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Charles Darwin: The origin of species
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CHAPTER IX.
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, Kolreuter and Gartner, who
almost devoted their lives to this subject, without being deeply impressed
with the high generality of some degree of sterility. Kolreuter 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. Gartner,
also, makes the rule equally universal; and he disputes the entire
fertility of Kolreuter's ten cases. But in these and in many other cases,
Gartner 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 Gartner 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 Gartner 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 Leguminosae, in which
there is an acknowledged difficulty in the manipulation) half of these
twenty plants had their fertility in some degree impaired. Moreover, as
Gartner 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 Kolreuter and Gartner, 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
Gartner 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 Gartner,
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
Gartner 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 Kolreuter and Gartner 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 Gartner. 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 Muller, 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, etc., 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
Gartner 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 Rutimeyer on their important osteological differences,
as well as from those by Mr. Blyth on their differences in habits, voice,
constitution, etc., 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
Gartner's admirable work on the hybridisation of plants. I have taken much
pains to ascertain how far they apply to animals, and, considering how
scanty our knowledge is in regard to hybrid animals, I have been surprised
to find how generally the same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility, both of first
crosses and of hybrids, graduates from zero to perfect fertility. It is
surprising in how many curious ways this gradation can be shown; but only
the barest outline of the facts can here be given. When pollen from a
plant of one family is placed on the stigma of a plant of a distinct
family, it exerts no more influence than so much inorganic dust. From this
absolute zero of fertility, the pollen of different species applied to the
stigma of some one species of the same genus, yields a perfect gradation in
the number of seeds produced, up to nearly complete or even quite complete
fertility; and, as we have seen, in certain abnormal cases, even to an
excess of fertility, beyond that which the plant's own pollen produces. So
in hybrids themselves, there are some which never have produced, and
probably never would produce, even with the pollen of the pure parents, a
single fertile seed: but in some of these cases a first trace of fertility
may be detected, by the pollen of one of the pure parent-species causing
the flower of the hybrid to wither earlier than it otherwise would have
done; and the early withering of the flower is well known to be a sign of
incipient fertilisation. From this extreme degree of sterility we have
self-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 Gartner found that N. acuminata,
which is not a particularly distinct species, obstinately failed to
fertilise, or to be fertilised, by no less than eight other species of
Nicotiana. Many analogous facts could be given.
No one has been able to point out what kind or what amount of difference,
in any recognisable character, is sufficient to prevent two species
crossing. It can be shown that plants most widely different in habit and
general appearance, and having strongly marked differences in every part of
the flower, even in the pollen, in the fruit, and in the cotyledons, can be
crossed. Annual and perennial plants, deciduous and evergreen trees,
plants inhabiting different stations and fitted for extremely different
climates, can often be crossed with ease.
By a reciprocal cross between two species, I mean the case, for instance,
of a female-ass being first crossed by a stallion, and then a mare by a
male-ass: these two species may then be said to have been reciprocally
crossed. There is often the widest possible difference in the facility of
making reciprocal crosses. Such cases are highly important, for they prove
that the capacity in any two species to cross is often completely
independent of their systematic affinity, that is of any difference in
their structure or constitution, excepting in their reproductive systems.
The diversity of the result in reciprocal crosses between the same two
species was long ago observed by Kolreuter. To give an instance:
Mirabilis jalapa can easily be fertilised by the pollen of M. longiflora,
and the hybrids thus produced are sufficiently fertile; but Kolreuter tried
more than two hundred times, during eight following years, to fertilise
reciprocally M. longiflora with the pollen of M. jalapa, and utterly
failed. Several other equally striking cases could be given. Thuret has
observed the same fact with certain sea-weeds or Fuci. Gartner, moreover,
found that this difference of facility in making reciprocal crosses is
extremely common in a lesser degree. He has observed it even between
closely related forms (as Matthiola annua and glabra) which many botanists
rank only as varieties. It is also a remarkable fact that hybrids raised
from reciprocal crosses, though of course compounded of the very same two
species, the one species having first been used as the father and then as
the mother, though they rarely differ in external characters, yet generally
differ in fertility in a small, and occasionally in a high degree.
Several other singular rules could be given from Gartner: for instance,
some species have a remarkable power of crossing with other species; other
species of the same genus have a remarkable power of impressing their
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, etc.; 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 Gartner found that there was sometimes an innate difference in different
INDIVIDUALS of the same two species in crossing; so Sagaret believes this
to be the case with different individuals of the same two species in being
grafted together. As in reciprocal crosses, the facility of effecting an
union is often very far from equal, so it sometimes is in grafting. The
common gooseberry, for instance, cannot be grafted on the currant, whereas
the currant will take, though with difficulty, on the gooseberry.
We have seen that the sterility of hybrids which have their reproductive
organs in an imperfect condition, is a 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, etc., 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 Gartner and Kolreuter have proved that in genera including numerous
species, a series can be formed from species which when crossed yield fewer
and fewer seeds, to species which never produce a single seed, but yet are
affected by the pollen of certain other species, for the germen swells. It
is here manifestly impossible to select the more sterile individuals, which
have already ceased to yield seeds; so that this acme of sterility, when
the germen alone is 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,
etc., 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, Gartner, 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 Gartner 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. Gartner 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 Gartner 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 Gartner: 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 Gartner'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.
Kolreuter, 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. Gartner,
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 Gartner 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. Gartner 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: Gartner 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, Gartner 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. Gartner 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
Kolreuter.
Such alone are the unimportant differences which Gartner 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 Gartner 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.
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