CHAPTER IX - HYBRIDISM
Laws governing the Sterility of first Crosses and of Hybrids
We will now consider a little more in detail the laws governing
the sterility of first crosses and of hybrids. Our chief object will
be to see whether or not these laws indicate that species have been
specially endowed with this quality, in order to prevent their
crossing and blending together in utter confusion. The following
conclusions are drawn up chiefly from Gartner's admirable work on
the hybridisation of plants. I have taken much pains to ascertain
how far they apply to animals, and, considering how scanty our
knowledge is in regard to hybrid animals, I have been surprised to
find how generally the same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility, both
of first crosses and of hybrids, graduates from zero to perfect
fertility. It is surprising in how many curious ways this gradation
can be shown; but only the barest outline of the facts can here be
given. When pollen from a plant of one family is placed on the
stigma of a plant of a distinct family, it exerts no more influence
than so much inorganic dust. From this absolute zero of fertility, the
pollen of different species applied to the stigma of some one
species of the same genus, yields a perfect gradation in the number of
seeds produced, up to nearly complete or even quite complete
fertility; and, as we have seen, in certain abnormal cases, even to an
excess of fertility, beyond that which the plant's own pollen
produces. So in hybrids themselves, there are some which never have
produced, and probably never would produce, even with the pollen of
the pure parents, a single fertile seed: but in some of these cases
a first trace of fertility may be detected, by the pollen of one of
the pure parent-species causing the flower of the hybrid to wither
earlier than it otherwise would have done; and the early withering
of the flower is well known to be a sign of incipient fertilisation.
From this extreme degree of sterility we have self-sterilised
hybrids producing a greater and greater number of seeds up to
perfect fertility.
The hybrids raised from two species which are very difficult to
cross, and which rarely produce any offspring, are generally very
sterile; but the parallelism between the difficulty of making a
first cross, and the sterility of the hybrids thus produced- two
classes of facts which are generally confounded together- is by no
means strict. There are many cases, in which two pure species, as in
the genus Verbascum, can be united with unusual facility, and
produce numerous hybrid offspring, yet these hybrids are remarkably
sterile. On the other hand, there are species which can be crossed
very rarely, or with extreme difficulty, but the hybrids, when at last
produced, are very fertile. Even within the limits of the same
genus, for instance in Dianthus, these two opposite cases occur.
The fertility, both of first crosses and of hybrids, is more
easily affected by unfavourable conditions, than is that of pure
species. But the fertility of first crosses is likewise innately
variable; for it is not always the same in degree when the same two
species are crossed under the same circumstances; it depends in part
upon the constitution of the individuals which happen to have been
chosen for the experiment. So it is with hybrids, for their degree
of fertility is often found to differ greatly in the several
individuals raised from seed out of the same capsule and exposed to
the same conditions.
By the term systematic affinity is meant, the general resemblance
between species in structure and constitution. Now the fertility of
first crosses, 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 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 fertilise, or to be fertilised
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 recognisable character, is sufficient to prevent
two species crossing. It can be shown that plants most widely
different in habit and general appearance, 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
difference 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 systematic
affinity, that is of any difference in their structure or
constitution, excepting in their reproductive systems. The diversity
of the result in reciprocal crosses between the same two species was
long ago observed by Kolreuter. To give an instance: Mirabilis
jalapa can easily be fertilised 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
fertilise 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
gilabra) 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 likeness 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 resemble one of
them; and such hybrids, though externally so like one of their pure
parent-species, are with rare exceptions extremely sterile. So again
amongst hybrids which are usually intermediate in structure between
their parents, exceptional and abnormal individuals sometimes are
born, 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 causes and of hybrids, we see that when forms,
which must be considered 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 favourable and unfavourable
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 reciprocal 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
reciprocal 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 produce 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 arrangement.
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
differences, 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, &c.; but in a multitude of cases we can assign no reason
whatever. 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 hybridisation, 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 hybridisation, 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
different 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 current 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 reminded by this latter fact of the extraordinary
cases of Hippeastrum, Passiflora, &c., which seed much more freely
when fertilised with the pollen of a distinct species, than when
fertilised 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 parallelism 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 differences in their vegetative
systems, so I believe that the still more complex laws governing the
facility of first crosses are incidental on unknown differences 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.
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