together in utter confusion. The following rules and conclusions are
chiefly drawn up from Gartner's admirable work on the hybridisation of
plants. I have taken much pains to ascertain how far the rules 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 to exist;
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 of the same
genus applied to the stigma of some one species, 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 will
produce. So in hybrids themselves, there are some which never have
produced, and probably never would produce, even with the pollen of either
pure parent, 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-fertilised hybrids producing a greater and greater
number of seeds up to perfect fertility.
Hybrids 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 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 the fertility of pure species.
But the degree of fertility is likewise innately variable; for it is not
always the same when the same two species are crossed under the same
circumstances, but 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 exactly the same conditions.
By the term systematic affinity is meant, the resemblance between species
in structure and in constitution, more especially in the structure of parts
which are of high physiological importance and which differ little in the
allied species. 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 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. Very 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 stallion-horse being first crossed with a female-ass, and then a
male-ass with a mare: 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, or of any recognisable
difference in their whole organisation. On the other hand, these cases
clearly show that the capacity for crossing is connected with
constitutional differences imperceptible by us, and confined to the
reproductive system. This difference in the result of reciprocal crosses
between the same two species was long ago observed by Kolreuter. To give
an instance: Mirabilis jalappa 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. jalappa, 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 forms so closely related (as Matthiola annua and glabra) that many
botanists rank them 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, generally diff
er 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 fertility in the hybrid is
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
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 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 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 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 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 so entirely unimportant for its 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, does 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 trees together
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 Sagaret 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 very different case from the
difficulty of uniting two pure species, which have their reproductive
organs perfect; yet these two distinct cases run to a certain 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 another 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 case of Hippeastrum, Lobelia, &c., which seeded much more
freely when fertilised with the pollen of distinct species, than when
self-fertilised with their own pollen.
We thus see, that although there is a clear and fundamental 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, chiefly in their reproductive systems. These
differences, in both cases, follow to a certain extent, as might have been
expected, systematic affinity, by which every kind of resemblance and
dissimilarity between organic beings is attempted to be expressed. The
facts by no means seem to me to indicate that the greater or lesser
difficulty of either grafting or crossing together 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.
Causes of the Sterility of first Crosses and of Hybrids. -- We may now look
a little closer at the probable causes of the sterility of first crosses
and of hybrids. These two cases are fundamentally different, for, as just
remarked, in the union of two pure species the male and female sexual
elements are perfect, whereas in hybrids they are imperfect. Even in first
crosses, the greater or lesser difficulty in effecting a union 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 pollen of one species is
placed on the stigma of a distantly allied 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 embryo
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
communicated to me by Mr. Hewitt, who has had great experience in
hybridising gallinaceous birds, that the early death of the embryo is a
very frequent cause of sterility in first crosses. I was at first very
unwilling to believe in this view; as 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 can live, they
are generally placed under suitable conditions of life. But a hybrid
partakes of only half of the nature and constitution of its mother, and
therefore before birth, as long as it is nourished within its mother's womb
or within the egg or seed produced by the mother, it may 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 seem eminently
sensitive to injurious or unnatural conditions of life.
In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is very different. I have more than once
alluded to a large body of facts, which I have collected, showing that when
animals and plants are removed from their natural conditions, they are