Charles Darwin.

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species ranked by systematists in distinct families; and on the other hand,
by very closely allied species generally uniting with facility. But the cor-
respondence between systematic affinity and the facility of crossing is by
no means strict. A multitude of cases could be given of very closely allied
species which will not unite, or only with extreme difficulty; and on the
other hand of very distinct species which unite with the utmost facility.
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. Even within the limits of the same genus, we meet
with this same difference; for instance, the many species of Nicotiana have
been more largely crossed than the species of almost any other genus; but
Gartner found that N. acuminata, which is not a particularly distinct species,
obstinately failed to fertilize, or to be fertilized by, no less than eight other
species of Nicotiana. Many analogous facts could be given.

No one has been able to point out what kind or what amount of difference,
in any recognizable character, is sufficient to prevent two species crossing.
It can be shown that plants most widely different in habit and general ap-
pearance, and having strongly marked differences in every part of the flower,
even in the pollen, in the fruit, and in the cotyledons, can be crossed. Annual
and perennial plants, deciduous and evergreen trees, plants inhabiting
different stations, and fitted for extremely different climates, can often be
crossed with ease.

By a reciprocal cross between two species, I mean the case, for instance,


of a female ass being first crossed by a stallion, and then a mare by a male ass;
these two species may then be said to have been reciprocally crossed. There
is often the widest possible difTerence in the facility of making reciprocal
crosses. Such cases are highly important, for they prove that the capacity
in any two species to cross is often completely independent of their system-
atic affinity, that is, of any difference in their structure or constitution,
excepting in their reproductive systems. The diversity of the result in re-
ciprocal crosses between the same two species was long ago observed by
Kolreuter. To give an instance; Mirabilis jalapa can easily be fertilized by
the pollen of M. longiflora, and the hybrids thus produced are sufficiently
fertile; but Kolreuter tried more than two hundred times, during eight
following years, to fertilize reciprocally M. longiflora with the pollen of M.
jalapa, and utterly failed. Several other equally striking cases could be
given. Thuret has observed the same fact with certain sea-weeds or Fuci.
Gartner, moreover, found that this difference of facility in making reciprocal
crosses is extremely common in a lesser degree. He has observed it even
between closely related forms (as Matthiola annua and glabra) which many
botanists rank only as varieties. It is also a remarkable fact, that hybrids
raised from reciprocal crosses, though of course compounded of the very
same two species, the one species having first been used as the father and
then as the mother, though they rarely differ in external characters, yet
generally differ in fertility in a small, and occasionally in a high, degree.

Several other singular rules could be given from Gartner: for instance,
some species have a remarkable power of crossing with other species; other
species of the same genus have a remarkable power of impressing their like-
ness on their hybrid offspring; but these two powers do not at all necessarily
go together. There are certain hybrids which, instead of having, as is usual,
an intermediate character between their two parents, always closely re-
semble one of them; and such hybrids, though externally so like one of
their pure parent-species, are with rare exceptions extremely sterile. So
again among hybrids which are usually intermediate in structure between
their parents, exceptional and abnormal individuals sometimes are bom,
which closely resemble one of their pure parents; and these hybrids are
almost always utterly sterile, even when the other hybrids raised from seed
from the same capsule have a considerable degree of fertility. These facts
show how completely the fertility of a hybrid may be independent of its
external resemblance to either pure parent.

Considering the several rules now given, which govern the fertility of
first crosses and of hybrids, we see that when forms, which must be con-
sidered as good and distinct species, are united, their fertility graduates
from zero to perfect fertility, or even to fertility under certain conditions
in excess ; that their fertility, besides being eminently susceptible to favorable
and unfavorable conditions, is innately variable; that it is by no means
always the same in degree in the first cross and in the hybrids produced
from this cross; that the fertility of hybrids is not related to the degree in
which they resemble in external appearance either parent; and lastly, that


the facility of making a first cross between any two species is not always
governed by their systematic affinity or degree of resemblance to each other.
This latter statement is clearly proved by the difference in the result of re-
ciprocal crosses between the same two species, for, according as the one
species or the other is used as the father or the mother, there is generally
some difference, and occasionally the widest possible difference, in the
facility of effecting an union. The hybrids, moreover, produced from re-
ciprocal crosses often differ in fertility.

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 the 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 pro-
duce fairly fertile hybrids? Why should there often be so great a difference
in the result of a reciprocal cross between the same two species? 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 a strange arrange-

The foregoing rules and facts, on the other hand, appear to me clearly
to indicate that the sterility, both of first crosses and of hybrids, is simply
incidental or dependent on unknown differences in their reproductive
systems; the differences being of so peculiar and limited a nature, that, in
reciprocal crosses between the same two species, the male sexual element
of the one will often freely act on the female sexual element of the other,
but not in a reversed direction. It will be advisable to explain a little more
fully, by an example, what I mean by sterility being incidental on other dif-
ferences, and not a specially endowed quality. As the capacity of one plant
to be grafted or budded on another is unimportant for their welfare in a
state of nature, I presume that no one will suppose that this capacity is a
specially endowed quality, but will admit that it is incidental on differences
in the laws of growth of the two plants. We can sometimes see the reason
why one tree will not take on another, from differences in their rate of
growth, in the hardness of their wood, in the period of the flow or nature
of their sap, etc. ; but in a multitude of cases we can assign no reason what-
ever. Great diversity in the size of two plants, one being woody and the other
herbaceous, one being evergreen and the other deciduous, and adaptation
to widely different climates, do not always prevent the two grafting together.
As in hybridization, so with grafting, the capacity is limited by systematic
affinity, for no one has been able to graft together trees belonging to quite
distinct families; and, on the other hand, closely allied species and varieties
of the same species can usually,, but not invariably, be grafted with ease.


But this capacity, as in hybridization, is by no means absolutely governed
by systematic affinity. Although many distinct genera within the same family
have been grafted together, in other cases species of the same genus will
not take on each other. The pear can be grafted far more readily on the
quince, which is ranked as a distinct genus, than on the apple, which is a
member of the same genus. Even different varieties of the pear take with
different degrees of facility on the quince; so do different varieties of the
apricot and peach on certain varieties of the plum.

As Gartner found that there was sometimes an innate difference in dif-
ferent individuals of the same two species in crossing; so Sageret believes
this to be the case with different individuals of the same two species in
being grafted together. As in reciprocal crosses, the facility of effecting an
union is often very far from equal, so it sometimes is in grafting. The common
gooseberry, for instance, cannot be grafted on the currant, whereas the
currant will take, though with difficulty, on the gooseberry.

We have seen that the sterility of hybrids which have their reproductive
organs in an imperfect condition, is a different case from the difficulty of
uniting two pure species which have their reproductive organs perfect; yet
these two distinct classes of cases run to a large extent parallel. Something
analogous occurs in grafting; for Thouin found that three species of Robinia,
which seeded freely on their own roots, and which could be grafted with
no great difficulty on a fourth species, when thus grafted were rendered
barren. On the other hand, certain species of Sorbus, when grafted on other
species, yielded twice as much fruit as when on their own roots. We are re-
minded by this latter fact of the extraordinary cases of hippeastrum, passi-
flora, etc., which seed much more freely when fertilized with the pollen of
a distinct species than when fertilized with pollen from the same plant.

We thus see, that, although there is a clear and great difference between
the mere adhesion of grafted stocks and the union of the male and female
elements in the act of reproduction, yet that there is a rude degree of paral-
lelism in the results of grafting and of crossing distinct species. And as we
must look at the curious and complex laws governing the facility with
which trees can be grafted on each other as incidental on unknown dif-
ferences in their vegetative systems, so I believe that the still more complex
laws governing the facility of first crosses are incidental on unknown dif-
ferences in their reproductive systems. These differences in both cases follow,
to a certain extent, as might have been expected, systematic affinity, by which
term every kind of resemblance and dissimilarity between organic beings is
attempted to be expressed. The facts by no means seem to indicate that the
greater or lesser difficulty of either grafting or crossing various species has
been a special endowment; although in the case of crossing, the difficulty
is as important for the endurance and stability of specific forms as in the
case of grafting it is unimportant for their welfare.




At one time it appeared to me probable, as it has to others, that the
sterility of first crosses and of hybrids might have been slowly acquired
through the natural selection of slightly lessened degrees of fertility, which,
like any other variation, spontaneously appeared in certain individuals of one
variety when crossed with those of another variety. For it would clearly be
advantageous to two varieties or incipient species if they could be kept from
blending, on the same principle that, when man is selecting at the same
time two varieties, it is necessary that he should keep them separate. In the
first place, it may be remarked that species inhabiting distinct regions are
often sterile when crossed; now it could clearly have been of no advantage
to such separated species to have been rendered mutually sterile, and con-
sequently this could not have been effected through natural selection; but it
may perhaps be argued, that, if a species was rendered sterile with some one
compatriot, sterility with other species would follow as a necessary con-
tingency. In the second place, it is almost as much opposed to the theory of
natural selection as to that of special creation, that in reciprocal crosses the
male element of one form should have been rendered utterly impotent on
a second form, while at the same time the male element of this second form
is enabled freely to fertilize the first form; for this peculiar state of tlie re-
productive system could hardly have been advantageous to either species.

In considering the probability of natural selection having come into action,
in rendering species mutually sterile, the greatest difficulty will be found to
He in the existence of many graduated steps, from slightly lessened fertiUty
to absolute sterility. It may be admitted that it would profit an incipient
species, if it were rendered in some slight degree sterile when crossed with
its parent form or with some other variety; for thus fewer bastardized and
deteriorated offspring would be produced to commingle their blood with
the new species in process of formation. But he who will take the trouble to
reflect on the steps by which this first degree of steriHty could be increased
through natural selection to that high degree which is common with so many
species, and which is universal with species which have been differentiated
to a generic or family rank, will find the subject extraordinarily complex.
After mature reflection, it seems to me that this could not have been effected
through natural selection. Take the case of any two species which, when
crossed, produced few and sterile offspring; now, what is there which could
favor the survival of those individuals which happened to be endowed in a
slightly higher degree with mutual infertility, and which thus approached
by one small step toward absolute sterility? Yet an advance of this kind, if
the theory of natural selection be brought to bear, must have incessantly oc-
curred with many species, for a multitude are mutually quite barren. With
sterile neuter insects we have reason to beheve that modifications in their
structure and fertility have been slowly accumulated by natural selection,


from an advantage having been thus indirectly given to the community to
which they belonged over other communities of the same species; but an in-
dividual animal not belonging to a social community, if rendered slightly
sterile when crossed with some other variety, would not thus itself gain any
advantage or indirectly give any advantage to the other individuals of the
same variety, thus leading to their preservation.

But it would be superfluous to discuss this question in detail: for with
plants we have conclusive evidence that the sterility of crossed species must
be due to some principle, quite independent of natural selection. Both Gart-
ner and Kolreuter have proved that in genera including numerous species, a
series can be formed from species which when crossed yield fewer and fewer
seeds, to species which never produce a single seed, but yet are affected by
the pollen of certain other species, for the germen swells. It is here manifestly
impossible to select the more sterile individuals, which have already ceased
to yield seeds; so that this acme of sterility, when the germen alone is af-
fected, cannot have been gained through selection; and from the laws gov-
erning the various grades of sterility being so uniform throughout the animal
and vegetable kingdoms, we may infer that the cause, whatever it may be, is
the same or nearly the same in all cases.

We will now look a little closer at the probable nature of the differences
between species which induce sterility in first crosses and in hybrids. In the
case of first crosses, the greater or less difficulty in effecting an union and in
obtaining offspring apparently depends on several distinct causes. There must
sometimes be a physical impossibility in the male element reaching the ovule,
as would be the case with a plant having a pistil too long for the pollen-tubes
to reach the ovarium. It has also been observed that when the pollen of one
species is placed on the stigma of a distantly alhed species, though the pollen-
tubes protrude, they do not penetrate the stigmatic surface. Again, the male
element may reach the female element, but be incapable of causing an em-
bryo to be developed, as seems to have been the case with some of Thuret's
experiments on Fuci. No explanation can be given of these facts, any more
than why certain trees cannot be grafted on others. Lastly, an embryo may
be developed, and then perish at an early period. This latter alternative has
not been sufficiently attended to; but I believe, from observations communi-
cated to me by Mr. Hewitt, who has had great experience in hybridizing
pheasants and fowls, that the early death of the embryo is a very frequent
cause of sterility in first crosses. Mr. Salter has recently given the results of
an examination of about 500 eggs produced from various crosses between
three species of Gallus and their hybrids; the majority of these eggs had been
fertilized; and in the majority of the fertilized eggs, the embryos had either
been partially developed and had then perished, or had become nearly ma-
ture, but the young chickens had been unable to break through the shell.
Of the chickens which were born, more than four-fifths died within the first
few days, or at latest weeks, "without any obvious cause, apparently from
mere inability to live" ; so that from the 500 eggs only twelve chickens were



reared. With plants, hybridized embryos probably often perish in a like man-
ner; at least it is known that hybrids raised from very distinct species are
sometimes weak and dwarfed, and perish at an early age ; of which fact Max
Wichura has recently given some striking cases with hybrid willows. It may
be here worth noticing that in some cases of parthenogenesis, the embryos
within the eggs of silk moths which had not been fertilized, pass through
their early stages of development and then perish like the embryos produced
by a cross between distinct species. Until becoming acquainted with these
facts, I was unwilling to believe in the frequent early death of hybrid em-
bryos; for hybrids, when once born, are generally healthy and long-lived, as
we see in the case of the common mule. Hybrids, however, are differently
circumstanced before and after birth: when born and living in a country
where their two parents live, they are generally placed under suitable con-
ditions of life. But a hybrid partakes of only half of the nature and constitu-
tion of its mother; it may therefore, before birth, as long as it is nourished
within its mother's womb, or within the egg or seed produced by the mother,
be exposed to conditions in some degree unsuitable, and consequently be
liable to perish at an early period; more especially as all very young beings
are eminently sensitive to injurious or unnatural conditions of life. But after
all, the cause more probably lies in some imperfection in the original act
of impregnation, causing the embryo to be imperfectly developed, rather than
in the conditions to which it is subsequently exposed.'

In regard to the sterility of hybrids, in which the sexual elements are im-
perfectly developed, the case is somewhat different. I have more than once
alluded to a large body of facts showing that, when animals and plants are
removed from their natural conditions, they are extremely liable to have
their reproductive systems seriously affected. This, in fact, is the great bar to
the domestication of animals. Between the sterility thus superinduced and
that of hybrids, there are many points of similarity. In both cases the sterility
is independent of general health, and is often accompanied by excess of size
or great luxuriance. In both cases the sterility occurs in various degrees; in
both, the male element is the most liable to be affected; but sometimes the
female more than the male. In both, the tendency goes to a certain extent
with systematic affinity, for whole groups of animals and plants are rendered
impotent by the same unnatural conditions; and whole groups of species
tend to produce sterile hybrids. On the other hand, one species in a group
will sometimes resist great changes of conditions with unimpaired fertility;
and certain species in a group will produce unusually fertile hybrids. No one
can tell till he tries, whether any particular animal will breed under confine-
ment, or any exotic plant seed freely under culture; nor can he tell till he
tries, whether any two species of a genus will produce more or less sterile
hybrids. Lastly, when organic beings are placed during several generations
under conditions not natural to them, they are extremely liable to vary,
which seems to be partly due to their reproductive systems having been
specially affected, though in a lesser degree than when sterility ensues. So it


is with hybrids, for their offspring in successive generations are eminently
liable to vary, as every experimentalist has observed.

Thus we see that when organic beings are placed under new and un-
natural conditions, and when hybrids are produced by the unnatural cross-
ing of two species, the reproductive system, independently of the general
state of health, is affected in a very similar manner. In the one case, the
conditions of life have been disturbed, though often in so slight a degree as
to be inappreciable by us; in the other case, or that of hybrids, the external
conditions have remained the same, but the organization has been disturbed
by two distinct structures and constitutions, including of course the repro-
ductive systems, having been blended into one. For it is scarcely possible that
two organizations should be compounded into one, without some disturbance
occurring in the development, or periodical action, or mutual relations of
the different parts and organs one to another or to the conditions of life.
When hybrids are able to breed inter se, they transmit to their offspring from
generation to generation the same compounded organization, and hence we
need not be surprised that their sterility, though in some degree variable,
does not diminish; it is even apt to increase, this being generally the result,
as before explained, of too close interbreeding. The above view of the sterility
of hybrids being caused by two constitutions being compounded into one
has been strongly maintained by Max Wichura.

It must, however, be owned that we cannot understand, on the above or
any other view, several facts with respect to the sterility of hybrids; for in-
stance, the unequal fertility of hybrids produced from reciprocal crosses; or
the increased sterility in those hybrids which occasionally and exceptionally
resemble closely either pure parent. Nor do I pretend that the foregoing
remarks go to the root of the matter; no explanation is offered why an organ-
ism, when placed under unnatural conditions, is rendered sterile. All that
I have attempted to show is, that in two cases, in some respects allied, sterility
is the common result — in the one case from the conditions of life having
been disturbed, in the other case from the organization having been disturbed
by two organizations being compounded into one.

A similar parallelism holds good with an allied yet very different class of
facts. It is an old and almost universal belief, founded on a considerable
body of evidence, which I have elsewhere given, that slight changes in the
conditions of life are beneficial to all living things. We see this acted on by
farmers and gardeners in their frequent exchanges of seed, tubers, etc., from

Online LibraryCharles DarwinThe origin of species → online text (page 28 of 50)