Distinction between the sterility of first crosses
and of hybrids -- Sterility various in degree, not universal, affected by
close interbreeding, removed by domestication -- Laws governing the sterility
of hybrids -- Sterility not a special endowment, but incidental on other
differences -- Causes of the sterility of first crosses and of hybrids --
Parallelism between the effects of changed conditions of life and crossing --
Fertility of varieties when crossed and of their mongrel offspring not
universal -- Hybrids and mongrels compared independently of their fertility --
Summary.
2 The view generally entertained by naturalists is that
species, when intercrossed, have been specially endowed with the quality of
sterility, in order to prevent the confusion of all organic forms. This view
certainly seems at first probable, for species within the same country could
hardly have kept distinct had they been capable of crossing freely. The
importance of the fact that hybrids are very generally sterile, has, I think,
been much underrated by some late writers. On the theory of natural selection
the case is especially important, inasmuch as the sterility of hybrids could
not possibly be of any advantage to them, and therefore could not have been
acquired by the continued preservation of successive profitable degrees of
sterility. I hope, however, to be able to show that sterility is not a
specially acquired or endowed quality, but is incidental on other acquired
differences.
3 In treating this subject, two classes of facts, to a
large extent fundamentally different, have generally been confounded together;
namely, the sterility of two species when first crossed, and the sterility of
the hybrids produced from them.
4 Pure species have of course their organs of
reproduction in a perfect condition, yet when intercrossed they produce either
few or no offspring. Hybrids, on the other hand, have their reproductive
organs functionally impotent, as may be clearly seen in the state of the male
element in both plants and animals; though the organs themselves are perfect
in structure, as far as the microscope reveals. In the first case the two
sexual elements which go to form the embryo are perfect; in the second case
they are either not at all developed, or are imperfectly developed. This
distinction is important, when the cause of the sterility, which is common to
the two cases, has to be considered. The distinction has probably been slurred
over, owing to the sterility in both cases being looked on as a special
endowment, beyond the province of our reasoning powers.
5 The fertility of varieties, that is of the forms
known or believed to have descended from common parents, when intercrossed,
and likewise the fertility of their mongrel offspring, is, on my theory, of
equal importance with the sterility of species; for it seems to make a broad
and clear distinction between varieties and species.
6 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 crossed and by their hybrid offspring, with the average
number produced by both pure parent-species in a state of nature. But a
serious cause of error seems to me to be here introduced: a plant to be
hybridised must be castrated, and, what is often more important, must be
secluded in order to prevent pollen being brought to it by insects from other
plants. Nearly all the plants experimentised on by Gartner were potted, and
apparently were kept in a chamber in his house. That these processes are often
injurious to the fertility of a plant cannot be doubted; for 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 during several years repeatedly crossed the
primrose and cowslip, which we have such good reason to believe to be
varieties, and only once or twice succeeded in getting fertile seed; as he
found the common red and blue pimpernels (Anagallis arvensis and coerulea),
which the best botanists rank as varieties, absolutely sterile together; and
as he came to the same conclusion in several other analogous cases; it seems
to me that we may well be permitted to doubt whether many other species are
really so sterile, when intercrossed, as Gartner believes.
7 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,
should have arrived at diametrically opposite conclusions in regard to the
very same species. It is also most instructive to compare--but I have not
space here to enter on details--the evidence advanced by our best botanists on
the question whether certain doubtful forms should be ranked as species or
varieties, with the evidence from fertility adduced by different hybridisers,
or by the same author, from experiments made during different years. It can
thus be shown that neither sterility nor fertility affords any clear
distinction between species and varieties; but that the evidence from this
source graduates away, and is doubtful in the same degree as is the evidence
derived from other constitutional and structural differences.
8 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 increased, but generally greatly decreased. I do not doubt that this is
usually the case, and that the fertility often suddenly decreases in the first
few generations. Nevertheless I believe that in all these experiments the
fertility has been diminished by an independent cause, namely, from close
interbreeding. I have collected so large a body of facts, showing that close
interbreeding lessens fertility, and, on the other hand, that an occasional
cross with a distinct individual or variety increases fertility, that I cannot
doubt the correctness of this almost universal belief amongst breeders.
Hybrids are seldom raised by experimentalists in great numbers; and as the
parent-species, or other allied hybrids, generally grow in the same garden,
the visits of insects must be carefully prevented during the flowering season:
hence hybrids will generally be fertilised during each generation by their own
individual pollen; and I am convinced that this would be injurious to their
fertility, already lessened by their hybrid origin. I am strengthened in this
conviction by a remarkable statement repeatedly made by 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 of
manipulation, sometimes decidedly increases, and goes on increasing. Now, in
artificial fertilisation pollen is as often taken by chance (as I know from my
own experience) from the anthers of another flower, as from the anthers of the
flower itself which is to be fertilised; so that a cross between two flowers,
though probably on the same plant, would be thus effected. Moreover, whenever
complicated experiments are in progress, so careful an observer as Gartner
would have castrated his hybrids, and this would have insured in each
generation a cross with the pollen from a distinct flower, either from the
same plant or from another plant of the same hybrid nature. And thus, the
strange fact of the increase of fertility in the successive generations of
artificially fertilised hybrids may, I believe, be accounted for by close
interbreeding having been avoided.
9 Now let us turn to the results arrived at by the
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
experimentised 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 hothouses at his command. Of his many
important statements I will here give only a single one as an example, namely,
that 'every ovule in a pod of Crinum capense fertilised by C. revolutum
produced a plant, which (he says) I never saw to occur in a case of its
natural fecundation.' So that we here have perfect, or even more than commonly
perfect, fertility in a first cross between two distinct species.
10 This case of the Crinum leads me to refer to a most
singular fact, namely, that there are individual plants, as with certain
species of Lobelia, and with all the species of the genus Hippeastrum, which
can be far more easily fertilised by the pollen of another and distinct
species, than by their own pollen. For these plants have been found to yield
seed to the pollen of a distinct species, though quite sterile with their own
pollen, notwithstanding that their own pollen was found to be perfectly good,
for it fertilised distinct species. So that certain individual plants and all
the individuals of certain species can actually be hybridised much more
readily than they can be self-fertilised! For instance, a bulb of Hippeastrum
aulicum produced four flowers; three were fertilised by Herbert with their own
pollen, and the fourth was subsequently fertilised by the pollen of a compound
hybrid descended from three other and distinct species: the result was that
'the ovaries of the three first flowers soon ceased to grow, and after a few
days perished entirely, whereas the pod impregnated by the pollen of the
hybrid made vigorous growth and rapid progress to maturity, and bore good
seed, which vegetated freely.' In a letter to me, in 1839, Mr. Herbert told me
that he had then tried the experiment during five years, and he continued to
try it during several subsequent years, and always with the same result. This
result has, also, been confirmed by other observers in the case of Hippeastrum
with its sub-genera, and in the case of some other genera, as Lobelia,
Passiflora and Verbascum. Although the plants in these experiments appeared
perfectly healthy, and although both the ovules and pollen of the same flower
were perfectly good with respect to other species, yet as they were
functionally imperfect in their mutual self-action, we must infer that the
plants were in an unnatural state. Nevertheless these facts show on what
slight and mysterious causes the lesser or greater fertility of species when
crossed, in comparison with the same species when self-fertilised, sometimes
depends.
11 The practical experiments of horticulturists, though
not made with scientific precision, deserve some notice. It is notorious in
how complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, &c., have been crossed, yet many of these hybrids
seed freely. For instance, Herbert asserts that a hybrid from Calceolaria
integrifolia and plantaginea, species most widely dissimilar in general habit,
'reproduced itself as perfectly as if it had been a natural species from the
mountains of Chile.' I have taken some pains to ascertain the degree of
fertility of some of the complex crosses of Rhododendrons, and I am assured
that many of them are perfectly fertile. Mr. C. Noble, for instance, informs
me that he raises stocks for grafting from a hybrid between Rhod. Ponticum and
Catawbiense, and that this hybrid 'seeds as freely as it is possible to
imagine.' Had hybrids, when fairly treated, gone on decreasing in fertility in
each successive generation, as Gartner believes to be the case, the fact would
have been notorious to nurserymen. Horticulturists raise large beds of the
same hybrids, and such alone are fairly treated, for by insect agency the
several individuals of the same hybrid variety are allowed to freely cross
with each other, and the injurious influence of close interbreeding is thus
prevented. Any one may readily convince himself of the efficiency of
insect-agency by examining the flowers of the more sterile kinds of hybrid
rhododendrons, which produce no pollen, for he will find on their stigmas
plenty of pollen brought from other flowers.
12 In regard to animals, much fewer experiments have
been carefully tried than with plants. If our systematic arrangements can be
trusted, that is if the genera of animals are as distinct from each other, as
are the genera of plants, then we may infer that animals more widely separated
in the scale of nature can be more easily crossed than in the case of plants;
but the hybrids themselves are, I think, more sterile. I doubt whether any
case of a perfectly fertile hybrid animal can be considered as thoroughly well
authenticated. It should, however, be borne in mind that, owing to few animals
breeding freely under confinement, few experiments have been fairly tried: for
instance, the canary-bird has been crossed with nine other finches, but as not
one of these nine species breeds freely in confinement, we have no right to
expect that the first crosses between them and the canary, or that their
hybrids, should be perfectly fertile. Again, with respect to the fertility in
successive generations of the more fertile hybrid animals, I hardly know of an
instance in which two families of the same hybrid have been raised at the same
time from different parents, so as to avoid the ill effects of close
interbreeding. On the contrary, brothers and sisters have usually been crossed
in each successive generation, in opposition to the constantly repeated
admonition of every breeder. And in this case, it is not at all surprising
that the inherent sterility in the hybrids should have gone on increasing. If
we were to act thus, and pair brothers and sisters in the case of any pure
animal, which from any cause had the least tendency to sterility, the breed
would assuredly be lost in a very few generations.
13 Although I do not know of any thoroughly
well-authenticated cases of perfectly fertile hybrid animals, I have some
reason to believe that the hybrids from Cervulus vaginalis and Reevesii, and
from Phasianus colchicus with P. torquatus and with P. versicolor are
perfectly fertile. 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 Capt. Hutton, that whole flocks of these crossed geese are kept in
various parts of the country; and as they are kept for profit, where neither
pure parent-species exists, they must certainly be highly fertile.
14 A doctrine which originated with Pallas, has been
largely accepted by modern naturalists; namely, that most of our domestic
animals have descended from two or more aboriginal species, since commingled
by intercrossing. On this view, the aboriginal species must either at first
have produced quite fertile hybrids, or the hybrids must have become in
subsequent generations quite fertile under domestication. This latter
alternative seems to me the most probable, and I am inclined to believe in its
truth, although it rests on no direct evidence. I believe, for instance, that
our dogs have descended from several wild stocks; yet, with perhaps the
exception of certain indigenous domestic dogs of South America, all are quite
fertile together; and analogy makes me greatly doubt, whether the several
aboriginal species would at first have freely bred together and have produced
quite fertile hybrids. So again there is reason to believe that our European
and the humped Indian cattle are quite fertile together; but from facts
communicated to me by Mr. Blyth, I think they must be considered as distinct
species. On this view of the origin of many of our domestic animals, we must
either give up the belief of the almost universal sterility of distinct
species of animals when crossed; or we must look at sterility, not as an
indelible characteristic, but as one capable of being removed by
domestication.
15 Finally, looking to all the ascertained facts on the
intercrossing of plants and animals, it may be concluded that some degree of
sterility, both in first crosses and in hybrids, is an extremely general
result; but that it cannot, under our present state of knowledge, be
considered as absolutely universal.
16 Laws governing the Sterility of first Crosses and of
Hybrids. -- We will now consider a little more in detail the circumstances and
rules governing the sterility of first crosses and of hybrids. Our chief
object will be to see whether or not the rules indicate that species have
specially been endowed with this quality, in order to prevent their crossing
and blending together in utter confusion. The following rules and conclusions
are chiefly drawn up from Gartner's admirable work on the hybridisation of
plants. I have taken much pains to ascertain how far the rules apply to
animals, and considering how scanty our knowledge is in regard to hybrid
animals, I have been surprised to find how generally the same rules apply to
both kingdoms.
17 It has been already remarked, that the degree of
fertility, both of first crosses and of hybrids, graduates from zero to
perfect fertility. It is surprising in how many curious ways this gradation
can be shown to exist; but only the barest outline of the facts can here be
given. When pollen from a plant of one family is placed on the stigma of a
plant of a distinct family, it exerts no more influence than so much inorganic
dust. From this absolute zero of fertility, the pollen of different species of
the same genus applied to the stigma of some one species, yields a perfect
gradation in the number of seeds produced, up to nearly complete or even quite
complete fertility; and, as we have seen, in certain abnormal cases, even to
an excess of fertility, beyond that which the plant's own pollen will produce.
So in hybrids themselves, there are some which never have produced, and
probably never would produce, even with the pollen of either pure parent, a
single fertile seed: but in some of these cases a first trace of fertility may
be detected, by the pollen of one of the pure parent-species causing the
flower of the hybrid to wither earlier than it otherwise would have done; and
the early withering of the flower is well known to be a sign of incipient
fertilisation. From this extreme degree of sterility we have self-fertilised
hybrids producing a greater and greater number of seeds up to perfect
fertility.
18 Hybrids from two species which are very difficult to
cross, and which rarely produce any offspring, are generally very sterile; but
the parallelism between the difficulty of making a first cross, and the
sterility of the hybrids thus produced--two classes of facts which are
generally confounded together--is by no means strict. There are many cases, in
which two pure species can be united with unusual facility, and produce
numerous hybrid-offspring, yet these hybrids are remarkably sterile. On the
other hand, there are species which can be crossed very rarely, or with
extreme difficulty, but the hybrids, when at last produced, are very fertile.
Even within the limits of the same genus, for instance in Dianthus, these two
opposite cases occur.
19 The fertility, both of first crosses and of hybrids,
is more easily affected by unfavourable conditions, than is the fertility of
pure species. But the degree of fertility is likewise innately variable; for
it is not always the same when the same two species are crossed under the same
circumstances, but depends in part upon the constitution of the individuals
which happen to have been chosen for the experiment. So it is with hybrids,
for their degree of fertility is often found to differ greatly in the several
individuals raised from seed out of the same capsule and exposed to exactly
the same conditions.
20 By the term systematic affinity is meant, the
resemblance between species in structure and in constitution, more especially
in the structure of parts which are of high physiological importance and which
differ little in the allied species. Now the fertility of first crosses
between species, and of the hybrids produced from them, is largely governed by
their systematic affinity. This is clearly shown by hybrids never having been
raised between species ranked by systematists in distinct families; and on the
other hand, by very closely allied species generally uniting with facility.
But the correspondence between systematic affinity and the facility of
crossing is by no means strict. A multitude of cases could be given of very
closely allied species which will not unite, or only with extreme difficulty;
and on the other hand of very distinct species which unite with the utmost
facility. In the same family there may be a genus, as Dianthus, in which very
many species can most readily be crossed; and another genus, as Silene, in
which the most persevering efforts have failed to produce between extremely
close species a single hybrid. Even within the limits of the same genus, we
meet with this same difference; for instance, the many species of Nicotiana
have been more largely crossed than the species of almost any other genus; but
Gartner found that N. acuminata, which is not a particularly distinct species,
obstinately failed to fertilise, or to be fertilised by, no less than eight
other species of Nicotiana. Very many analogous facts could be given.
21 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.
22 By a reciprocal cross between two species, I mean the
case, for instance, of a stallion-horse being first crossed with a female-ass,
and then a male-ass with a mare: these two species may then be said to have
been reciprocally crossed. There is often the widest possible difference in
the facility of making reciprocal crosses. Such cases are highly important,
for they prove that the capacity in any two species to cross is often
completely independent of their systematic affinity, or of any recognisable
difference in their whole organisation. On the other hand, these cases clearly
show that the capacity for crossing is connected with constitutional
differences imperceptible by us, and confined to the reproductive system. This
difference in the result of reciprocal crosses between the same two species
was long ago observed by Kolreuter. To give an instance: Mirabilis jalappa can
easily be fertilised by the pollen of M. longiflora, and the hybrids thus
produced are sufficiently fertile; but Kolreuter tried more than two hundred
times, during eight following years, to fertilise reciprocally M. longiflora
with the pollen of M. jalappa, and utterly failed. Several other equally
striking cases could be given. Thuret has observed the same fact with certain
sea-weeds or Fuci. Gartner, moreover, found that this difference of facility
in making reciprocal crosses is extremely common in a lesser degree. He has
observed it even between forms so closely related (as Matthiola annua and
glabra) that many botanists rank them only as varieties. It is also a
remarkable fact, that hybrids raised from reciprocal crosses, though of course
compounded of the very same two species, the one species having first been
used as the father and then as the mother, generally differ in fertility in a
small, and occasionally in a high degree.
23 Several other singular rules could be given from
Gartner: for instance, some species have a remarkable power of crossing with
other species; other species of the same genus have a remarkable power of
impressing their likeness on their hybrid offspring; but these two powers do
not at all necessarily go together. There are certain hybrids which instead of
having, as is usual, an intermediate character between their two parents,
always closely resemble one of them; and such hybrids, though externally so
like one of their pure parent-species, are with rare exceptions extremely
sterile. So again amongst hybrids which are usually intermediate in structure
between their parents, exceptional and abnormal individuals sometimes are
born, which closely resemble one of their pure parents; and these hybrids are
almost always utterly sterile, even when the other hybrids raised from seed
from the same capsule have a considerable degree of fertility. These facts
show how completely fertility in the hybrid is independent of its external
resemblance to either pure parent.
24 Considering the several rules now given, which govern
the fertility of first crosses and of hybrids, we see that when forms, which
must be considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to fertility under certain
conditions in excess. That their fertility, besides being eminently
susceptible to favourable and unfavourable conditions, is innately variable.
That it is by no means always the same in degree in the first cross and in the
hybrids produced from this cross. That the fertility of hybrids is not related
to the degree in which they resemble in external appearance either parent. And
lastly, that the facility of making a first cross between any two species is
not always governed by their systematic affinity or degree of resemblance to
each other. This latter statement is clearly proved by reciprocal crosses
between the same two species, for according as the one species or the other is
used as the father or the mother, there is generally some difference, and
occasionally the widest possible difference, in the facility of effecting an
union. The hybrids, moreover, produced from reciprocal crosses often differ in
fertility.
25 Now do these complex and singular rules indicate that
species have been endowed with sterility simply to prevent their becoming
confounded in nature? I think not. For why should the sterility be so
extremely different in degree, when various species are crossed, all of which
we must suppose it would be equally important to keep from blending together?
Why should the degree of sterility be innately variable in the individuals of
the same species? Why should some species cross with facility, and yet produce
very sterile hybrids; and other species cross with extreme difficulty, and yet
produce fairly fertile hybrids? Why should there often be so great a
difference in the result of a reciprocal cross between the same two species?
Why, it may even be asked, has the production of hybrids been permitted? to
grant to species the special power of producing hybrids, and then to stop
their further propagation by different degrees of sterility, not strictly
related to the facility of the first union between their parents, seems to be
a strange arrangement.
26 The foregoing rules and facts, on the other hand,
appear to me clearly to indicate that the sterility both of first crosses and
of hybrids is simply incidental or dependent on unknown differences, chiefly
in the reproductive systems, of the species which are crossed. The differences
being of so peculiar and limited a nature, that, in reciprocal crosses between
two species the male sexual element of the one will often freely act on the
female sexual element of the other, but not in a reversed direction. It will
be advisable to explain a little more fully by an example what I mean by
sterility being incidental on other differences, and not a specially endowed
quality. As the capacity of one plant to be grafted or budded on another is so
entirely unimportant for its welfare in a state of nature, I presume that no
one will suppose that this capacity is a specially endowed quality, but will
admit that it is incidental on differences in the laws of growth of the two
plants. We can sometimes see the reason why one tree will not take on another,
from differences in their rate of growth, in the hardness of their wood, in
the period of the flow or nature of their sap, &c.; but in a multitude of
cases we can assign no reason whatever. Great diversity in the size of two
plants, one being woody and the other herbaceous, one being evergreen and the
other deciduous, and adaptation to widely different climates, does not always
prevent the two grafting together. As in hybridisation, so with grafting, the
capacity is limited by systematic affinity, for no one has been able to graft
trees together belonging to quite distinct families; and, on the other hand,
closely allied species, and varieties of the same species, can usually, but
not invariably, be grafted with ease. But this capacity, as in hybridisation,
is by no means absolutely governed by systematic affinity. Although many
distinct genera within the same family have been grafted together, in other
cases species of the same genus will not take on each other. The pear can be
grafted far more readily on the quince, which is ranked as a distinct genus,
than on the apple, which is a member of the same genus. Even different
varieties of the pear take with different degrees of facility on the quince;
so do different varieties of the apricot and peach on certain varieties of the
plum.
27 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.
28 We have seen that the sterility of hybrids, which
have their reproductive organs in an imperfect condition, is a very different
case from the difficulty of uniting two pure species, which have their
reproductive organs perfect; yet these two distinct cases run to a certain
extent parallel. Something analogous occurs in grafting; for Thouin found that
three species of Robinia, which seeded freely on their own roots, and which
could be grafted with no great difficulty on another species, when thus
grafted were rendered barren. On the other hand, certain species of Sorbus,
when grafted on other species, yielded twice as much fruit as when on their
own roots. We are reminded by this latter fact of the extraordinary case of
Hippeastrum, Lobelia, &c., which seeded much more freely when fertilised
with the pollen of distinct species, than when self-fertilised with their own
pollen.
29 We thus see, that although there is a clear and
fundamental difference between the mere adhesion of grafted stocks, and the
union of the male and female elements in the act of reproduction, yet that
there is a rude degree of parallelism in the results of grafting and of
crossing distinct species. And as we must look at the curious and complex laws
governing the facility with which trees can be grafted on each other as
incidental on unknown differences in their vegetative systems, so I believe
that the still more complex laws governing the facility of first crosses, are
incidental on unknown differences, chiefly in their reproductive systems.
These differences, in both cases, follow to a certain extent, as might have
been expected, systematic affinity, by which every kind of resemblance and
dissimilarity between organic beings is attempted to be expressed. The facts
by no means seem to me to indicate that the greater or lesser difficulty of
either grafting or crossing together various species has been a special
endowment; although in the case of crossing, the difficulty is as important
for the endurance and stability of specific forms, as in the case of grafting
it is unimportant for their welfare.
30 Causes of the Sterility of first Crosses and of
Hybrids. -- We may now look a little closer at the probable causes of the
sterility of first crosses and of hybrids. These two cases are fundamentally
different, for, as just remarked, in the union of two pure species the male
and female sexual elements are perfect, whereas in hybrids they are imperfect.
Even in first crosses, the greater or lesser difficulty in effecting a union
apparently depends on several distinct causes. There must sometimes be a
physical impossibility in the male element reaching the ovule, as would be the
case with a plant having a pistil too long for the pollen-tubes to reach the
ovarium. It has also been observed that when pollen of one species is placed
on the stigma of a distantly allied species, though the pollen-tubes protrude,
they do not penetrate the stigmatic surface. Again, the male element may reach
the female element, but be incapable of causing an embryo to be developed, as
seems to have been the case with some of Thuret's experiments on Fuci. No
explanation can be given of these facts, any more than why certain trees
cannot be grafted on others. Lastly, an embryo may be developed, and then
perish at an early period. This latter alternative has not been sufficiently
attended to; but I believe, from observations communicated to me by Mr.
Hewitt, who has had great experience in hybridising gallinaceous birds, that
the early death of the embryo is a very frequent cause of sterility in first
crosses. I was at first very unwilling to believe in this view; as hybrids,
when once born, are generally healthy and long-lived, as we see in the case of
the common mule. Hybrids, however, are differently circumstanced before and
after birth: when born and living in a country where their two parents can
live, they are generally placed under suitable conditions of life. But a
hybrid partakes of only half of the nature and constitution of its mother, and
therefore before birth, as long as it is nourished within its mother's womb or
within the egg or seed produced by the mother, it may be exposed to conditions
in some degree unsuitable, and consequently be liable to perish at an early
period; more especially as all very young beings seem eminently sensitive to
injurious or unnatural conditions of life.
31 In regard to the sterility of hybrids, in which the
sexual elements are imperfectly developed, the case is very different. I have
more than once alluded to a large body of facts, which I have collected,
showing that when animals and plants are removed from their natural
conditions, they are 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, or 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 plant seed freely under culture; nor can he tell, till he
tries, whether any two species of a genus will produce more or less sterile
hybrids. Lastly, when organic beings are placed during several generations
under conditions not natural to them, they are extremely liable to vary, which
is due, as I believe, to their reproductive systems having been specially
affected, though in a lesser degree than when sterility ensues. So it is with
hybrids, for hybrids in successive generations are eminently liable to vary,
as every experimentalist has observed.
32 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 by sterility in a very similar manner. In the one
case, the conditions of life have been disturbed, though often in so slight a
degree as to be inappreciable by us; in the other case, or that of hybrids,the
external conditions have remained the same, but the organisation has been
disturbed by two different structures and constitutions having been blended
into one. For it is scarcely possible that two organisations should be
compounded into one, without some disturbance occurring in the development, or
periodical action, or mutual relation of the different parts and organs one to
another, or to the conditions of life. When hybrids are able to breed inter
se, they transmit to their offspring from generation to generation the same
compounded organisation, and hence we need not be surprised that their
sterility, though in some degree variable, rarely diminishes.
33 It must, however, be confessed that we cannot
understand, excepting on vague hypotheses, several facts with respect to the
sterility of hybrids; for instance, the unequal fertility of hybrids produced
from reciprocal crosses; or the increased sterility in those hybrids which
occasionally and exceptionally resemble closely either pure parent. Nor do I
pretend that the foregoing remarks go to the root of the matter: no
explanation is offered why an organism, when placed under unnatural
conditions, is rendered sterile. All that I have attempted to show, is that in
two cases, in some respects allied, sterility is the common result,--in the
one case from the conditions of life having been disturbed, in the other case
from the organisation having been disturbed by two organisations having been
compounded into one.
34 It may seem fanciful, but I suspect that a similar
parallelism extends to an allied yet very different class of facts. It is an
old and almost universal belief, founded, I think, on a considerable body of
evidence, that slight changes in the conditions of life are beneficial to all
living things. We see this acted on by farmers and gardeners in their frequent
exchanges of seed, tubers, &c., from one soil or climate to another, and
back again. During the convalescence of animals, we plainly see that great
benefit is derived from almost any change in the habits of life. Again, both
with plants and animals, there is abundant evidence, that a cross between very
distinct individuals of the same species, that is between members of different
strains or sub-breeds, gives vigour and fertility to the offspring. I believe,
indeed, from the facts alluded to in our fourth chapter, that a certain amount
of crossing is indispensable even with hermaphrodites; and that close
interbreeding continued during several generations between the nearest
relations, especially if these be kept under the same conditions of life,
always induces weakness and sterility in the progeny.
35 Hence it seems that, on the one hand, slight changes
in the conditions of life benefit all organic beings, and on the other hand,
that slight crosses, that is crosses between the males and females of the same
species which have varied and become slightly different, give vigour and
fertility to the offspring. But we have seen that greater changes, or changes
of a particular nature, often render organic beings in some degree sterile;
and that greater crosses, that is crosses between males and females which have
become widely or specifically different, produce hybrids which are generally
sterile in some degree. I cannot persuade myself that this parallelism is an
accident or an illusion. Both series of facts seem to be connected together by
some common but unknown bond, which is essentially related to the principle of
life.
36 Fertility of Varieties when crossed, and of their
Mongrel off-spring. -- It may be urged, as a most forcible argument, that
there must be some essential distinction between species and varieties, and
that there must be some error in all the foregoing remarks, inasmuch as
varieties, however much they may differ from each other in external
appearance, cross with perfect facility, and yield perfectly fertile
offspring. I fully admit that this is almost invariably the case. But if we
look to varieties produced under nature, we are immediately involved in
hopeless difficulties; for if two hitherto reputed varieties be found in any
degree sterile together, they are at once ranked by most naturalists as
species. For instance, the blue and red pimpernel, the primrose and cowslip,
which are considered by many of our best botanists as varieties, are said by
Gartner not to be quite fertile when crossed, and he consequently ranks them
as undoubted species. If we thus argue in a circle, the fertility of all
varieties produced under nature will assuredly have to be granted.
37 If we turn to varieties, produced, or supposed to
have been produced, under domestication, we are still involved in doubt. For
when it is stated, for instance, that the German Spitz dog unites more easily
than other dogs with foxes, or that certain South American indigenous domestic
dogs do not readily cross with European dogs, the explanation which will occur
to everyone, and probably the true one, is that these dogs have descended from
several aboriginally distinct species. Nevertheless the perfect fertility of
so many domestic varieties, differing widely from each other in appearance,
for instance of the pigeon or of the cabbage, is a remarkable fact; more
especially when we reflect how many species there are, which, though
resembling each other most closely, are utterly sterile when intercrossed.
Several considerations, however, render the fertility of domestic varieties
less remarkable than at first appears. It can, in the first place, be clearly
shown that mere external dissimilarity between two species does not determine
their greater or lesser degree of sterility when crossed; and we may apply the
same rule to domestic varieties. In the second place, some eminent naturalists
believe that a long course of domestication tends to eliminate sterility in
the successive generations of hybrids, which were at first only slightly
sterile; and if this be so, we surely ought not to expect to find sterility
both appearing and disappearing under nearly the same conditions of life.
Lastly, and this seems to me by far the most important consideration, new
races of animals and plants are produced under domestication by man's
methodical and unconscious power of selection, for his own use and pleasure:
he neither wishes to select, nor could select, slight differences in the
reproductive system, or other constitutional differences correlated with the
reproductive system. He supplies his several varieties with the same food;
treats them in nearly the same manner, and does not wish to alter their
general habits of life. Nature acts uniformly and slowly during vast periods
of time on the whole organisation, in any way which may be for each creature's
own good; and thus she may, either directly, or more probably indirectly,
through correlation, modify the reproductive system in the several descendants
from any one species. Seeing this difference in the process of selection, as
carried on by man and nature, we need not be surprised at some difference in
the result.
38 I have as yet spoken as if the varieties of the same
species were invariably fertile when intercrossed. But it seems to me
impossible to resist the evidence of the existence of a certain amount of
sterility in the few following cases, which I will briefly abstract. The
evidence is at least as good as that from which we believe in the sterility of
a multitude of species. The evidence is, also, derived from hostile witnesses,
who in all other cases consider fertility and sterility as safe criterions of
specific distinction. 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 with the
pollen of the other; but only a single head produced any seed, and this one
head produced only five grains. Manipulation in this case could not have been
injurious, as the plants have separated sexes. No one, I believe, has
suspected that these varieties of maize are distinct species; and it is
important to notice that the hybrid plants thus raised were themselves
perfectly fertile; so that even Gartner did not venture to consider the two
varieties as specifically distinct.
39 Girou de Buzareingues crossed three varieties of
gourd, which like the maize has separated sexes, and he asserts that their
mutual fertilisation is by so much the less easy as their differences are
greater. How far these experiments may be trusted, I know not; but the forms
experimentised on, are ranked by Sagaret, who mainly founds his classification
by the test of infertility, as varieties.
40 The following case is far more remarkable, and seems
at first quite incredible; but it is the result of an astonishing number of
experiments made during many years on nine species of Verbascum, by so good an
observer and so hostile a witness, as Gartner: namely, that yellow and white
varieties of the same species of Verbascum when intercrossed produce less
seed, than do either coloured varieties when fertilised with pollen from their
own coloured flowers. Moreover, he asserts that when yellow and white
varieties of one species are crossed with yellow and white varieties of a
distinct species, more seed is produced by the crosses between the same
coloured flowers, than between those which are differently coloured. Yet these
varieties of Verbascum present no other difference besides the mere colour of
the flower; and one variety can sometimes be raised from the seed of the
other.
41 From observations which I have made on certain
varieties of hollyhock, I am inclined to suspect that they present analogous
facts.
42 Kolreuter, whose accuracy has been confirmed by every
subsequent observer, has proved the remarkable fact, that one variety of the
common tobacco is more fertile, when crossed with a widely distinct species,
than are the other varieties. He experimentised on five forms, which are
commonly reputed to be varieties, and which he tested by the severest trial,
namely, by reciprocal crosses, and he found their mongrel offspring perfectly
fertile. But one of these five varieties, when used either as father or
mother, and crossed with the Nicotiana glutinosa, always yielded hybrids not
so sterile as those which were produced from the four other varieties when
crossed with N. glutinosa. Hence the reproductive system of this one variety
must have been in some manner and in some degree modified.
43 From these facts; from the great difficulty of
ascertaining the infertility of varieties in a state of nature, for a supposed
variety if infertile in any degree would generally be ranked as species; from
man selecting only external characters in the production of the most distinct
domestic varieties, and from not wishing or being able to produce recondite
and functional differences in the reproductive system; from these several
considerations and facts, I do not think that the very general fertility of
varieties can be proved to be of universal occurrence, or to form a
fundamental distinction between varieties and species. The general fertility
of varieties does not seem to me sufficient to overthrow the view which I have
taken with respect to the very general, but not invariable, sterility of first
crosses and of hybrids, namely, that it is not a special endowment, but is
incidental on slowly acquired modifications, more especially in the
reproductive systems of the forms which are crossed.
44 Hybrids and Mongrels compared, independently of their
fertility. -- Independently of the question of fertility, the offspring of
species when crossed and of varieties when crossed may be compared in several
other respects. Gartner, whose strong wish was to draw a marked line of
distinction between species and varieties, could find very few and, as it
seems to me, quite unimportant differences between the so-called hybrid
offspring of species, and the so-called mongrel offspring of varieties. And,
on the other hand, they agree most closely in very many important respects.
45 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 their offspring is notorious; but some few cases both of
hybrids and mongrels long retaining uniformity of character could be given.
The variability, however, in the successive generations of mongrels is,
perhaps, greater than in hybrids.
46 This greater variability of mongrels than of hybrids
does not seem to me at all surprising. For the parents of mongrels are
varieties, and mostly domestic varieties (very few experiments having been
tried on natural varieties), and this implies in most cases that there has
been recent variability; and therefore we might expect that such variability
would often continue and be super-added to that arising from the mere act of
crossing. The slight degree of variability in hybrids from the first cross or
in the first generation, in contrast with their extreme variability in the
succeeding generations, is a curious fact and deserves attention. For it bears
on and corroborates the view which I have taken on the cause of ordinary
variability; namely, that it is due to the reproductive system being eminently
sensitive to any change in the conditions of life, being thus often rendered
either impotent or at least incapable of its proper function of producing
offspring identical with the parent-form. Now hybrids in the first generation
are descended from species (excluding those long cultivated) which have not
had their reproductive systems in any way affected, and they are not variable;
but hybrids themselves have their reproductive systems seriously affected, and
their descendants are highly variable.
47 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. Gartner further insists that when any two species, although most
closely allied to each other, are crossed with a third species, the hybrids
are widely different from each other; whereas if two very distinct varieties
of one species are crossed with another species, the hybrids do not differ
much. But this conclusion, as far as I can make out, is founded on a single
experiment; and seems directly opposed to the results of several experiments
made by Kolreuter.
48 These alone are the unimportant differences, which
Gartner is able to point out, between hybrid and mongrel plants. On the other
hand, the resemblance in mongrels and in hybrids to their respective parents,
more especially in hybrids produced from nearly related species, follows
according to Gartner the same laws. When two species are crossed, one has
sometimes a prepotent power of impressing its likeness on the hybrid; and so I
believe it to be with varieties of plants. With animals one variety certainly
often has this prepotent power over another variety. Hybrid plants produced
from a reciprocal cross, generally resemble each other closely; and so it is
with mongrels from a reciprocal cross. Both hybrids and mongrels can be
reduced to either pure parent-form, by repeated crosses in successive
generations with either parent.
49 These several remarks are apparently applicable to
animals; but the subject is here excessively complicated, partly owing to the
existence of secondary sexual characters; but more especially owing to
prepotency in transmitting likeness running more strongly in one sex than in
the other, both when one species is crossed with another, and when one variety
is crossed with another variety. For instance, I think those authors are
right, who maintain that the ass has a prepotent power over the horse, so that
both the mule and the hinny more resemble the ass than the horse; but that the
prepotency runs more strongly in the male-ass than in the female, so that the
mule, which is the offspring of the male-ass and mare, is more like an ass,
than is the hinny, which is the offspring of the female-ass and stallion.
50 Much stress has been laid by some authors on the
supposed fact, that mongrel animals alone are born closely like one of their
parents; but it can be shown that this does sometimes occur with hybrids; yet
I grant much less frequently with hybrids than with mongrels. Looking to the
cases which I have collected of cross-bred animals closely resembling one
parent, the resemblances seem chiefly confined to characters almost monstrous
in their nature, and which have suddenly appeared--such as albinism, melanism,
deficiency of tail or horns, or additional fingers and toes; and do not relate
to characters which have been slowly acquired by selection. Consequently,
sudden reversions to the perfect character of either parent would be more
likely to occur with mongrels, which are descended from varieties often
suddenly produced and semi-monstrous in character, than with hybrids, which
are descended from species slowly and naturally produced. On the whole I
entirely agree with Dr. Prosper Lucas, who, after arranging an enormous body
of facts with respect to animals, comes to the conclusion, that the laws of
resemblance of the child to its parents are the same, whether the two parents
differ much or little from each other, namely in the union of individuals of
the same variety, or of different varieties, or of distinct species.
51 Laying aside 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.
52 Summary of Chapter -- First crosses between forms
sufficiently distinct to be ranked as species, and their hybrids, are very
generally, but not universally, sterile. The sterility is of all degrees, and
is often so slight that the two most careful experimentalists who have ever
lived, have come to diametrically opposite conclusions in ranking forms by
this test. The sterility is innately variable in individuals of the same
species, and is eminently susceptible of favourable and unfavourable
conditions. The degree of sterility does not strictly follow systematic
affinity, but is governed by several curious and complex laws. It is generally
different, and sometimes widely different, in reciprocal crosses between the
same two species. It is not always equal in degree in a first cross and in the
hybrid produced from this cross.
53 In the same manner as in grafting trees, the capacity
of one species or variety to take on another, is incidental on generally
unknown differences in their vegetative systems, so in crossing, the greater
or less facility of one species to unite with another, is incidental on
unknown differences in their reproductive systems. There is no more reason to
think that species have been specially endowed with various degrees of
sterility to prevent them crossing and blending in nature, than to think that
trees have been specially endowed with various and somewhat analogous degrees
of difficulty in being grafted together in order to prevent them becoming
inarched in our forests.
54 The sterility of first crosses between pure species,
which have their reproductive systems perfect, seems to depend on several
circumstances; in some cases largely on the early death of the embryo. The
sterility of hybrids, which have their reproductive systems imperfect, and
which have had this system and their whole organisation disturbed by being
compounded of two distinct species, seems closely allied to that sterility
which so frequently affects pure species, when their natural conditions of
life have been disturbed. This view is supported by a parallelism of another
kind;--namely, that the crossing of forms only slightly different is
favourable to the vigour and fertility of their offspring; and that slight
changes in the conditions of life are apparently favourable to the vigour and
fertility of all organic beings. It is not surprising that the degree of
difficulty in uniting two species, and the degree of sterility of their
hybrid-offspring should generally correspond, though due to distinct causes;
for both depend on the amount of difference of some kind between the species
which are crossed. Nor is it surprising that the facility of effecting a first
cross, the fertility of the hybrids produced, and the capacity of being
grafted together--though this latter capacity evidently depends on widely
different circumstances--should all run, to a certain extent, parallel with
the systematic affinity of the forms which are subjected to experiment; for
systematic affinity attempts to express all kinds of resemblance between all
species.
55 First crosses between forms known to be varieties, or
sufficiently alike to be considered as varieties, and their mongrel offspring,
are very generally, but not quite universally, fertile. Nor is this nearly
general and perfect fertility surprising, when we remember how liable we are
to argue in a circle with respect to varieties in a state of nature; and when
we remember that the greater number of varieties have been produced under
domestication by the selection of mere external differences, and not of
differences in the reproductive system. In all other respects, excluding
fertility, there is a close general resemblance between hybrids and mongrels.
Finally, then, the facts briefly given in this chapter do not seem to me
opposed to, but even rather to support the view, that there is no fundamental
distinction between species and varieties.