belonging to distinct orders present
two forms, which exist in about equal numbers and which differ in no respect
except in their reproductive organs ; one form having a long pistil with short
stamens, the other a short pistil with long stamens; the two having differently
sized pollen-grains. With trimorphic plants there are three forms likewise dif-
fering in the lengths of their pistils and stamens, in the size and color of the
pollen-grains, and in some other respects; and as in each of the three forms
there are two sets of stamens, the three forms possess altogether six sets of
stamens and three kinds of pistils. These organs are so proportioned in length |
to each other that half the stamens in two of the forms stand on a level with i
the stigma of the third form. Now I have shown, and the result has beeni
confirmed by other observers, that in order to obtain full fertility with these '
plants, it is necessary that the stigma of the one form should be fertilized!
by pollen taken from the stamens of corresponding height in another form. ,
So that with dimorphic species two unions, which may be called legitimate, ,
are fully fertile; and two, which may be called illegitimate, are more or lesss
infertile. With trimorphic species six unions are legitimate, or fully fertile, ,
and twelve are illegitimate, or more or less infertile.


The infertility which may be observed in various dimorphic and trimor-
phic plants^ when they are illegitimately fertilized, that is, by pollen taken
from stamens not corresponding in height with the pistil, differs much in
degree, up to absolute and utter sterility; just in the same manner as occurs
in crossing distinct species. As the degree of sterility in the latter case de-
pends in an eminent degree on the conditions of life being more or less favor-
able, so I have found it with illegitimate unions. It is well known that if
pollen of a distinct species be placed on the stigma of a flower, and its own
pollen be afterward, even after a considerable interval of time, placed on
the same stigma, its action is so strongly prepotent that it generally annihi-
lates the effect of the foreign pollen; so it is with the pollen of the several
forms of the same species, for legitimate pollen is strongly prepotent over
illegitimate pollen, when both are placed on the same stigma. I ascertained
this by fertilizing several flowers, first illegitimately and twenty-four hours
afterward legitimately, with pollen taken from a peculiarly colored variety,
and all the seedlings were similarly colored; this shows that the legitimate
pollen, though applied twenty-four hours subsequently, had wholly destroyed
or prevented the action of the previously applied illegitimate pollen. Again,
as in making reciprocal crosses between the same two species, there is occa-
sionally a great difference in the result, so the same thing occurs with trimor-
phic plants; for instance, the mid-styled form of Lythrum salicaria was
illegitimately fertilized with the greatest ease by pollen from the longer sta-
mens of the short-styled form, and yielded many seeds; but the latter form
did not yield a single seed when fertilized by the longer stamens of the mid-
styled form.

In all these respects, and in others which might be added, the forms of
the same undoubted species, when illegitimately united, behave in exactly
the same manner as do two distinct species when crossed. This led me care-
fully to observe during four years many seedlings, raised from several illegiti-
mate unions. The chief result is that these illegitimate plants, as they may
be called, are not fully fertile. It is possible to raise from dimorphic species,
both long-styled and short-styled illegitimate plants, and from trimorphic
plants all three illegitimate forms. These can then be properly united in a
legitimate manner. When this is done, there is no apparent reason why they
should not yield as many seeds as did their parents when legitimately fer-
tilized. But such is not the case. They are all infertile, in various degrees;
some being so utterly and incurably sterile th^t they did not yield during
four seasons a single seed or even seed-capsule. The sterility of these illegiti-
mate plants, when united with each other in a legitimate manner, may be
strictly compared with that of hybrids when crossed inter se. If, on the other
hand, a hybrid is crossed with either pure parent-species, the sterility is usu-
ally much lessened and so it is when an illegitimate plant is fertilized by a
legitimate plant. In the same manner as the sterility of hybrids does not
always run parallel with the difficulty of making the first cross between the
two parent-species, so that sterility of certain illegitimate plants was unusu -
ally great, while the sterility of the union from which they were derived was


by no means great. With hybrids raised from the same seed-capsule the de-
gree of sterility is innately variable, so it is in a marked manner with illegiti-
mate plants. Lastly, many hybrids are profuse and persistent flowerers, while
other and more sterile hybrids produce few flowers, and are weak, miserable
dwarfs; exactly similar cases occur with the illegitimate offspring of various
dimorphic and trimorphic plants.

Altogether there is the closest identity in character and behavior between
illegitimate plants and hybrids. It is hardly an exaggeration to maintain that
illegitimate plants are hybrids, produced within the limits of the same species
by the improper union of certain forms, while ordinary hybrids are produced
from an improper union between so-called distinct species. We have also
already seen that there is the closest similarity in all respects between first
illegitimate unions and first crosses between distinct species. This will per-
haps be made more fully apparent by an illustration; we may suppose that a
botanist found two well-marked varieties (and such occur) of the long-styled
form of the trimorphic Lythrum salicaria, and that he determined to try by
crossing whether they were specifically distinct. He would find that they
yielded only about one-fifth of the proper number of seed, and that they be-
haved in all the other above specified respects as if they had been two dis-
tinct species. But to make the case sure, he would raise plants from his sup-
posed hybridized seed, and he would find that the seedlings were miserably
dwarfed and utterly sterile, and that they behaved in all other respects like
ordinary hybrids. He might then maintain that he had actually proved, in
accordance with the common view, that his two varieties were as good and
as distinct species as any in the world ; but he would be completely mistaken.

The facts now given on dimorphic and trimorphic plants are important,
because they show us, first, that the physiological test of lessened fertility,
both in first crosses and in hybrids, is no safe criterion of specific distinction;
secondly, because we may conclude that there is some unknown bond which
connects the infertility of illegitimate unions with chat of their illegitimate
offspring, and we are led to extend the same view to first crosses and hybrids;
thirdly, because we find, and this seems to me of especial importance, that
two or three forms of the same species may exist and may differ in no respect
whatever, either in structure or in constitution, relatively to external condi-
tions, and yet be sterile when united in certain ways. For we must remember
that it is the union of the sexual elements of individuals of the same form,
for instance, of two long-styled forms, which results in sterility; while it is the
union of the sexual elements proper to two distinct forms which is fertile.
Hence the case appears at first sight exactly the reverse of what occurs, in
the ordinary unions of the individuals of the same species and with crosses
between distinct species. It is, however, doubtful whether this is really so;
but I will not enlarge on this obscure subject.

We may, however, infer as probable from the consideration of dimorphic
and trimorphic plants, that the sterility of distinct species when crossed and
of their hybrid progeny, depends exclusively on the nature of their sexual
elements, and not on any difference in their structure or general constitu-


tion. We are also led to this same conclusion by considering reciprocal crosses,
in which the male of one species cannot be united, or can be united with
great difficulty, with the female of a second species, while the converse cross
can be effected with perfect facility. That excellent observer, Gartner, like-
wise concluded that species when crossed are sterile owing to differences con-
fined to their reproductive systems.



It may be urged as an overwhelming argument that there must be some es-
sential distinction between species and varieties, inasmuch as the latter, how-
ever much they may differ from each other in external appearance, cross
with perfect facility, and yield perfectly fertile offspring. With some excep-
tions, presently to be given, I fully admit that this is the rule. But the sub-
ject is surrounded by difficulties, for, looking to varieties produced under
nature, if two forms hitherto reputed to be 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, which are considered by most botanists
as varieties, are said by Gartner to be quite sterile when crossed, and he con-
sequently ranks them as undoubted species. If we thus argue in a circle, the
fertility of all varieties produced under nature will assuredly have to be

If we turn to varieties, produced, or supposed to have been produced,
under domestication, we are still involved in some doubt. For when it is
stated, for instance, that certain South American indigenous domestic dogs
do not readily unite with European dogs, the explanation which will occur
to every one, and probably the true one, is that they are descended from
aboriginally distinct species. Nevertheless the perfect fertility of so many do-
mestic races, differing widely from each other in appearance, for instance,
those of the pigeon, or of the cabbage, is a remarkable fact; more especially
when we reflect how many species there are, which, though resembling each
other most closely, are utterly sterile when intercrossed. Several considera-
tions, however, render the fertility of domestic varieties less remarkable. In
the first place, it may be observed that the amount of external difference be-
tween two species is no sure guide to their degree of mutual sterility, so
that similar differences in the case of varieties would be no sure guide. It is
certain that with species the cause lies exclusively in differences in their sex-
ual constitution. Now the varying conditions to which domesticated animals
and cultivated plants have been subjected, have had so little tendency toward
modifying the reproductive system in a manner leading to mutual sterility,
that we have good grounds for admitting the directly opposite doctrine of
Pallas, namely, that such conditions generally eliminate this tendency; so
that the domesticated descendants of species, which in their natural state
probably would have been in some degree sterile when crossed, become per-
fectly fertile together. With plants, so far is cultivation from giving a tend-


ency toward sterility between distinct species, that in several well-authenti-
cated cases already alluded to, certain plants have been affected in an oppo-
site manner, for they have become self-impotent, while still retaining the
capacity of fertilizing, and being fertilized by, other species. If the Pallasian
doctrine of the elimination of sterility through long-continued domestication
be admitted, and it can hardly be rejected, it becomes in the highest degree
improbable that similar conditions long-continued should likewise induce
this tendency; though in certain cases, with species having a peculiar consti-
tution, sterility might occasionally be thus caused. Thus, as I believe, we can
understand why, with domesticated animals, varieties have not been pro-
duced which are mutually sterile ; and why with plants only a few such cases,
immediately to be given, have been observed.

The real difficulty in our present subject is not, as it appears to me, why
domestic varieties have not become mutually infertile when crossed, but why
this has so generally occurred with natural varieties, as soon as they have
been permanently modified in a sufficient degree to take rank as species. We
are far from precisely knowing the cause; nor is this surprising, seeing how
profoundly ignorant we are in regard to the normal and abnormal action of
the reproductive system. But we can see that species, owing to their struggle
for existence with numerous competitors, will have been exposed during
long periods of time to more uniform conditions, than have domestic vari-
eties; and this may well make a wide difference in the result. For we know
how commonly wild animals and plants, when taken from their natural con-
ditions and subjected to captivity, are rendered sterile; and the reproductive
functions of organic beings which have always lived under natural condi-
tions would probably in like manner be eminently sensitive to the influence
of an unnatural cross. Domesticated productions, on the other hand, which,
as shown by the mere fact of their domestication, were not originally highly
sensitive to changes in their conditions of life, and which can now generally
resist with undiminished fertility repeated changes of conditions, might be
expected to produce varieties, which would be little liable to have their re-
productive powers injuriously affected by the act of crossing with other vari-
eties which had originated in a like manner.

I have as yet spoken as if the varieties of the same species were invariably
fertile when intercrossed. But it is impossible to resist the evidence of the ex-
istence 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. Gartner kept, during several years, a dwarf
kind of maize with yellow seeds, and a tall variety with red seeds growing
near each other in his garden; and although these plants have separated
sexes, they never naturally crossed. He then fertilized thirteen flowers of the
one kind with pollen of the other; but only a single head produced any seed, ,
and this one head produced only five grains. Manipulation in this case could


not have been injurious, as the plants have separated sexes. No one, I believe,
has suspected that these varieties of maize are distinct species; and it is im-
portant to notice that the hybrid plants thus raised were themselves perfectly
fertile; so that even Gartner did not venture to consider the two varieties as
specifically distinct.

Girou de Buzareingues crossed three varieties of gourd, which like the maize
has separate sexes, and he asserts that their mutual fertilization is by so much
the less easy as their differences are greater. How far these experiments may
be trusted, I know not ; but the forms experimented on are ranked by Sageret,
who mainly founds his classification by the test of infertility, as varieties, and
Naudin has come to the same conclusion.

The following case is far more remarkable, and seems at first incredible;
but it is the result of an astonishing number of experiments made during
many years on nine species of Verbascum, by so good an observer and so hos-
tile a witness as Gartner: namely, that the yellow and white varieties when
crossed produce less seed than the similarly colored varieties of the same
species. Moreover, he asserts that, when yellow and white varieties of one
species are crossed with yellow and white varieties of a distinct species, more
seed is produced by the crosses between the similarly colored flowers, than
between those which are differently colored. Mr. Scott also has experimented
on the species and varieties of Verbascum; and although unable to confirm
Gartner's results on the crossing of the distinct species, he finds that the dis-
similarly colored varieties of the same species yield fewer seeds, in the pro-
portion of eighty-six to one hundred, than the similarly colored varieties.
Yet these varieties differ in no respect, except in the color of their flowers;
and one variety can sometimes be raised from the seed of another.

Kolreuter, whose accuracy has been confirmed by every subsequent ob-
server, has proved the remarkable fact that one particular variety of the com-
mon tobacco was more fertile than the other varieties, when crossed with a
widely distinct species. He experimented on five forms which are commonly
reputed to be varieties, and which he tested by the severest trial, namely,
by reciprocal crosses, and he found their mongrel offspring perfectly fertile.
But one of these five varieties, when used either as the father or mother, and
crossed with the Nicotiana glutinosa, always yielded hybrids not so sterile as
those which were produced from the four other varieties when crossed with
N. glutinosa. Hence, the reproductive system of this one variety must have
been in some manner and in some degree modified.

From these facts it can no longer be maintained that varieties when crossed
are invariably quite fertile. From the great difficulty of ascertaining the in-
fertility of varieties in a state of nature, for a supposed variety, if proved to
be infertile in any degree, would almost universally be ranked as a species;
from man attending only to external characters in his domestic varieties, and
from such varieties not having been exposed for very long periods to uniform
conditions of life; from these several considerations we may conclude that
fertility does not constitute a fundamental distinction between varieties and


species when crossed. The general sterility of crossed species may safely be
looked at, not as a special acquirement or endowment, but as incidental on
changes of an unknown nature in their sexual elements.


Independently of the question of fertility, the offspring of species "and of
varieties when crossed may be compared in several other respects. Gartner,
whose strong wish it was to draw a distinct line between species and varieties,
could find very few, and, as it seems to me, quite unimportant differences
between the so-called hybrid offspring of species, and the so-called mongrel
offspring of varieties. And, on the other hand, they agree most closely in
many important respects.

I shall here discuss this subject with extreme brevity. The most important
distinction is, that in the first generation mongrels are more variable than
hybrids; but Gartner admits that hybrids from species which have long been
cultivated are often variable in the first generation; and I have myself seen
striking instances of this fact. Gartner further admits that hybrids between
very closely allied species are more variable than those from very distinct
species; and this shows that the difference in degree of variability graduates
away. When mongrels and the more fertile hybrids are propagated for sev-
eral generations, an extreme amount of variability in the offspring in both
cases is notorious ; but some few instances of both hybrids and mongrels long
retaining a uniform character could be given. The variability, however, in
the successive generations of mongrels is, perhaps, greater than in hybrids.

This greater variability in mongrels than in hybrids does not seem at all
surprising. For the parents of mongrels are varieties, and mostly domestic
varieties (very few experiments having been tried on natural varieties), and
this implies that there has been recent variability, which would often continue
and would augment that arising from the act of crossing. The slight variability
of hybrids in the first generation, in contrast with that in the succeeding
generations, is a curious fact and deserves attention. For it bears on the view
which I have taken of one of the causes of ordinary variability, namely, that
the reproductive system, from being eminently sensitive to changed condi-
tions of life, fails under these circumstances to perform its proper function of
producing offspring closely similar in all respects to the parent form. Now,
hybrids in the first generation are descended from species (excluding those
long cultivated) which have not had their reproductive systems in any way
affected, and they are not variable; but hybrids themselves have the repro-
ductive systems seriously affected and their descendants are highly variable.

But to return to our comparison of mongrels and hybrids: Gartner states
that mongrels are more liable than hybrids to revert to either parent form;
but this, if it be true, is certainly only a difference in degree. Moreover, Gart-
ner expressly states that the hybrids from long cultivated plants are more
subject to reversion than hybrids from species in their natural state; and this
probably explains the singular difference in the results arrived at by differ-


ent observers. Thus Max Wichura doubts whether hybrids ever revert to their
parent forms, and he experimented on uncultivated species of willows, while
Naudin, on the other hand, insists in the strongest terms on the almost uni-
versal tendency to reversion in hybrids, and he experimented chiefly on culti-

Online LibraryCharles DarwinThe origin of species → online text (page 29 of 50)