HYBRIDISM.
Distinction between the sterility of first crosses and of
hybrids--Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication--Laws governing the sterility
of hybrids--Sterility not a special endowment, but incidental on other
differences--Causes of the sterility of first crosses and of
hybrids--Parallelism between the effects of changed conditions of life
and crossing--Fertility of varieties when crossed and of their mongrel
offspring not universal--Hybrids and mongrels compared independently of
their fertility--Summary.
The view generally entertained by naturalists is that species, when
intercrossed, have been specially endowed with the quality of sterility, in
order to prevent the confusion of all organic forms. This view certainly
seems at first probable, for species within the same country could hardly
have kept distinct had they been capable of crossing freely. The importance
of the fact that hybrids are very generally sterile, has, I think, been
much underrated by some late writers. On the theory of natural selection
the case is especially important, inasmuch as the sterility of hybrids
could not possibly be of any advantage to them, and therefore could not
have been acquired by the continued preservation of successive profitable
degrees of sterility. I hope, however, to be able to show that sterility is
not a specially acquired or endowed quality, but is incidental on other
acquired differences.
In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded together; namely,
the sterility of two species {246} when first crossed, and the sterility of
the hybrids produced from them.
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, as may be clearly seen in the state of the male element in both
plants and animals; though the organs themselves are perfect in structure,
as far as the microscope reveals. In the first case the two sexual elements
which go to form the embryo are perfect; in the second case they are either
not at all developed, or are imperfectly developed. This distinction is
important, when the cause of the sterility, which is common to the two
cases, has to be considered. The distinction has probably been slurred
over, owing to the sterility in both cases being looked on as a special
endowment, beyond the province of our reasoning powers.
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.
First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of those
two conscientious and admirable observers, Kölreuter and Gärtner, who
almost devoted their lives to this subject, without being deeply impressed
with the high generality of some degree of sterility. Kölreuter makes the
rule 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 {247} them as varieties. Gärtner,
also, makes the rule equally universal; and he disputes the entire
fertility of Kölreuter's ten cases. But in these and in many other cases,
Gärtner is obliged carefully to count the seeds, in order to show that
there is any degree of sterility. He always compares the maximum number of
seeds produced by two species when crossed and by their hybrid offspring,
with the average number produced by both pure parent-species in a state of
nature. But a serious cause of error seems to me to be here introduced: a
plant to be hybridised must be castrated, and, what is often more
important, must be secluded in order to prevent pollen being brought to it
by insects from other plants. Nearly all the plants experimentised on by
Gärtner were potted, and apparently were kept in a chamber in his house.
That these processes are often injurious to the fertility of a plant cannot
be doubted; for Gärtner gives in his table about a score of cases of plants
which he castrated, and artificially fertilised with their own pollen, and
(excluding all cases such as the Leguminosæ, in which there is an
acknowledged difficulty in the manipulation) half of these twenty plants
had their fertility in some degree impaired. Moreover, as Gärtner during
several years repeatedly crossed the primrose and cowslip, which we have
such good reason to believe to be varieties, and only once or twice
succeeded in getting fertile seed; as he found the common red and blue
pimpernels (Anagallis arvensis and coerulea), which the best botanists rank
as varieties, absolutely sterile together; and as he came to the same
conclusion in several other analogous cases; it seems to me that we may
well be permitted to doubt whether many other species are really so
sterile, when intercrossed, as Gärtner believes. {248}
It is certain, on the one hand, that the sterility of various species when
crossed is so different in degree and graduates away so insensibly, and, on
the other hand, that the fertility of pure species is so easily affected by
various circumstances, that 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. It is also most instructive to compare--but I have not space here
to enter on details--the evidence advanced by our best botanists on the
question whether certain doubtful forms should be ranked as species or
varieties, with the evidence from fertility adduced by different
hybridisers, or by the same author, from experiments made during different
years. It can thus be shown that neither sterility nor fertility affords
any clear distinction between species and varieties; but that the evidence
from this source graduates away, and is doubtful in the same degree as is
the evidence derived from other constitutional and structural differences.
In regard to the sterility of hybrids in successive generations; though
Gärtner was enabled to rear some hybrids, carefully guarding them from a
cross with either pure parent, for six or seven, and in one case for ten
generations, yet he asserts positively that their fertility never
increased, but generally greatly decreased. I do not doubt that this is
usually the case, and that the fertility often suddenly decreases in the
first few generations. Nevertheless I believe that in all these experiments
the fertility has been diminished by an independent cause, namely, from
close interbreeding. I have collected so large a body of facts, showing
{249} that close interbreeding lessens fertility, and, on the other hand,
that an occasional cross with a distinct individual or variety increases
fertility, that I cannot doubt the correctness of this almost universal
belief amongst breeders. Hybrids are seldom raised by experimentalists in
great numbers; and as the parent-species, or other allied hybrids,
generally grow in the same garden, the visits of insects must be carefully
prevented during the flowering season: hence hybrids will generally be
fertilised during each generation by their own individual pollen; and I am
convinced that this would be injurious to their fertility, already lessened
by their hybrid origin. I am strengthened in this conviction by a
remarkable statement repeatedly made by Gärtner, namely, that if even the
less fertile hybrids be artificially fertilised with hybrid pollen of the
same kind, their fertility, notwithstanding the frequent ill effects of
manipulation, sometimes decidedly increases, and goes on increasing. Now,
in artificial fertilisation pollen is as often taken by chance (as I know
from my own experience) from the anthers of another flower, as from the
anthers of the flower itself which is to be fertilised; so that a cross
between two flowers, though probably on the same plant, would be thus
effected. Moreover, whenever complicated experiments are in progress, so
careful an observer as Gärtner would have castrated his hybrids, and this
would have insured in each generation a cross with a pollen from a distinct
flower, either from the same plant or from another plant of the same hybrid
nature. And thus, the strange fact of the increase of fertility in the
successive generations of _artificially fertilised_ hybrids may, I believe,
be accounted for by close interbreeding having been avoided.
Now let us turn to the results arrived at by the third most experienced
hybridiser, namely, the Hon. and {250} Rev. W. Herbert. He is as emphatic
in his conclusion that some hybrids are perfectly fertile--as fertile as
the pure parent-species--as are Kölreuter and Gärtner that some degree of
sterility between distinct species is a universal law of nature. He
experimentised on some of the very same species as did Gärtner. The
difference in their results may, I think, be in part accounted for by
Herbert's great horticultural skill, and by his having hothouses at his
command. Of his many important statements I will here give only a single
one as an example, namely, that "every ovule in a pod of Crinum capense
fertilised by C. revolutum produced a plant, which (he says) I never saw to
occur in a case of its natural fecundation." So that we here have perfect,
or even more than commonly perfect, fertility in a first cross between two
distinct species.
This case of the Crinum leads me to refer to a most singular fact, namely,
that there are individual plants of certain species of Lobelia and of some
other genera, which can be far more easily fertilised by the pollen of
another and distinct species, than by their own pollen; and all the
individuals of nearly all the species of Hippeastrum seem to be in this
predicament. For these plants have been found to yield seed to the pollen
of a distinct species, though quite sterile with their own pollen,
notwithstanding that their own pollen was found to be perfectly good, for
it fertilised distinct species. So that certain individual plants and all
the individuals of certain species can actually be hybridised much more
readily than they can be self-fertilised! For instance, a bulb of
Hippeastrum aulicum produced four flowers; three were fertilised by Herbert
with their own pollen, and the fourth was subsequently fertilised by the
pollen of a compound hybrid descended from three other and distinct {251}
species: the result was that "the ovaries of the three first flowers soon
ceased to grow, and after a few days perished entirely, whereas the pod
impregnated by the pollen of the hybrid made vigorous growth and rapid
progress to maturity, and bore good seed, which vegetated freely." In a
letter to me, in 1839, Mr. Herbert told me that he had then tried the
experiment during five years, and he continued to try it during several
subsequent years, and always with the same result. This result has, also,
been confirmed by other observers in the case of Hippeastrum with its
sub-genera, and in the case of some other genera, as Lobelia, Passiflora
and Verbascum. Although the plants in these experiments appeared perfectly
healthy, and although both the ovules and pollen of the same flower were
perfectly good with respect to other species, yet as they were functionally
imperfect in their mutual self-action, we must infer that the plants were
in an unnatural state. Nevertheless these facts show on what slight and
mysterious causes the lesser or greater fertility of species when crossed,
in comparison with the same species when self-fertilised, sometimes
depends.
The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, &c., have been crossed, yet many of these hybrids
seed freely. For instance, Herbert asserts that a hybrid from Calceolaria
integrifolia and plantaginea, species most widely dissimilar in general
habit, "reproduced itself as perfectly as if it had been a natural species
from the mountains of Chile." I have taken some pains to ascertain the
degree of fertility of some of the complex crosses of Rhododendrons, and I
am assured that many of them {252} are perfectly fertile. Mr. C. Noble, for
instance, informs me that he raises stocks for grafting from a hybrid
between Rhod. Ponticum and Catawbiense, and that this hybrid "seeds as
freely as it is possible to imagine." Had hybrids, when fairly treated,
gone on decreasing in fertility in each successive generation, as Gärtner
believes to be the case, the fact would have been notorious to nurserymen.
Horticulturists raise large beds of the same hybrids, and such alone are
fairly treated, for by insect agency the several individuals of the same
hybrid variety are allowed to freely cross with each other, and the
injurious influence of close interbreeding is thus prevented. Any one may
readily convince himself of the efficiency of insect-agency by examining
the flowers of the more sterile kinds of hybrid rhododendrons, which
produce no pollen, for he will find on their stigmas plenty of pollen
brought from other flowers.
In regard to animals, much fewer experiments have been carefully tried than
with plants. If our systematic arrangements can be trusted, that is if the
genera of animals are as distinct from each other, as are the genera of
plants, then we may infer that animals more widely separated in the scale
of nature can be more easily crossed than in the case of plants; but the
hybrids themselves are, I think, more sterile. I doubt whether any case of
a perfectly fertile hybrid animal can be considered as thoroughly well
authenticated. It should, however, be borne in mind that, owing to few
animals breeding freely under confinement, few experiments have been fairly
tried: for instance, the canary-bird has been crossed with nine other
finches, but as not one of these nine species breeds freely in confinement,
we have no right to expect that the first crosses between them and the
canary, or that their hybrids, {253} should be perfectly fertile. Again,
with respect to the fertility in successive generations of the more fertile
hybrid animals, I hardly know of an instance in which two families of the
same hybrid have been raised at the same time from different parents, so as
to avoid the ill effects of close interbreeding. On the contrary, brothers
and sisters have usually been crossed in each successive generation, in
opposition to the constantly repeated admonition of every breeder. And in
this case, it is not at all surprising that the inherent sterility in the
hybrids should have gone on increasing. If we were to act thus, and pair
brothers and sisters in the case of any pure animal, which from any cause
had the least tendency to sterility, the breed would assuredly be lost in a
very few generations.
Although I do not know of any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have some reason to believe that the
hybrids from Cervulus vaginalis and Reevesii, and from Phasianus colchicus
with P. torquatus and with P. versicolor are perfectly fertile. There is no
doubt that these three pheasants, namely, the common, the true ring-necked,
and the Japan, intercross, and are becoming blended together in the woods
of several parts of England. The hybrids from the common and Chinese geese
(A. cygnoides), species which are so different that they are generally
ranked in distinct genera, have often bred in this country with either pure
parent, and in one single instance they have bred _inter se_. This was
effected by Mr. Eyton, who raised two hybrids from the same parents but
from different hatches; and from these two birds he raised no less than
eight hybrids (grandchildren of the pure geese) from one nest. In India,
however, these cross-bred geese must be far more fertile; for I am assured
by two eminently capable judges, namely {254} Mr. Blyth and Capt. Hutton,
that whole flocks of these crossed geese are kept in various parts of the
country; and as they are kept for profit, where neither pure parent-species
exists, they must certainly be highly fertile.
A doctrine which originated with Pallas, has been largely accepted by
modern naturalists; namely, that most of our domestic animals have
descended from two or more wild species, since commingled by intercrossing.
On this view, the aboriginal species must either at first have produced
quite fertile hybrids, or the hybrids must have become in subsequent
generations quite fertile under domestication. This latter alternative
seems to me the most probable, and I am inclined to believe in its truth,
although it rests on no direct evidence. I believe, for instance, that our
dogs have descended from several wild stocks; yet, with perhaps the
exception of certain indigenous domestic dogs of South America, all are
quite fertile together; and analogy makes me greatly doubt, whether the
several aboriginal species would at first have freely bred together and
have produced quite fertile hybrids. So again there is reason to believe
that our European and the humped Indian cattle are quite fertile together;
but from facts communicated to me by Mr. Blyth, I think they must be
considered as distinct species. On this view of the origin of many of our
domestic animals, we must either give up the belief of the almost universal
sterility of distinct species of animals when crossed; or we must look at
sterility, not as an indelible characteristic, but as one capable of being
removed by domestication.
Finally, looking to all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility, both
in first crosses {255} and in hybrids, is an extremely general result; but
that it cannot, under our present state of knowledge, be considered as
absolutely universal.
_Laws governing the Sterility of first Crosses and of Hybrids._--We will
now consider a little more in detail the circumstances and rules governing
the sterility of first crosses and of hybrids. Our chief object will be to
see whether or not the rules indicate that species have specially been
endowed with this quality, in order to prevent their crossing and blending
together in utter confusion. The following rules and conclusions are
chiefly drawn up from Gärtner'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 {256} 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. {257}
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 Gärtner 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 {258} 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 Kölreuter. 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
Kölreuter 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. Gärtner,
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 {259} 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 differ in fertility in a small,
and occasionally in a high degree.
Several other singular rules could be given from Gärtner: 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 {260} 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, {261} 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
{262} 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 Gärtner 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. {263}
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 {264} 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 {265} 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
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
confinement 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 is due, as I believe, to their reproductive systems having
been specially affected, though in a lesser degree than when sterility
ensues. So it is with hybrids, for hybrids 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 unnatural
conditions, and when hybrids {266} are produced by the unnatural crossing
of two species, the reproductive system, independently of the general state
of health, is affected by sterility 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
organisation has been disturbed by two different structures and
constitutions having been blended into one. For it is scarcely possible
that two organisations should be compounded into one, without some
disturbance occurring in the development, or periodical action, or mutual
relation 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 organisation, and hence we need not be surprised that their
sterility, though in some degree variable, rarely diminishes.
It must, however, be confessed that we cannot understand, excepting on
vague hypotheses, several facts with respect to the sterility of hybrids;
for instance, 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 organism, 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 organisation having been disturbed by two organisations having been
compounded into one.
It may seem fanciful, but I suspect that a similar {267} parallelism
extends to an allied yet very different class of facts. It is an old and
almost universal belief, founded, I think, on a considerable body of
evidence, 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, &c., from one soil or climate to
another, and back again. During the convalescence of animals, we plainly
see that great benefit is derived from almost any change in the habits of
life. Again, both with plants and animals, there is abundant evidence, that
a cross between very distinct individuals of the same species, that is
between members of different strains or sub-breeds, gives vigour and
fertility to the offspring. I believe, indeed, from the facts alluded to in
our fourth chapter, that a certain amount of crossing is indispensable even
with hermaphrodites; and that close interbreeding continued during several
generations between the nearest relations, especially if these be kept
under the same conditions of life, always induces weakness and sterility in
the progeny.
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 varied and become slightly different, give vigour and fertility
to the offspring. But we have seen that greater changes, or changes of a
particular nature, often render organic beings in some degree sterile; and
that greater crosses, that is crosses between males and females which have
become widely or specifically different, produce hybrids which are
generally sterile in some degree. I cannot persuade myself that this
parallelism is an accident or an illusion. Both series of facts seem to be
connected together by some {268} common but unknown bond, which is
essentially related to the principle of life.
_Fertility of Varieties when crossed, and of their Mongrel offspring._--It
may be urged, as a most forcible 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, and yield perfectly fertile offspring. 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 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. If we thus argue in a circle, the fertility of all varieties
produced under nature will assuredly have to be granted.
If we turn to varieties, produced, or supposed to have been produced, under
domestication, we are still involved in doubt. For when it is stated, for
instance, that the German Spitz dog unites more easily than other dogs with
foxes, or that certain South American indigenous domestic dogs do not
readily cross with European dogs, the explanation which will occur to every
one, and probably the true one, is that these dogs have descended from
several aboriginally distinct species. Nevertheless the perfect fertility
of so many domestic varieties, differing widely from each other in
appearance, for instance of the pigeon or of the cabbage, is {269} 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 considerations, however, render the fertility of
domestic varieties less remarkable than at first appears. It can, in the
first place, be clearly shown that mere external dissimilarity between two
species does not determine their greater or lesser degree of sterility when
crossed; and we may apply the same rule to domestic varieties. In the
second place, some eminent naturalists believe that a long course of
domestication tends to eliminate sterility in the successive generations of
hybrids which were at first only slightly sterile; and if this be so, we
surely ought not to expect to find sterility both appearing and
disappearing under nearly the same conditions of life. Lastly, and this
seems to me by far the most important consideration, new races of animals
and plants are produced under domestication by man's methodical and
unconscious power of selection, for his own use and pleasure: he neither
wishes to select, nor could select, slight differences in the reproductive
system, or other constitutional differences correlated with the
reproductive system. He supplies his several varieties with the same food;
treats them in nearly the same manner, and does not wish to alter their
general habits of life. Nature acts uniformly and slowly during vast
periods of time on the whole organisation, in any way which may be for each
creature's own good; and thus she may, either directly, or more probably
indirectly, through correlation, modify the reproductive system in the
several descendants from any one species. Seeing this difference in the
process of selection, as carried on by man and nature, we need not be
surprised at some difference in the result.
I have as yet spoken as if the varieties of the same {270} species were
invariably fertile when intercrossed. But it seems to me impossible to
resist the evidence of the existence 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 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 fertilised thirteen flowers of the
one with the 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 important to notice that the hybrid plants thus raised
were themselves _perfectly_ fertile; so that even Gärtner did not venture
to consider the two varieties as specifically distinct.
Girou de Buzareingues crossed three varieties of gourd, which like the
maize has separated sexes, and he asserts that their mutual fertilisation
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 experimentised on,
are ranked by Sagaret, who mainly founds his classification by the test of
infertility, as varieties.
The following case is far more remarkable, and seems at first quite
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
{271} and so hostile a witness, as Gärtner: namely, that yellow and white
varieties of the same species of Verbascum when intercrossed produce less
seed, than do either coloured varieties when fertilised with pollen from
their own coloured flowers. 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 coloured flowers, than between those which are differently
coloured. Yet these varieties of Verbascum present no other difference
besides the mere colour of the flower; and one variety can sometimes be
raised from the seed of the other.
From observations which I have made on certain varieties of hollyhock, I am
inclined to suspect that they present analogous facts.
Kölreuter, whose accuracy has been confirmed by every subsequent observer,
has proved the remarkable fact, that one variety of the common tobacco is
more fertile, when crossed with a widely distinct species, than are the
other varieties. He experimentised 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 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; 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
{272} 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.
_Hybrids and Mongrels compared, independently of their
fertility._--Independently of the question of fertility, the offspring of
species when crossed and of varieties when crossed may be compared in
several other respects. Gärtner, whose strong wish was to draw a marked
line of distinction 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 very 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 Gärtner 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. Gärtner further admits that
hybrids between very closely allied species are more variable {273} than
those from very distinct species; and this shows that the difference in the
degree of variability graduates away. When mongrels and the more fertile
hybrids are propagated for several generations an extreme amount of
variability in their offspring is notorious; but some few cases both of
hybrids and mongrels long retaining uniformity of character could be given.
The variability, however, in the successive generations of mongrels is,
perhaps, greater than in hybrids.
This greater variability of mongrels than of hybrids does not seem to me 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 in most cases that there has been recent
variability; and therefore we might expect that such variability would
often continue and be superadded to that arising from the mere act of
crossing. The slight degree of variability in hybrids from the first cross
or in the first generation, in contrast with their extreme variability in
the succeeding generations, is a curious fact and deserves attention. For
it bears on and corroborates the view which I have taken on the cause of
ordinary variability; namely, that it is due to the reproductive system
being eminently sensitive to any change in the conditions of life, being
thus often rendered either impotent or at least incapable of its proper
function of producing offspring identical with 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 their
reproductive systems seriously affected, and their descendants are highly
variable.
But to return to our comparison of mongrels and {274} hybrids: Gärtner
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. Gärtner further insists that when any two species, although most
closely allied to each other, are crossed with a third species, the hybrids
are widely different from each other; whereas if two very distinct
varieties of one species are crossed with another species, the hybrids do
not differ much. But this conclusion, as far as I can make out, is founded
on a single experiment; and seems directly opposed to the results of
several experiments made by Kölreuter.
These alone are the unimportant differences, which Gärtner is able to point
out, between hybrid and mongrel plants. On the other hand, the resemblance
in mongrels and in hybrids to their respective parents, more especially in
hybrids produced from nearly related species, follows according to Gärtner
the same laws. When two species are crossed, one has sometimes a prepotent
power of impressing its likeness on the hybrid; and so I believe it to be
with varieties of plants. With animals one variety certainly often has this
prepotent power over another variety. Hybrid plants produced from a
reciprocal cross, generally resemble each other closely; and so it is with
mongrels from a reciprocal cross. Both hybrids and mongrels can be reduced
to either pure parent-form, by repeated crosses in successive generations
with either parent.
These several remarks are apparently applicable to animals; but the subject
is here excessively complicated, partly owing to the existence of secondary
sexual characters; but more especially owing to prepotency in transmitting
likeness running more strongly in one sex than in the other, both when one
species is crossed with another, and when, one variety is crossed with
{275} another variety. For instance, I think those authors are right, who
maintain that the ass has a prepotent power over the horse, so that both
the mule and the hinny more resemble the ass than the horse; but that the
prepotency runs more strongly in the male-ass than in the female, so that
the mule, which is the offspring of the male-ass and mare, is more like an
ass, than is the hinny, which is the offspring of the female-ass and
stallion.
Much stress has been laid by some authors on the supposed fact, that
mongrel animals alone are born closely like one of their parents; but it
can be shown that this does sometimes occur with hybrids; yet I grant much
less frequently with hybrids than with mongrels. Looking to the cases which
I have collected of cross-bred animals closely resembling one parent, the
resemblances seem chiefly confined to characters almost monstrous in their
nature, and which have suddenly appeared--such as albinism, melanism,
deficiency of tail or horns, or additional fingers and toes; and do not
relate to characters which have been slowly acquired by selection.
Consequently, sudden reversions to the perfect character of either parent
would be more likely to occur with mongrels, which are descended from
varieties often suddenly produced and semi-monstrous in character, than
with hybrids, which are descended from species slowly and naturally
produced. On the whole I entirely agree with Dr. Prosper Lucas, who, after
arranging an enormous body of facts with respect to animals, comes to the
conclusion, that the laws of resemblance of the child to its parents are
the same, whether the two parents differ much or little from each other,
namely in the union of individuals of the same variety, or of different
varieties, or of distinct species.
Laying aside the question of fertility and sterility, {276} in all other
respects there seems to be a general and close similarity in the offspring
of crossed species, and of crossed varieties. If we look at species as
having been specially created, and at varieties as having been produced by
secondary laws, this similarity would be an astonishing fact. But it
harmonises perfectly with the view that there is no essential distinction
between species and varieties.
_Summary of Chapter._--First crosses between forms sufficiently distinct to
be ranked as species, and their hybrids, are very generally, but not
universally, sterile. The sterility is of all degrees, and is often so
slight that the two most careful experimentalists who have ever lived, have
come to diametrically opposite conclusions in ranking forms by this test.
The sterility is innately variable in individuals of the same species, and
is eminently susceptible of favourable and unfavourable conditions. The
degree of sterility does not strictly follow systematic affinity, but is
governed by several curious and complex laws. It is generally different,
and sometimes widely different, in reciprocal crosses between the same two
species. It is not always equal in degree in a first cross and in the
hybrid produced from this cross.
In the same manner as in grafting trees, the capacity of one species or
variety to take on another, is incidental on generally unknown differences
in their vegetative systems, so in crossing, the greater or less facility
of one species to unite with another, is incidental on unknown differences
in their reproductive systems. There is no more reason to think that
species have been specially endowed with various degrees of sterility to
prevent them crossing and blending in nature, than to think that trees have
been specially endowed with various and {277} somewhat analogous degrees of
difficulty in being grafted together in order to prevent them becoming
inarched in our forests.
The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems to depend on several circumstances; in
some cases largely on the early death of the embryo. The sterility of
hybrids, which have their reproductive systems imperfect, and which have
had this system and their whole organisation disturbed by being compounded
of two distinct species, seems closely allied to that sterility which so
frequently affects pure species, when their natural conditions of life have
been disturbed. This view is supported by a parallelism of another
kind;--namely, that the crossing of forms only slightly different is
favourable to the vigour and fertility of their offspring; and that slight
changes in the conditions of life are apparently favourable to the vigour
and fertility of all organic beings. It is not surprising that the degree
of difficulty in uniting two species, and the degree of sterility of their
hybrid-offspring should generally correspond, though due to distinct
causes; for both depend on the amount of difference of some kind between
the species which are crossed. Nor is it surprising that the facility of
effecting a first cross, the fertility of the hybrids produced from it, and
the capacity of being grafted together--though this latter capacity
evidently depends on widely different circumstances--should all run, to a
certain extent, parallel with the systematic affinity of the forms which
are subjected to experiment; for systematic affinity attempts to express
all kinds of resemblance between all species.
First crosses between forms known to be varieties, or sufficiently alike to
be considered as varieties, and their mongrel offspring, are very
generally, but not quite {278} universally, fertile. Nor is this nearly
general and perfect fertility surprising, when we remember how liable we
are to argue in a circle with respect to varieties in a state of nature;
and when we remember that the greater number of varieties have been
produced under domestication by the selection of mere external differences,
and not of differences in the reproductive system. In all other respects,
excluding fertility, there is a close general resemblance between hybrids
and mongrels. 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.
* * * * *
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