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, not accumulated by natural selection—Causes of the
sterility of first crosses and of hybrids—Parallelism between the
effects of changed conditions of life and of crossing—Dimorphism and
trimorphism—Fertility of varieties when crossed and of their mongrel
offspring not universal—Hybrids and mongrels compared independently of
their fertility—Summary.
The view commonly entertained by naturalists is that species, when
intercrossed, have been specially endowed with sterility, in order to
prevent their confusion. This view certainly seems at first highly
probable, for species living together could hardly have been kept
distinct had they been capable of freely crossing. The subject is in
many ways important for us, more especially as the sterility of species
when first crossed, and that of their hybrid offspring, cannot have
been acquired, as I shall show, by the preservation of successive
profitable degrees of sterility. It is an incidental result of
differences in the reproductive systems of the parent-species.
In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded; namely, the
sterility of species 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 formative 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 probably has 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
be descended from common parents, when crossed, and likewise the
fertility of their mongrel offspring, is, with reference to my theory,
of equal importance with the sterility of species; for it seems to make
a broad and clear distinction between varieties and species.
_Degrees of Sterility._—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 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 first crossed, and the maximum produced by
their hybrid offspring, with the average number produced by both pure
parent-species in a state of nature. But causes of serious error here
intervene: 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
experimented on by Gärtner were potted, and 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 repeatedly crossed some forms, such as the common
red and blue pimpernels (Anagallis arvensis and coerulea), which the
best botanists rank as varieties, and found them absolutely sterile, we
may doubt whether many species are really so sterile, when
intercrossed, as he believed.
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, arrived at diametrically opposite
conclusions in regard to some of the very same forms. 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 observer from experiments made during
different years. It can thus be shown that neither sterility nor
fertility affords any certain distinction between species and
varieties. 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
increases, but generally decreases greatly and suddenly. With respect
to this decrease, it may first be noticed that when any deviation in
structure or constitution is common to both parents, this is often
transmitted in an augmented degree to the offspring; and both sexual
elements in hybrid plants are already affected in some degree. But I
believe that their fertility has been diminished in nearly all these
cases by an independent cause, namely, by too close interbreeding. I
have made so many experiments and collected so many facts, showing on
the one hand that an occasional cross with a distinct individual or
variety increases the vigour and fertility of the offspring, and on the
other hand that very close interbreeding lessens their vigour and
fertility, that I cannot doubt the correctness of this conclusion.
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, if left to themselves, will generally
be fertilised during each generation by pollen from the same flower;
and this would probably 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 from manipulation, sometimes decidedly increases, and goes
on increasing. Now, in the process of 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 often 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 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 an increase of fertility
in the successive generations of _artificially fertilised_ hybrids, in
contrast with those spontaneously self-fertilised, may, as I believe,
be accounted for by too close interbreeding having been avoided.
Now let us turn to the results arrived at by a third most experienced
hybridiser, namely, the Hon. and 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
experimented 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 hot-houses 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 I never saw
to occur in a case of its natural fecundation.” So that here we 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 singular fact, namely,
that individual plants of certain species of Lobelia, Verbascum and
Passiflora, can easily be fertilised by the pollen from a distinct
species, but not by pollen from the same plant, though this pollen can
be proved to be perfectly sound by fertilising other plants or species.
In the genus Hippeastrum, in Corydalis as shown by Professor
Hildebrand, in various orchids as shown by Mr. Scott and Fritz Müller,
all the individuals are in this peculiar condition. So that with some
species, certain abnormal individuals, and in other species all the
individuals, can actually be hybridised much more readily than they can
be fertilised by pollen from the same individual plant! To give one
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 distinct 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.” Mr. Herbert tried similar experiments during
many years, and always with the same result. These cases serve to show
on what slight and mysterious causes the lesser or greater fertility of
a species 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, “reproduces 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 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, always gone on decreasing
in fertility in each successive generation, as Gärtner believed to be
the case, the fact would have been notorious to nurserymen.
Horticulturists raise large beds of the same hybrid, and such alone are
fairly treated, for by insect agency the several individuals are
allowed to cross freely 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 distinct
in the scale of nature can be crossed more easily than in the case of
plants; but the hybrids themselves are, I think, more sterile. 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 distinct species
of finches, but, as not one of these breeds freely in confinement, we
have no right to expect that the first crosses between them and the
canary, or that their hybrids, 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.
Although I know of hardly any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have reason to believe that the
hybrids from Cervulus vaginalis and Reevesii, and from Phasianus
colchicus with P. torquatus, are perfectly fertile. M. Quatrefages
states that the hybrids from two moths (Bombyx cynthia and arrindia)
were proved in Paris to be fertile _inter se_ for eight generations. It
has lately been asserted that two such distinct species as the hare and
rabbit, when they can be got to breed together, produce offspring,
which are highly fertile when crossed with one of the parent-species.
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 Mr. Blyth and Captain 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 or perfectly
fertile.
With our domesticated animals, the various races when crossed together
are quite fertile; yet in many cases they are descended from two or
more wild species. From this fact we must conclude either that the
aboriginal parent-species at first produced perfectly fertile hybrids,
or that the hybrids subsequently reared under domestication became
quite fertile. This latter alternative, which was first propounded by
Pallas, seems by far the most probable, and can, indeed, hardly be
doubted. It is, for instance, almost certain that our dogs are
descended from several wild stocks; yet, with perhaps the exception of
certain indigenous domestic dogs of South America, all are quite
fertile together; but 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 I have lately acquired
decisive evidence that the crossed offspring from the Indian humped and
common cattle are inter se perfectly fertile; and from the observations
by Rütimeyer on their important osteological differences, as well as
from those by Mr. Blyth on their differences in habits, voice,
constitution, &c., these two forms must be regarded as good and
distinct species. The same remarks may be extended to the two chief
races of the pig. We must, therefore, either give up the belief of the
universal sterility of species when crossed; or we must look at this
sterility in animals, not as an indelible characteristic, but as one
capable of being removed by domestication.
Finally, considering all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility,
both in first crosses 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 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 Gärtner’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-fertilised 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 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.
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
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 closely
related forms (as Matthiola annua and glabra) which many botanists rank
only as varieties. It is also a remarkable fact that hybrids raised
from reciprocal crosses, though of course compounded of the very same
two species, the one species having first been used as the father and
then as the mother, though they rarely differ in external characters,
yet generally differ in fertility in a small, and occasionally in a
high degree.
Several other singular rules could be given from 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 among 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 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 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, 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 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 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 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.
_Origin and Causes of the Sterility of first Crosses and of Hybrids._
At one time it appeared to me probable, as it has to others, that the
sterility of first crosses and of hybrids might have been slowly
acquired through the natural selection of slightly lessened degrees of
fertility, which, like any other variation, spontaneously appeared in
certain individuals of one variety when crossed with those of another
variety. For it would clearly be advantageous to two varieties or
incipient species if they could be kept from blending, on the same
principle that, when man is selecting at the same time two varieties,
it is necessary that he should keep them separate. In the first place,
it may be remarked that species inhabiting distinct regions are often
sterile when crossed; now it could clearly have been of no advantage to
such separated species to have been rendered mutually sterile, and
consequently this could not have been effected through natural
selection; but it may perhaps be argued, that, if a species was
rendered sterile with some one compatriot, sterility with other species
would follow as a necessary contingency. In the second place, it is
almost as much opposed to the theory of natural selection as to that of
special creation, that in reciprocal crosses the male element of one
form should have been rendered utterly impotent on a second form, while
at the same time the male element of this second form is enabled freely
to fertilise the first form; for this peculiar state of the
reproductive system could hardly have been advantageous to either
species.
In considering the probability of natural selection having come into
action, in rendering species mutually sterile, the greatest difficulty
will be found to lie in the existence of many graduated steps, from
slightly lessened fertility to absolute sterility. It may be admitted
that it would profit an incipient species, if it were rendered in some
slight degree sterile when crossed with its parent form or with some
other variety; for thus fewer bastardised and deteriorated offspring
would be produced to commingle their blood with the new species in
process of formation. But he who will take the trouble to reflect on
the steps by which this first degree of sterility could be increased
through natural selection to that high degree which is common with so
many species, and which is universal with species which have been
differentiated to a generic or family rank, will find the subject
extraordinarily complex. After mature reflection, it seems to me that
this could not have been effected through natural selection. Take the
case of any two species which, when crossed, produced few and sterile
offspring; now, what is there which could favour the survival of those
individuals which happened to be endowed in a slightly higher degree
with mutual infertility, and which thus approached by one small step
towards absolute sterility? Yet an advance of this kind, if the theory
of natural selection be brought to bear, must have incessantly occurred
with many species, for a multitude are mutually quite barren. With
sterile neuter insects we have reason to believe that modifications in
their structure and fertility have been slowly accumulated by natural
selection, from an advantage having been thus indirectly given to the
community to which they belonged over other communities of the same
species; but an individual animal not belonging to a social community,
if rendered slightly sterile when crossed with some other variety,
would not thus itself gain any advantage or indirectly give any
advantage to the other individuals of the same variety, thus leading to
their preservation.
But it would be superfluous to discuss this question in detail: for
with plants we have conclusive evidence that the sterility of crossed
species must be due to some principle, quite independent of natural
selection. Both Gärtner and Kölreuter have proved that in genera
including numerous species, a series can be formed from species which
when crossed yield fewer and fewer seeds, to species which never
produce a single seed, but yet are affected by the pollen of certain
other species, for the germen swells. It is here manifestly impossible
to select the more sterile individuals, which have already ceased to
yield seeds; so that this acme of sterility, when the germen alone is
effected, cannot have been gained through selection; and from the laws
governing the various grades of sterility being so uniform throughout
the animal and vegetable kingdoms, we may infer that the cause,
whatever it may be, is the same or nearly the same in all cases.
We will now look a little closer at the probable nature of the
differences between species which induce sterility in first crosses and
in hybrids. In the case of first crosses, the greater or less
difficulty in effecting a union and in obtaining offspring apparently
depends on several distinct causes. There must sometimes be a physical
impossibility in the male element reaching the ovule, as would be the
case with a plant having a pistil too long for the pollen-tubes to
reach the ovarium. It has also been observed that when the pollen of
one species is placed on the stigma of a distantly 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 pheasants and
fowls, that the early death of the embryo is a very frequent cause of
sterility in first crosses. Mr. Salter has recently given the results
of an examination of about 500 eggs produced from various crosses
between three species of Gallus and their hybrids; the majority of
these eggs had been fertilised; and in the majority of the fertilised
eggs, the embryos had either been partially developed and had then
perished, or had become nearly mature, but the young chickens had been
unable to break through the shell. Of the chickens which were born,
more than four-fifths died within the first few days, or at latest
weeks, “without any obvious cause, apparently from mere inability to
live;” so that from the 500 eggs only twelve chickens were reared. With
plants, hybridized embryos probably often perish in a like manner; at
least it is known that hybrids raised from very distinct species are
sometimes weak and dwarfed, and perish at an early age; of which fact
Max Wichura has recently given some striking cases with hybrid willows.
It may be here worth noticing that in some cases of parthenogenesis,
the embryos within the eggs of silk moths which had not been
fertilised, pass through their early stages of development and then
perish like the embryos produced by a cross between distinct species.
Until becoming acquainted with these facts, I was unwilling to believe
in the frequent early death of hybrid embryos; for hybrids, when once
born, are generally healthy and long-lived, as we see in the case of
the common mule. Hybrids, however, are differently circumstanced before
and after birth: when born and living in a country where their two
parents live, they are generally placed under suitable conditions of
life. But a hybrid partakes of only half of the nature and constitution
of its mother; it may therefore, before birth, as long as it is
nourished within its mother’s womb, or within the egg or seed produced
by the mother, be exposed to conditions in some degree unsuitable, and
consequently be liable to perish at an early period; more especially as
all very young beings are eminently sensitive to injurious or unnatural
conditions of life. But after all, the cause more probably lies in some
imperfection in the original act of impregnation, causing the embryo to
be imperfectly developed, rather than in the conditions to which it is
subsequently exposed.
In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is somewhat different. I have more than
once alluded to a large body of facts showing that, when animals and
plants are removed from their natural conditions, they are extremely
liable to have their reproductive systems seriously affected. This, in
fact, is the great bar to the domestication of animals. Between the
sterility thus superinduced and that of hybrids, there are many points
of similarity. In both cases the sterility is independent of general
health, and is often accompanied by excess of size or great luxuriance.
In both cases the sterility occurs in various degrees; in both, the
male element is the most liable to be affected; but sometimes the
female more than the male. In both, the tendency goes to a certain
extent with systematic affinity, for whole groups of animals and plants
are rendered impotent by the same unnatural conditions; and whole
groups of species tend to produce sterile hybrids. On the other hand,
one species in a group will sometimes resist great changes of
conditions with unimpaired fertility; and certain species in a group
will produce unusually fertile hybrids. No one can tell till he tries,
whether any particular animal will breed under 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 seems to be partly due to their reproductive
systems having been specially affected, though in a lesser degree than
when sterility ensues. So it is with hybrids, for their offspring in
successive generations are eminently liable to vary, as every
experimentalist has observed.
Thus we see that when organic beings are placed under new and unnatural
conditions, and when hybrids are produced by the unnatural crossing of
two species, the reproductive system, independently of the general
state of health, is affected in a very similar manner. In the one case,
the conditions of life have been disturbed, though often in so slight a
degree as to be inappreciable by us; in the other case, or that of
hybrids, the external conditions have remained the same, but the
organisation has been disturbed by two distinct structures and
constitutions, including of course the reproductive systems, 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 relations
of the different parts and organs one to another or to the conditions
of life. When hybrids are able to breed _inter se_, they transmit to
their offspring from generation to generation the same compounded
organisation, and hence we need not be surprised that their sterility,
though in some degree variable, does not diminish; it is even apt to
increase, this being generally the result, as before explained, of too
close interbreeding. The above view of the sterility of hybrids being
caused by two constitutions being compounded into one has been strongly
maintained by Max Wichura.
It must, however, be owned that we cannot understand, on the above or
any other view, several facts with respect to the sterility of hybrids;
for 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 being compounded into one.
A similar parallelism holds good with an allied yet very different
class of facts. It is an old and almost universal belief, founded on a
considerable body of evidence, which I have elsewhere given, that
slight changes in the conditions of life are beneficial to all living
things. We see this acted on by farmers and gardeners in their frequent
exchanges of seed, tubers, &c., from one soil or climate to another,
and back again. During the convalescence of animals, great benefit is
derived from almost any change in their habits of life. Again, both
with plants and animals, there is the clearest evidence that a cross
between individuals of the same species, which differ to a certain
extent, gives vigour and fertility to the offspring; and that close
interbreeding continued during several generations between the nearest
relations, if these be kept under the same conditions of life, almost
always leads to decreased size, weakness, or sterility.
Hence it seems that, on the one hand, slight changes in the conditions
of life benefit all organic beings, and on the other hand, that slight
crosses, that is, crosses between the males and females of the same
species, which have been subjected to slightly different conditions, or
which have slightly varied, give vigour and fertility to the offspring.
But, as we have seen, organic beings long habituated to certain uniform
conditions under a state of nature, when subjected, as under
confinement, to a considerable change in their conditions, very
frequently are rendered more or less sterile; and we know that a cross
between two forms that have become widely or specifically different,
produce hybrids which are almost always in some degree sterile. I am
fully persuaded that this double parallelism is by no means an accident
or an illusion. He who is able to explain why the elephant, and a
multitude of other animals, are incapable of breeding when kept under
only partial confinement in their native country, will be able to
explain the primary cause of hybrids being so generally sterile. He
will at the same time be able to explain how it is that the races of
some of our domesticated animals, which have often been subjected to
new and not uniform conditions, are quite fertile together, although
they are descended from distinct species, which would probably have
been sterile if aboriginally crossed. The above two parallel series of
facts seem to be connected together by some common but unknown bond,
which is essentially related to the principle of life; this principle,
according to Mr. Herbert Spencer, being that life depends on, or
consists in, the incessant action and reaction of various forces,
which, as throughout nature, are always tending towards an equilibrium;
and when this tendency is slightly disturbed by any change, the vital
forces gain in power.
_Reciprocal Dimorphism and Trimorphism._
This subject may be here briefly discussed, and will be found to throw
some light on hybridism. Several plants belonging to distinct orders
present two forms, which exist in about equal numbers and which differ
in no respect except in their reproductive organs; one form having a
long pistil with short stamens, the other a short pistil with long
stamens; the two having differently sized pollen-grains. With
trimorphic plants there are three forms likewise differing in the
lengths of their pistils and stamens, in the size and colour of the
pollen-grains, and in some other respects; and as in each of the three
forms there are two sets of stamens, the three forms possess altogether
six sets of stamens and three kinds of pistils. These organs are so
proportioned in length to each other, that half the stamens in two of
the forms stand on a level with the stigma of the third form. Now I
have shown, and the result has been confirmed by other observers, that
in order to obtain full fertility with these plants, it is necessary
that the stigma of the one form should be fertilised by pollen taken
from the stamens of corresponding height in another form. So that with
dimorphic species two unions, which may be called legitimate, are fully
fertile; and two, which may be called illegitimate, are more or less
infertile. With trimorphic species six unions are legitimate, or fully
fertile, and twelve are illegitimate, or more or less infertile.
The infertility which may be observed in various dimorphic and
trimorphic plants, when they are illegitimately fertilised, that is by
pollen taken from stamens not corresponding in height with the pistil,
differs much in degree, up to absolute and utter sterility; just in the
same manner as occurs in crossing distinct species. As the degree of
sterility in the latter case depends in an eminent degree on the
conditions of life being more or less favourable, so I have found it
with illegitimate unions. It is well known that if pollen of a distinct
species be placed on the stigma of a flower, and its own pollen be
afterwards, even after a considerable interval of time, placed on the
same stigma, its action is so strongly prepotent that it generally
annihilates the effect of the foreign pollen; so it is with the pollen
of the several forms of the same species, for legitimate pollen is
strongly prepotent over illegitimate pollen, when both are placed on
the same stigma. I ascertained this by fertilising several flowers,
first illegitimately, and twenty-four hours afterwards legitimately,
with pollen taken from a peculiarly coloured variety, and all the
seedlings were similarly coloured; this shows that the legitimate
pollen, though applied twenty-four hours subsequently, had wholly
destroyed or prevented the action of the previously applied
illegitimate pollen. Again, as in making reciprocal crosses between the
same two species, there is occasionally a great difference in the
result, so the same thing occurs with trimorphic plants; for instance,
the mid-styled form of Lythrum salicaria was illegitimately fertilised
with the greatest ease by pollen from the longer stamens of the
short-styled form, and yielded many seeds; but the latter form did not
yield a single seed when fertilised by the longer stamens of the
mid-styled form.
In all these respects, and in others which might be added, the forms of
the same undoubted species, when illegitimately united, behave in
exactly the same manner as do two distinct species when crossed. This
led me carefully to observe during four years many seedlings, raised
from several illegitimate unions. The chief result is that these
illegitimate plants, as they may be called, are not fully fertile. It
is possible to raise from dimorphic species, both long-styled and
short-styled illegitimate plants, and from trimorphic plants all three
illegitimate forms. These can then be properly united in a legitimate
manner. When this is done, there is no apparent reason why they should
not yield as many seeds as did their parents when legitimately
fertilised. But such is not the case. They are all infertile, in
various degrees; some being so utterly and incurably sterile that they
did not yield during four seasons a single seed or even seed-capsule.
The sterility of these illegitimate plants, when united with each other
in a legitimate manner, may be strictly compared with that of hybrids
when crossed _inter se_. If, on the other hand, a hybrid is crossed
with either pure parent-species, the sterility is usually much
lessened: and so it is when an illegitimate plant is fertilised by a
legitimate plant. In the same manner as the sterility of hybrids does
not always run parallel with the difficulty of making the first cross
between the two parent-species, so that sterility of certain
illegitimate plants was unusually great, while the sterility of the
union from which they were derived was by no means great. With hybrids
raised from the same seed-capsule the degree of sterility is innately
variable, so it is in a marked manner with illegitimate plants. Lastly,
many hybrids are profuse and persistent flowerers, while other and more
sterile hybrids produce few flowers, and are weak, miserable dwarfs;
exactly similar cases occur with the illegitimate offspring of various
dimorphic and trimorphic plants.
Altogether there is the closest identity in character and behaviour
between illegitimate plants and hybrids. It is hardly an exaggeration
to maintain that illegitimate plants are hybrids, produced within the
limits of the same species by the improper union of certain forms,
while ordinary hybrids are produced from an improper union between
so-called distinct species. We have also already seen that there is the
closest similarity in all respects between first illegitimate unions
and first crosses between distinct species. This will perhaps be made
more fully apparent by an illustration; we may suppose that a botanist
found two well-marked varieties (and such occur) of the long-styled
form of the trimorphic Lythrum salicaria, and that he determined to try
by crossing whether they were specifically distinct. He would find that
they yielded only about one-fifth of the proper number of seed, and
that they behaved in all the other above specified respects as if they
had been two distinct species. But to make the case sure, he would
raise plants from his supposed hybridised seed, and he would find that
the seedlings were miserably dwarfed and utterly sterile, and that they
behaved in all other respects like ordinary hybrids. He might then
maintain that he had actually proved, in accordance with the common
view, that his two varieties were as good and as distinct species as
any in the world; but he would be completely mistaken.
The facts now given on dimorphic and trimorphic plants are important,
because they show us, first, that the physiological test of lessened
fertility, both in first crosses and in hybrids, is no safe criterion
of specific distinction; secondly, because we may conclude that there
is some unknown bond which connects the infertility of illegitimate
unions with that of their illegitimate offspring, and we are led to
extend the same view to first crosses and hybrids; thirdly, because we
find, and this seems to me of especial importance, that two or three
forms of the same species may exist and may differ in no respect
whatever, either in structure or in constitution, relatively to
external conditions, and yet be sterile when united in certain ways.
For we must remember that it is the union of the sexual elements of
individuals of the same form, for instance, of two long-styled forms,
which results in sterility; while it is the union of the sexual
elements proper to two distinct forms which is fertile. Hence the case
appears at first sight exactly the reverse of what occurs, in the
ordinary unions of the individuals of the same species and with crosses
between distinct species. It is, however, doubtful whether this is
really so; but I will not enlarge on this obscure subject.
We may, however, infer as probable from the consideration of dimorphic
and trimorphic plants, that the sterility of distinct species when
crossed and of their hybrid progeny, depends exclusively on the nature
of their sexual elements, and not on any difference in their structure
or general constitution. We are also led to this same conclusion by
considering reciprocal crosses, in which the male of one species cannot
be united, or can be united with great difficulty, with the female of a
second species, while the converse cross can be effected with perfect
facility. That excellent observer, Gärtner, likewise concluded that
species when crossed are sterile owing to differences confined to their
reproductive systems.
_Fertility of Varieties when Crossed, and of their Mongrel Offspring,
not universal._
It may be urged as an overwhelming argument that there must be some
essential distinction between species and varieties inasmuch as the
latter, however much they may differ from each other in external
appearance, cross with perfect facility, and yield perfectly fertile
offspring. With some exceptions, presently to be given, I fully admit
that this is the rule. But the subject is surrounded by difficulties,
for, looking to varieties produced under nature, if two forms hitherto
reputed to be varieties be found in any degree sterile together, they
are at once ranked by most naturalists as species. For instance, the
blue and red pimpernel, which are considered by most botanists as
varieties, are said by Gärtner to be quite sterile 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 some doubt. For when it
is stated, for instance, that certain South American indigenous
domestic dogs do not readily unite with European dogs, the explanation
which will occur to everyone, and probably the true one, is that they
are descended from aboriginally distinct species. Nevertheless the
perfect fertility of so many domestic races, differing widely from each
other in appearance, for instance, those of the pigeon, or of the
cabbage, is a remarkable fact; more especially when we reflect how many
species there are, which, though resembling each other most closely,
are utterly sterile when intercrossed. Several considerations, however,
render the fertility of domestic varieties less remarkable. In the
first place, it may be observed that the amount of external difference
between two species is no sure guide to their degree of mutual
sterility, so that similar differences in the case of varieties would
be no sure guide. It is certain that with species the cause lies
exclusively in differences in their sexual constitution. Now the
varying conditions to which domesticated animals and cultivated plants
have been subjected, have had so little tendency towards modifying the
reproductive system in a manner leading to mutual sterility, that we
have good grounds for admitting the directly opposite doctrine of
Pallas, namely, that such conditions generally eliminate this tendency;
so that the domesticated descendants of species, which in their natural
state probably would have been in some degree sterile when crossed,
become perfectly fertile together. With plants, so far is cultivation
from giving a tendency towards sterility between distinct species, that
in several well-authenticated cases already alluded to, certain plants
have been affected in an opposite manner, for they have become
self-impotent, while still retaining the capacity of fertilising, and
being fertilised by, other species. If the Pallasian doctrine of the
elimination of sterility through long-continued domestication be
admitted, and it can hardly be rejected, it becomes in the highest
degree improbable that similar conditions long-continued should
likewise induce this tendency; though in certain cases, with species
having a peculiar constitution, sterility might occasionally be thus
caused. Thus, as I believe, we can understand why, with domesticated
animals, varieties have not been produced which are mutually sterile;
and why with plants only a few such cases, immediately to be given,
have been observed.
The real difficulty in our present subject is not, as it appears to me,
why domestic varieties have not become mutually infertile when crossed,
but why this has so generally occurred with natural varieties, as soon
as they have been permanently modified in a sufficient degree to take
rank as species. We are far from precisely knowing the cause; nor is
this surprising, seeing how profoundly ignorant we are in regard to the
normal and abnormal action of the reproductive system. But we can see
that species, owing to their struggle for existence with numerous
competitors, will have been exposed during long periods of time to more
uniform conditions, than have domestic varieties; and this may well
make a wide difference in the result. For we know how commonly wild
animals and plants, when taken from their natural conditions and
subjected to captivity, are rendered sterile; and the reproductive
functions of organic beings which have always lived under natural
conditions would probably in like manner be eminently sensitive to the
influence of an unnatural cross. Domesticated productions, on the other
hand, which, as shown by the mere fact of their domestication, were not
originally highly sensitive to changes in their conditions of life, and
which can now generally resist with undiminished fertility repeated
changes of conditions, might be expected to produce varieties, which
would be little liable to have their reproductive powers injuriously
affected by the act of crossing with other varieties which had
originated in a like manner.
I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it is impossible to resist
the evidence of the 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 kind with pollen of the other;
but only a single head produced any seed, and this one head produced
only five grains. Manipulation in this case could not have been
injurious, as the plants have separated sexes. No one, I believe, has
suspected that these varieties of maize are distinct species; and it is
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 experimented on are ranked by Sagaret, who mainly founds his
classification by the test of infertility, as varieties, and Naudin has
come to the same conclusion.
The following case is far more remarkable, and seems at first
incredible; but it is the result of an astonishing number of
experiments made during many years on nine species of Verbascum, by so
good an observer and so hostile a witness as Gärtner: namely, that the
yellow and white varieties when crossed produce less seed than the
similarly coloured varieties of the same species. Moreover, he asserts
that, when yellow and white varieties of one species are crossed with
yellow and white varieties of a _distinct_ species, more seed is
produced by the crosses between the similarly coloured flowers, than
between those which are differently coloured. Mr. Scott also has
experimented on the species and varieties of Verbascum; and although
unable to confirm Gärtner’s results on the crossing of the distinct
species, he finds that the dissimilarly coloured varieties of the same
species yield fewer seeds, in the proportion of eighty-six to 100, than
the similarly coloured varieties. Yet these varieties differ in no
respect, except in the colour of their flowers; and one variety can
sometimes be raised from the seed of another.
Kölreuter, whose accuracy has been confirmed by every subsequent
observer, has proved the remarkable fact that one particular variety of
the common tobacco was more fertile than the other varieties, when
crossed with a widely distinct species. He experimented on five forms
which are commonly reputed to be varieties, and which he tested by the
severest trial, namely, by reciprocal crosses, and he found their
mongrel offspring perfectly fertile. But one of these five varieties,
when used either as the father or mother, and crossed with the
Nicotiana glutinosa, always yielded hybrids not so sterile as those
which were produced from the four other varieties when crossed with N.
glutinosa. Hence the reproductive system of this one variety must have
been in some manner and in some degree modified.
From these facts it can no longer be maintained that varieties when
crossed are invariably quite fertile. From the great difficulty of
ascertaining the infertility of varieties in a state of nature, for a
supposed variety, if proved to be infertile in any degree, would almost
universally be ranked as a species; from man attending only to external
characters in his domestic varieties, and from such varieties not
having been exposed for very long periods to uniform conditions of
life; from these several considerations we may conclude that fertility
does not constitute a fundamental distinction between varieties and
species when crossed. The general sterility of crossed species may
safely be looked at, not as a special acquirement or endowment, but as
incidental on changes of an unknown nature in their sexual elements.
_Hybrids and Mongrels compared, independently of their fertility._
Independently of the question of fertility, the offspring of species
and of varieties when crossed may be compared in several other
respects. Gärtner, whose strong wish it was to draw a distinct line
between species and varieties, could find very few, and, as it seems to
me, quite unimportant differences between the so-called hybrid
offspring of species, and the so-called mongrel offspring of varieties.
And, on the other hand, they agree most closely in many important
respects.
I shall here discuss this subject with extreme brevity. The most
important distinction is, that in the first generation mongrels are
more variable than hybrids; but 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 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 the offspring in both
cases is notorious; but some few instances of both hybrids and mongrels
long retaining a uniform character could be given. The variability,
however, in the successive generations of mongrels is, perhaps, greater
than in hybrids.
This greater variability in mongrels than in hybrids does not seem at
all surprising. For the parents of mongrels are varieties, and mostly
domestic varieties (very few experiments having been tried on natural
varieties), and this implies that there has been recent variability;
which would often continue and would augment that arising from the act
of crossing. The slight variability of hybrids in the first generation,
in contrast with that in the succeeding generations, is a curious fact
and deserves attention. For it bears on the view which I have taken of
one of the causes of ordinary variability; namely, that the
reproductive system, from being eminently sensitive to changed
conditions of life, fails under these circumstances to perform its
proper function of producing offspring closely similar in all respects
to the parent-form. Now, hybrids in the first generation are descended
from species (excluding those long cultivated) which have not had their
reproductive systems in any way affected, and they are not variable;
but hybrids themselves have their reproductive systems seriously
affected, and their descendants are highly variable.
But to return to our comparison of mongrels and 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. Moreover, Gärtner expressly states that the hybrids from long
cultivated plants are more subject to reversion than hybrids from
species in their natural state; and this probably explains the singular
difference in the results arrived at by different observers. Thus Max
Wichura doubts whether hybrids ever revert to their parent forms, and
he experimented on uncultivated species of willows, while Naudin, on
the other hand, insists in the strongest terms on the almost universal
tendency to reversion in hybrids, and he experimented chiefly on
cultivated plants. Gärtner further states 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.
Such alone are the unimportant differences which Gärtner is able to
point out between hybrid and mongrel plants. On the other hand, the
degrees and kinds of resemblance in mongrels and in hybrids to their
respective parents, more especially in hybrids produced from nearly
related species, follow, according to Gärtner the same laws. When two
species are crossed, one has sometimes a prepotent power of impressing
its likeness on the hybrid. So I believe it to be with varieties of
plants; and 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 mongrel plants 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 much 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 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 resemble more closely
the ass than the horse; but that the prepotency runs more strongly in
the male than in the female ass, so that the mule, which is an
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 it
is only with mongrels that the offspring are not intermediate in
character, but closely resemble one of their parents; but this does
sometimes occur with hybrids, yet I grant much less frequently 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 through selection. A
tendency to sudden reversions to the perfect character of either parent
would, also, be much 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
little or much from each other, namely, in the union of individuals of
the same variety, or of different varieties, or of distinct species.
Independently of the question of fertility and sterility, 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 most careful experimentalists have arrived at diametrically
opposite conclusions in ranking forms by this test. The sterility is
innately variable in individuals of the same species, and is eminently
susceptible to action 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 hybrids produced from this cross.
In the same manner as in grafting trees, the capacity in one species or
variety to take on another, is incidental on differences, generally of
an unknown nature, 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 their crossing and
blending in nature, than to think that trees have been specially
endowed with various and somewhat analogous degrees of difficulty in
being grafted together in order to prevent their inarching in our
forests.
The sterility of first crosses and of their hybrid progeny has not been
acquired through natural selection. In the case of first crosses it
seems to depend on several circumstances; in some instances in chief
part on the early death of the embryo. In the case of hybrids, it
apparently depends on their whole organisation having been disturbed by
being compounded from two distinct forms; the sterility being closely
allied to that which so frequently affects pure species, when exposed
to new and unnatural conditions of life. He who will explain these
latter cases will be able to explain the sterility of hybrids. This
view is strongly supported by a parallelism of another kind: namely,
that, firstly, slight changes in the conditions of life add to the
vigour and fertility of all organic beings; and secondly, that the
crossing of forms, which have been exposed to slightly different
conditions of life, or which have varied, favours the size, vigour and
fertility of their offspring. The facts given on the sterility of the
illegitimate unions of dimorphic and trimorphic plants and of their
illegitimate progeny, perhaps render it probable that some unknown bond
in all cases connects the degree of fertility of first unions with that
of their offspring. The consideration of these facts on dimorphism, as
well as of the results of reciprocal crosses, clearly leads to the
conclusion that the primary cause of the sterility of crossed species
is confined to differences in their sexual elements. But why, in the
case of distinct species, the sexual elements should so generally have
become more or less modified, leading to their mutual infertility, we
do not know; but it seems to stand in some close relation to species
having been exposed for long periods of time to nearly uniform
conditions of life.
It is not surprising that the difficulty in crossing any two species,
and the sterility of their hybrid offspring, should in most cases
correspond, even if due to distinct causes: for both depend on the
amount of difference between the species which are crossed. Nor is it
surprising that the facility of effecting a first cross, and the
fertility of the hybrids thus produced, 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 subjected to
experiment; for systematic affinity includes resemblances of all kinds.
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, as is so often stated, invariably fertile. Nor
is this almost universal and perfect fertility surprising, when it is
remembered 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 that they have not been
long exposed to uniform conditions of life. It should also be
especially kept in mind, that long-continued domestication tends to
eliminate sterility, and is therefore little likely to induce this same
quality. Independently of the question of fertility, in all other
respects there is the closest general resemblance between hybrids and
mongrels, in their variability, in their power of absorbing each other
by repeated crosses, and in their inheritance of characters from both
parent-forms. Finally, then, although we are as ignorant of the precise
cause of the sterility of first crosses and of hybrids as we are why
animals and plants removed from their natural conditions become
sterile, yet the facts given in this chapter do not seem to me opposed
to the belief that species aboriginally existed as varieties.