HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids.
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication. Laws governing the sterility
of hybrids. Sterility not a special endowment, but incidental on other
differences. Causes of the sterility of first crosses and of hybrids.
Parallelism between the effects of changed conditions of life and
crossing. Fertility of varieties when crossed and of their mongrel
offspring not universal. Hybrids and mongrels compared independently of
their fertility. Summary.
The view generally entertained by naturalists is that species, when
intercrossed, have been specially endowed with the quality of
sterility, in order to prevent the confusion of all organic forms. This
view certainly seems at first probable, for species within the same
country could hardly have kept distinct had they been capable of
crossing freely. The importance of the fact that hybrids are very
generally sterile, has, I think, been much underrated by some late
writers. On the theory of natural selection the case is especially
important, inasmuch as the sterility of hybrids could not possibly be
of any advantage to them, and therefore could not have been acquired by
the continued preservation of successive profitable degrees of
sterility. I hope, however, to be able to show that sterility is not a
specially acquired or endowed quality, but is incidental on other
acquired differences.
In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded together;
namely, the sterility of two
species when first crossed, and the sterility of the hybrids produced
from them.
Pure species have of course their organs of reproduction in a perfect
condition, yet when intercrossed they produce either few or no
offspring. Hybrids, on the other hand, have their reproductive organs
functionally impotent, as may be clearly seen in the state of the male
element in both plants and animals; though the organs themselves are
perfect in structure, as far as the microscope reveals. In the first
case the two sexual elements which go to form the embryo are perfect;
in the second case they are either not at all developed, or are
imperfectly developed. This distinction is important, when the cause of
the sterility, which is common to the two cases, has to be considered.
The distinction has probably been slurred over, owing to the sterility
in both cases being looked on as a special endowment, beyond the
province of our reasoning powers.
The fertility of varieties, that is of the forms known or believed to
have descended from common parents, when intercrossed, and likewise the
fertility of their mongrel offspring, is, on my theory, of equal
importance with the sterility of species; for it seems to make a broad
and clear distinction between varieties and species.
First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of
those two conscientious and admirable observers, Kölreuter and Gärtner,
who almost devoted their lives to this subject, without being deeply
impressed with the high generality of some degree of sterility.
Kölreuter makes the rule universal; but then he cuts the knot, for in
ten cases in which he found two forms, considered by most authors as
distinct species, quite fertile together, he
unhesitatingly ranks them as varieties. Gärtner, also, makes the rule
equally universal; and he disputes the entire fertility of Kölreuter’s
ten cases. But in these and in many other cases, Gärtner is obliged
carefully to count the seeds, in order to show that there is any degree
of sterility. He always compares the maximum number of seeds produced
by two species when crossed and by their hybrid offspring, with the
average number produced by both pure parent-species in a state of
nature. But a serious cause of error seems to me to be here introduced:
a plant to be hybridised must be castrated, and, what is often more
important, must be secluded in order to prevent pollen being brought to
it by insects from other plants. Nearly all the plants experimentised
on by Gärtner were potted, and apparently were kept in a chamber in his
house. That these processes are often injurious to the fertility of a
plant cannot be doubted; for Gärtner gives in his table about a score
of cases of plants which he castrated, and artificially fertilised with
their own pollen, and (excluding all cases such as the Leguminosæ, in
which there is an acknowledged difficulty in the manipulation) half of
these twenty plants had their fertility in some degree impaired.
Moreover, as Gärtner during several years repeatedly crossed the
primrose and cowslip, which we have such good reason to believe to be
varieties, and only once or twice succeeded in getting fertile seed; as
he found the common red and blue pimpernels (Anagallis arvensis and
coerulea), which the best botanists rank as varieties, absolutely
sterile together; and as he came to the same conclusion in several
other analogous cases; it seems to me that we may well be permitted to
doubt whether many other species are really so sterile, when
intercrossed, as Gärtner believes.
It is certain, on the one hand, that the sterility of various species
when crossed is so different in degree and graduates away so
insensibly, and, on the other hand, that the fertility of pure species
is so easily affected by various circumstances, that for all practical
purposes it is most difficult to say where perfect fertility ends and
sterility begins. I think no better evidence of this can be required
than that the two most experienced observers who have ever lived,
namely, Kölreuter and Gärtner, should have arrived at diametrically
opposite conclusions in regard to the very same species. It is also
most instructive to compare—but I have not space here to enter on
details—the evidence advanced by our best botanists on the question
whether certain doubtful forms should be ranked as species or
varieties, with the evidence from fertility adduced by different
hybridisers, or by the same author, from experiments made during
different years. It can thus be shown that neither sterility nor
fertility affords any clear distinction between species and varieties;
but that the evidence from this source graduates away, and is doubtful
in the same degree as is the evidence derived from other constitutional
and structural differences.
In regard to the sterility of hybrids in successive generations; though
Gärtner was enabled to rear some hybrids, carefully guarding them from
a cross with either pure parent, for six or seven, and in one case for
ten generations, yet he asserts positively that their fertility never
increased, but generally greatly decreased. I do not doubt that this is
usually the case, and that the fertility often suddenly decreases in
the first few generations. Nevertheless I believe that in all these
experiments the fertility has been diminished by an independent cause,
namely, from close interbreeding. I have collected so large a body of
facts, showing
that close interbreeding lessens fertility, and, on the other hand,
that an occasional cross with a distinct individual or variety
increases fertility, that I cannot doubt the correctness of this almost
universal belief amongst breeders. Hybrids are seldom raised by
experimentalists in great numbers; and as the parent-species, or other
allied hybrids, generally grow in the same garden, the visits of
insects must be carefully prevented during the flowering season: hence
hybrids will generally be fertilised during each generation by their
own individual pollen; and I am convinced that this would be injurious
to their fertility, already lessened by their hybrid origin. I am
strengthened in this conviction by a remarkable statement repeatedly
made by Gärtner, namely, that if even the less fertile hybrids be
artificially fertilised with hybrid pollen of the same kind, their
fertility, notwithstanding the frequent ill effects of manipulation,
sometimes decidedly increases, and goes on increasing. Now, in
artificial fertilisation pollen is as often taken by chance (as I know
from my own experience) from the anthers of another flower, as from the
anthers of the flower itself which is to be fertilised; so that a cross
between two flowers, though probably on the same plant, would be thus
effected. Moreover, whenever complicated experiments are in progress,
so careful an observer as Gärtner would have castrated his hybrids, and
this would have insured in each generation a cross with 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.
Now let us turn to the results arrived at by the third most experienced
hybridiser, namely, the Honourable and
Reverend W. Herbert. He is as emphatic in his conclusion that some
hybrids are perfectly fertile—as fertile as the pure parent-species—as
are Kölreuter and Gärtner that some degree of sterility between
distinct species is a universal law of nature. He experimentised on
some of the very same species as did Gärtner. The difference in their
results may, I think, be in part accounted for by Herbert’s great
horticultural skill, and by his having hothouses at his command. Of his
many important statements I will here give only a single one as an
example, namely, that “every ovule in a pod of Crinum capense
fertilised by C. revolutum produced a plant, which (he says) I never
saw to occur in a case of its natural fecundation.” So that we here
have perfect, or even more than commonly perfect, fertility in a first
cross between two distinct species.
This case of the Crinum leads me to refer to a most singular fact,
namely, that there are individual plants, 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.
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, “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 Rhododendron Ponticum and Catawbiense, and that this hybrid
“seeds as freely as it is possible to imagine.” Had hybrids, when
fairly treated, gone on decreasing in fertility in each successive
generation, as Gärtner believes to be the case, the fact would have
been notorious to nurserymen. Horticulturists raise large beds of the
same hybrids, and such alone are fairly treated, for by insect agency
the several individuals of the same hybrid variety are allowed to
freely cross with each other, and the injurious influence of close
interbreeding is thus prevented. Any one may readily convince himself
of the efficiency of insect-agency by examining the flowers of the more
sterile kinds of hybrid rhododendrons, which produce no pollen, for he
will find on their stigmas plenty of pollen brought from other flowers.
In regard to animals, much fewer experiments have been carefully tried
than with plants. If our systematic arrangements can be trusted, that
is if the genera of animals are as distinct from each other, as are the
genera of plants, then we may infer that animals more widely separated
in the scale of nature can be more easily crossed than in the case of
plants; but the hybrids themselves are, I think, more sterile. I doubt
whether any case of a perfectly fertile hybrid animal can be considered
as thoroughly well authenticated. It should, however, be borne in mind
that, owing to few animals breeding freely under confinement, few
experiments have been fairly tried: for instance, the canary-bird has
been crossed with nine other finches, but as not one of these nine
species breeds freely in confinement, we have no right to expect that
the first crosses between them and the canary, or that their hybrids,
should be perfectly fertile. Again, with respect to the fertility in
successive generations of the more fertile
hybrid animals, I hardly know of an instance in which two families of
the same hybrid have been raised at the same time from different
parents, so as to avoid the ill effects of close interbreeding. On the
contrary, brothers and sisters have usually been crossed in each
successive generation, in opposition to the constantly repeated
admonition of every breeder. And in this case, it is not at all
surprising that the inherent sterility in the hybrids should have gone
on increasing. If we were to act thus, and pair brothers and sisters in
the case of any pure animal, which from any cause had the least
tendency to sterility, the breed would assuredly be lost in a very few
generations.
Although I do not know of any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have some reason to believe that
the hybrids from Cervulus vaginalis and Reevesii, and from Phasianus
colchicus with P. torquatus and with P. versicolor are perfectly
fertile. 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.
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.
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.
_Laws governing the Sterility of first Crosses and of Hybrids_.—We will
now consider a little more in detail the
circumstances and rules governing the sterility of first crosses and of
hybrids. Our chief object will be to see whether or not the rules
indicate that species have specially been endowed with this quality, in
order to prevent their crossing and blending together in utter
confusion. The following rules and conclusions are chiefly drawn up
from Gärtner’s admirable work on the hybridisation of plants. I have
taken much pains to ascertain how far the rules apply to animals, and
considering how scanty our knowledge is in regard to hybrid animals, I
have been surprised to find how generally the same rules apply to both
kingdoms.
It has been already remarked, that the degree of fertility, both of
first crosses and of hybrids, graduates from zero to perfect fertility.
It is surprising in how many curious ways this gradation can be shown
to exist; but only the barest outline of the facts can here be given.
When pollen from a plant of one family is placed on the stigma of a
plant of a distinct family, it exerts no more influence than so much
inorganic dust. From this absolute zero of fertility, the pollen of
different species of the same genus applied to the stigma of some one
species, yields a perfect gradation in the number of seeds produced, up
to nearly complete or even quite complete fertility; and, as we have
seen, in certain abnormal cases, even to an excess of fertility, beyond
that which the plant’s own pollen will produce. So in hybrids
themselves, there are some which never have produced, and probably
never would produce, even with the pollen of either pure parent, a
single fertile seed: but in some of these cases a first trace of
fertility may be detected, by the pollen of one of the pure
parent-species causing the flower of the hybrid to wither earlier than
it otherwise would have done; and the early withering of the flower is
well known to be a sign
of incipient fertilisation. From this extreme degree of sterility we
have self-fertilised hybrids producing a greater and greater number of
seeds up to perfect fertility.
Hybrids from two species which are very difficult to cross, and which
rarely produce any offspring, are generally very sterile; but the
parallelism between the difficulty of making a first cross, and the
sterility of the hybrids thus produced—two classes of facts which are
generally confounded together—is by no means strict. There are many
cases, in which two pure species can be united with unusual facility,
and produce numerous hybrid-offspring, yet these hybrids are remarkably
sterile. On the other hand, there are species which can be crossed very
rarely, or with extreme difficulty, but the hybrids, when at last
produced, are very fertile. Even within the limits of the same genus,
for instance in Dianthus, these two opposite cases occur.
The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is the fertility of pure
species. But the degree of fertility is likewise innately variable; for
it is not always the same when the same two species are crossed under
the same circumstances, but depends in part upon the constitution of
the individuals which happen to have been chosen for the experiment. So
it is with hybrids, for their degree of fertility is often found to
differ greatly in the several individuals raised from seed out of the
same capsule and exposed to exactly the same conditions.
By the term systematic affinity is meant, the resemblance between
species in structure and in constitution, more especially in the
structure of parts which are of high physiological importance and which
differ little in the allied species. Now the fertility of first crosses
between species, and of the hybrids produced from them, is largely
governed by their systematic affinity. This is clearly shown by hybrids
never having been raised between species ranked by systematists in
distinct families; and on the other hand, by very closely allied
species generally uniting with facility. But the correspondence between
systematic affinity and the facility of crossing is by no means strict.
A multitude of cases could be given of very closely allied species
which will not unite, or only with extreme difficulty; and on the other
hand of very distinct species which unite with the utmost facility. In
the same family there may be a genus, as Dianthus, in which very many
species can most readily be crossed; and another genus, as Silene, in
which the most persevering efforts have failed to produce between
extremely close species a single hybrid. Even within the limits of the
same genus, we meet with this same difference; for instance, the many
species of Nicotiana have been more largely crossed than the species of
almost any other genus; but Gärtner found that N. acuminata, which is
not a particularly distinct species, obstinately failed to fertilise,
or to be fertilised by, no less than eight other species of Nicotiana.
Very many analogous facts could be given.
No one has been able to point out what kind, or what amount, of
difference in any recognisable character is sufficient to prevent two
species crossing. It can be shown that plants most widely different in
habit and general appearance, and having strongly marked differences in
every part of the flower, even in the pollen, in the fruit, and in the
cotyledons, can be crossed. Annual and perennial plants, deciduous and
evergreen trees, plants inhabiting different stations and fitted for
extremely different climates, can often be crossed with ease.
By a reciprocal cross between two species, I mean the case, for
instance, of a stallion-horse being first crossed with a female-ass,
and then a male-ass with a mare: these two species may then be said to
have been reciprocally crossed. There is often the widest possible
difference in the facility of making reciprocal crosses. Such cases are
highly important, for they prove that the capacity in any two species
to cross is often completely independent of their systematic affinity,
or of any recognisable difference in their whole organisation. On the
other hand, these cases clearly show that the capacity for crossing is
connected with constitutional differences imperceptible by us, and
confined to the reproductive system. This difference in the result of
reciprocal crosses between the same two species was long ago observed
by Kölreuter. To give an instance: Mirabilis jalappa can easily be
fertilised by the pollen of M. longiflora, and the hybrids thus
produced are sufficiently fertile; but Kölreuter tried more than two
hundred times, during eight following years, to fertilise reciprocally
M. longiflora with the pollen of M. jalappa, and utterly failed.
Several other equally striking cases could be given. Thuret has
observed the same fact with certain sea-weeds or Fuci. Gärtner,
moreover, found that this difference of facility in making reciprocal
crosses is extremely common in a lesser degree. He has observed it even
between forms so closely related (as Matthiola annua and glabra) that
many botanists rank them only as varieties. It is also a remarkable
fact, that hybrids raised from reciprocal crosses, though of course
compounded of the very same two species, the one species having first
been used as the father and then as the mother, generally differ in
fertility in a small, and occasionally in a high degree.
Several other singular rules could be given from
Gärtner: for instance, some species have a remarkable power of crossing
with other species; other species of the same genus have a remarkable
power of impressing their likeness on their hybrid offspring; but these
two powers do not at all necessarily go together. There are certain
hybrids which instead of having, as is usual, an intermediate character
between their two parents, always closely resemble one of them; and
such hybrids, though externally so like one of their pure
parent-species, are with rare exceptions extremely sterile. So again
amongst hybrids which are usually intermediate in structure between
their parents, exceptional and abnormal individuals sometimes are born,
which closely resemble one of their pure parents; and these hybrids are
almost always utterly sterile, even when the other hybrids raised from
seed from the same capsule have a considerable degree of fertility.
These facts show how completely fertility in the hybrid is independent
of its external resemblance to either pure parent.
Considering the several rules now given, which govern the fertility of
first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to fertility under
certain conditions in excess. That their fertility, besides being
eminently susceptible to favourable and unfavourable conditions, is
innately variable. That it is by no means always the same in degree in
the first cross and in the hybrids produced from this cross. That the
fertility of hybrids is not related to the degree in which they
resemble in external appearance either parent. And lastly, that the
facility of making a first cross between any two species is not always
governed by their systematic affinity or
degree of resemblance to each other. This latter statement is clearly
proved by reciprocal crosses between the same two species, for
according as the one species or the other is used as the father or the
mother, there is generally some difference, and occasionally the widest
possible difference, in the facility of effecting an union. The
hybrids, moreover, produced from reciprocal crosses often differ in
fertility.
Now do these complex and singular rules indicate that species have been
endowed with sterility simply to prevent their becoming confounded in
nature? I think not. For why should the sterility be so extremely
different in degree, when various species are crossed, all of which we
must suppose it would be equally important to keep from blending
together? Why should the degree of sterility be innately variable in
the individuals of the same species? Why should some species cross with
facility, and yet produce very sterile hybrids; and other species cross
with extreme difficulty, and yet produce fairly fertile hybrids? Why
should there often be so great a difference in the result of a
reciprocal cross between the same two species? Why, it may even be
asked, has the production of hybrids been permitted? to grant to
species the special power of producing hybrids, and then to stop their
further propagation by different degrees of sterility, not strictly
related to the facility of the first union between their parents, seems
to be a strange arrangement.
The foregoing rules and facts, on the other hand, appear to me clearly
to indicate that the sterility both of first crosses and of hybrids is
simply incidental or dependent on unknown differences, chiefly in the
reproductive systems, of the species which are crossed. The differences
being of so peculiar and limited a nature,
that, in reciprocal crosses between two species the male sexual element
of the one will often freely act on the female sexual element of the
other, but not in a reversed direction. It will be advisable to explain
a little more fully by an example what I mean by sterility being
incidental on other differences, and not a specially endowed quality.
As the capacity of one plant to be grafted or budded on another is so
entirely unimportant for its welfare in a state of nature, I presume
that no one will suppose that this capacity is a _specially_ endowed
quality, but will admit that it is incidental on differences in the
laws of growth of the two plants. We can sometimes see the reason why
one tree will not take on another, from differences in their rate of
growth, in the hardness of their wood, in the period of the flow or
nature of their sap, 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 trees together belonging to quite distinct
families; and, on the other hand, closely allied species, and varieties
of the same species, can usually, but not invariably, be grafted with
ease. But this capacity, as in hybridisation, is by no means absolutely
governed by systematic affinity. Although many distinct genera within
the same family have been grafted together, in other cases species of
the same genus will not take on each other. The pear can be grafted far
more readily on the quince, which is ranked as a distinct genus, than
on the apple, which is a member of the same genus. Even different
varieties of the pear take
with different degrees of facility on the quince; so do different
varieties of the apricot and peach on certain varieties of the plum.
As Gärtner found that there was sometimes an innate difference in
different _individuals_ of the same two species in crossing; so Sagaret
believes this to be the case with different individuals of the same two
species in being grafted together. As in reciprocal crosses, the
facility of effecting an union is often very far from equal, so it
sometimes is in grafting; the common gooseberry, for instance, cannot
be grafted on the currant, whereas the currant will take, though with
difficulty, on the gooseberry.
We have seen that the sterility of hybrids, which have their
reproductive organs in an imperfect condition, is a very different case
from the difficulty of uniting two pure species, which have their
reproductive organs perfect; yet these two distinct cases run to a
certain extent parallel. Something analogous occurs in grafting; for
Thouin found that three species of Robinia, which seeded freely on
their own roots, and which could be grafted with no great difficulty on
another species, when thus grafted were rendered barren. On the other
hand, certain species of Sorbus, when grafted on other species, yielded
twice as much fruit as when on their own roots. We are reminded by this
latter fact of the extraordinary case of Hippeastrum, Lobelia, etc.,
which seeded much more freely when fertilised with the pollen of
distinct species, than when self-fertilised with their own pollen.
We thus see, that although there is a clear and fundamental difference
between the mere adhesion of grafted stocks, and the union of the male
and female elements in the act of reproduction, yet that there is a
rude degree of parallelism in the results of grafting and
of crossing distinct species. And as we must look at the curious and
complex laws governing the facility with which trees can be grafted on
each other as incidental on unknown differences in their vegetative
systems, so I believe that the still more complex laws governing the
facility of first crosses, are incidental on unknown differences,
chiefly in their reproductive systems. These differences, in both
cases, follow to a certain extent, as might have been expected,
systematic affinity, by which every kind of resemblance and
dissimilarity between organic beings is attempted to be expressed. The
facts by no means seem to me to indicate that the greater or lesser
difficulty of either grafting or crossing together various species has
been a special endowment; although in the case of crossing, the
difficulty is as important for the endurance and stability of specific
forms, as in the case of grafting it is unimportant for their welfare.
_Causes of the Sterility of first Crosses and of Hybrids_.—We may now
look a little closer at the probable causes of the sterility of first
crosses and of hybrids. These two cases are fundamentally different,
for, as just remarked, in the union of two pure species the male and
female sexual elements are perfect, whereas in hybrids they are
imperfect. Even in first crosses, the greater or lesser difficulty in
effecting a union apparently depends on several distinct causes. There
must sometimes be a physical impossibility in the male element reaching
the ovule, as would be the case with a plant having a pistil too long
for the pollen-tubes to reach the ovarium. It has also been observed
that when pollen of one species is placed on the stigma of a distantly
allied species, though the pollen-tubes protrude, they do not penetrate
the stigmatic surface. Again, the
male element may reach the female element, but be incapable of causing
an embryo to be developed, as seems to have been the case with some of
Thuret’s experiments on Fuci. No explanation can be given of these
facts, any more than why certain trees cannot be grafted on others.
Lastly, an embryo may be developed, and then perish at an early period.
This latter alternative has not been sufficiently attended to; but I
believe, from observations communicated to me by Mr. Hewitt, who has
had great experience in hybridising gallinaceous birds, that the early
death of the embryo is a very frequent cause of sterility in first
crosses. I was at first very unwilling to believe in this view; as
hybrids, when once born, are generally healthy and long-lived, as we
see in the case of the common mule. Hybrids, however, are differently
circumstanced before and after birth: when born and living in a country
where their two parents can live, they are generally placed under
suitable conditions of life. But a hybrid partakes of only half of the
nature and constitution of its mother, and therefore before birth, as
long as it is nourished within its mother’s womb or within the egg or
seed produced by the mother, it may be exposed to conditions in some
degree unsuitable, and consequently be liable to perish at an early
period; more especially as all very young beings seem eminently
sensitive to injurious or unnatural conditions of life.
In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is very different. I have more than
once alluded to a large body of facts, which I have collected, showing
that when animals and plants are removed from their natural conditions,
they are 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 plant
seed freely under culture; nor can he tell, till he tries, whether any
two species of a genus will produce more or less sterile hybrids.
Lastly, when organic beings are placed during several generations under
conditions not natural to them, they are extremely liable to vary,
which is due, as I believe, to their reproductive systems having been
specially affected, though in a lesser degree than when sterility
ensues. So it is with hybrids, for hybrids in successive generations
are eminently liable to vary, as every experimentalist has observed.
Thus we see that when organic beings are placed under new and unnatural
conditions, and when hybrids are produced by the unnatural crossing of
two species, the reproductive system, independently of the general
state of health, is affected by sterility in a very similar manner. In
the one case, the conditions of life have been disturbed, though often
in so slight a degree as to
be inappreciable by us; in the other case, or that of hybrids, the
external conditions have remained the same, but the organisation has
been disturbed by two different structures and constitutions having
been blended into one. For it is scarcely possible that two
organisations should be compounded into one, without some disturbance
occurring in the development, or periodical action, or mutual relation
of the different parts and organs one to another, or to the conditions
of life. When hybrids are able to breed _inter se_, they transmit to
their offspring from generation to generation the same compounded
organisation, and hence we need not be surprised that their sterility,
though in some degree variable, rarely diminishes.
It must, however, be confessed that we cannot understand, excepting on
vague hypotheses, several facts with respect to the sterility of
hybrids; for instance, the unequal fertility of hybrids produced from
reciprocal crosses; or the increased sterility in those hybrids which
occasionally and exceptionally resemble closely either pure parent. Nor
do I pretend that the foregoing remarks go to the root of the matter:
no explanation is offered why an organism, when placed under unnatural
conditions, is rendered sterile. All that I have attempted to show, is
that in two cases, in some respects allied, sterility is the common
result,—in the one case from the conditions of life having been
disturbed, in the other case from the organisation having been
disturbed by two organisations having been compounded into one.
It may seem fanciful, but I suspect that a similar 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,
etc., from one soil or climate to another, and back again. During the
convalescence of animals, we plainly see that great benefit is derived
from almost any change in the habits of life. Again, both with plants
and animals, there is abundant evidence, that a cross between very
distinct individuals of the same species, that is between members of
different strains or sub-breeds, gives vigour and fertility to the
offspring. I believe, indeed, from the facts alluded to in our fourth
chapter, that a certain amount of crossing is indispensable even with
hermaphrodites; and that close interbreeding continued during several
generations between the nearest relations, especially if these be kept
under the same conditions of life, always induces weakness and
sterility in the progeny.
Hence it seems that, on the one hand, slight changes in the conditions
of life benefit all organic beings, and on the other hand, that slight
crosses, that is crosses between the males and females of the same
species which have varied and become slightly different, give vigour
and fertility to the offspring. But we have seen that greater changes,
or changes of a particular nature, often render organic beings in some
degree sterile; and that greater crosses, that is crosses between males
and females which have become widely or specifically different, produce
hybrids which are generally sterile in some degree. I cannot persuade
myself that this parallelism is an accident or an illusion. Both series
of facts seem to be connected together by some common but unknown bond,
which is essentially related to the principle of life.
_Fertility of Varieties when crossed, and of their Mongrel
offspring_.—It may be urged, as a most forcible argument,
that there must be some essential distinction between species and
varieties, and that there must be some error in all the foregoing
remarks, inasmuch as varieties, however much they may differ from each
other in external appearance, cross with perfect facility, and yield
perfectly fertile offspring. I fully admit that this is almost
invariably the case. But if we look to varieties produced under nature,
we are immediately involved in hopeless difficulties; for if two
hitherto reputed varieties be found in any degree sterile together,
they are at once ranked by most naturalists as species. For instance,
the blue and red pimpernel, the primrose and cowslip, which are
considered by many of our best botanists as varieties, are said by
Gärtner not to be quite fertile when crossed, and he consequently ranks
them as undoubted species. If we thus argue in a circle, the fertility
of all varieties produced under nature will assuredly have to be
granted.
If we turn to varieties, produced, or supposed to have been produced,
under domestication, we are still involved in doubt. For when it is
stated, for instance, that the German Spitz dog unites more easily than
other dogs with foxes, or that certain South American indigenous
domestic dogs do not readily cross with European dogs, the explanation
which will occur to 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.
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.
Gärtner kept during several years a dwarf kind of maize with yellow
seeds, and a tall variety with red seeds, growing near each other in
his garden; and although these plants have separated sexes, they never
naturally crossed. He then fertilised thirteen flowers of the one with
the pollen of the other; but only a single head produced any seed, and
this one head produced only five grains. Manipulation in this case
could not have been injurious, as the plants have separated sexes. No
one, I believe, has suspected that these varieties of maize are
distinct species; and it is important to notice that the hybrid plants
thus raised were themselves _perfectly_ fertile; so that even Gärtner
did not venture to consider the two varieties as specifically distinct.
Girou de Buzareingues crossed three varieties of gourd, which like the
maize has separated sexes, and he asserts that their mutual
fertilisation is by so much the less easy as their differences are
greater. How far these experiments may be trusted, I know not; but the
forms experimentised on, are ranked by Sagaret, who mainly founds his
classification by the test of infertility, as varieties.
The following case is far more remarkable, and seems at first quite
incredible; but it is the result of an astonishing number of
experiments made during many years on nine species of Verbascum, by so
good an observer and so hostile a witness, as Gärtner: namely, that
yellow and white varieties of the same species of Verbascum when
intercrossed produce less seed, than do either coloured varieties when
fertilised with pollen from their own coloured flowers. Moreover, he
asserts that when
yellow and white varieties of one species are crossed with yellow and
white varieties of a _distinct_ species, more seed is produced by the
crosses between the 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.
From observations which I have made on certain varieties of hollyhock,
I am inclined to suspect that they present analogous facts.
Kölreuter, whose accuracy has been confirmed by every subsequent
observer, has proved the remarkable fact, that one variety of the
common tobacco is more fertile, when crossed with a widely distinct
species, than are the other varieties. He experimentised on five forms,
which are commonly reputed to be varieties, and which he tested by the
severest trial, namely, by reciprocal crosses, and he found their
mongrel offspring perfectly fertile. But one of these five varieties,
when used either as father or mother, and crossed with the Nicotiana
glutinosa, always yielded hybrids not so sterile as those which were
produced from the four other varieties when crossed with N. glutinosa.
Hence the reproductive system of this one variety must have been in
some manner and in some degree modified.
From these facts; from the great difficulty of ascertaining the
infertility of varieties in a state of nature, for a supposed variety
if infertile in any degree would generally be ranked as species; from
man selecting only external characters in the production of the most
distinct domestic varieties, and from not wishing or being able to
produce recondite and functional differences in the reproductive
system; from these several considerations and facts, I do not think
that the very general
fertility of varieties can be proved to be of universal occurrence, or
to form a fundamental distinction between varieties and species. The
general fertility of varieties does not seem to me sufficient to
overthrow the view which I have taken with respect to the very general,
but not invariable, sterility of first crosses and of hybrids, namely,
that it is not a special endowment, but is incidental on slowly
acquired modifications, more especially in the reproductive systems of
the forms which are crossed.
_Hybrids and Mongrels compared, independently of their
fertility_.—Independently of the question of fertility, the offspring
of species when crossed and of varieties when crossed may be compared
in several other respects. Gärtner, whose strong wish was to draw a
marked line of distinction between species and varieties, could find
very few and, as it seems to me, quite unimportant differences between
the so-called hybrid offspring of species, and the so-called mongrel
offspring of varieties. And, on the other hand, they agree most closely
in very many important respects.
I shall here discuss this subject with extreme brevity. The most
important distinction is, that in the first generation mongrels are
more variable than hybrids; but Gärtner admits that hybrids from
species which have long been cultivated are often variable in the first
generation; and I have myself seen striking instances of this fact.
Gärtner further admits that hybrids between very closely allied species
are more variable than those from very distinct species; and this shows
that the difference in the degree of variability graduates away. When
mongrels and the more fertile hybrids are propagated for several
generations an extreme amount of variability in their offspring is
notorious;
but some few cases both of hybrids and mongrels long retaining
uniformity of character could be given. The variability, however, in
the successive generations of mongrels is, perhaps, greater than in
hybrids.
This greater variability of mongrels than of hybrids does not seem to
me at all surprising. For the parents of mongrels are varieties, and
mostly domestic varieties (very few experiments having been tried on
natural varieties), and this implies in most cases that there has been
recent variability; and therefore we might expect that such variability
would often continue and be 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.
But to return to our comparison of mongrels and hybrids: Gärtner states
that mongrels are more liable than hybrids to revert to either
parent-form; but this, if it be true, is certainly only a difference in
degree. Gärtner further insists that when any two species, although
most closely allied to each other, are
crossed with a third species, the hybrids are widely different from
each other; whereas if two very distinct varieties of one species are
crossed with another species, the hybrids do not differ much. But this
conclusion, as far as I can make out, is founded on a single
experiment; and seems directly opposed to the results of several
experiments made by Kölreuter.
These alone are the unimportant differences, which Gärtner is able to
point out, between hybrid and mongrel plants. On the other hand, the
resemblance in mongrels and in hybrids to their respective parents,
more especially in hybrids produced from nearly related species,
follows according to Gärtner the same laws. When two species are
crossed, one has sometimes a prepotent power of impressing its likeness
on the hybrid; and so I believe it to be with varieties of plants. With
animals one variety certainly often has this prepotent power over
another variety. Hybrid plants produced from a reciprocal cross,
generally resemble each other closely; and so it is with mongrels from
a reciprocal cross. Both hybrids and mongrels can be reduced to either
pure parent-form, by repeated crosses in successive generations with
either parent.
These several remarks are apparently applicable to animals; but the
subject is here excessively complicated, partly owing to the existence
of secondary sexual characters; but more especially owing to prepotency
in transmitting likeness running more strongly in one sex than in the
other, both when one species is crossed with another, and when one
variety is crossed with another variety. For instance, I think those
authors are right, who maintain that the ass has a prepotent power over
the horse, so that both the mule and the hinny more resemble the ass
than the horse; but that the prepotency runs more strongly in the
male-ass than in
the female, so that the mule, which is the offspring of the male-ass
and mare, is more like an ass, than is the hinny, which is the
offspring of the female-ass and stallion.
Much stress has been laid by some authors on the supposed fact, that
mongrel animals alone are born closely like one of their parents; but
it can be shown that this does sometimes occur with hybrids; yet I
grant much less frequently with hybrids than with mongrels. Looking to
the cases which I have collected of cross-bred animals closely
resembling one parent, the resemblances seem chiefly confined to
characters almost monstrous in their nature, and which have suddenly
appeared—such as albinism, melanism, deficiency of tail or horns, or
additional fingers and toes; and do not relate to characters which have
been slowly acquired by selection. Consequently, sudden reversions to
the perfect character of either parent would be more likely to occur
with mongrels, which are descended from varieties often suddenly
produced and semi-monstrous in character, than with hybrids, which are
descended from species slowly and naturally produced. On the whole I
entirely agree with Dr. Prosper Lucas, who, after arranging an enormous
body of facts with respect to animals, comes to the conclusion, that
the laws of resemblance of the child to its parents are the same,
whether the two parents differ much or little from each other, namely
in the union of individuals of the same variety, or of different
varieties, or of distinct species.
Laying aside the question of fertility and sterility, in all other
respects there seems to be a general and close similarity in the
offspring of crossed species, and of crossed varieties. If we look at
species as having been specially created, and at varieties as having
been produced by secondary laws, this similarity would be an
astonishing fact. But it harmonises perfectly with the view that there
is no essential distinction between species and varieties.
_Summary of Chapter_.—First crosses between forms sufficiently distinct
to be ranked as species, and their hybrids, are very generally, but not
universally, sterile. The sterility is of all degrees, and is often so
slight that the two most careful experimentalists who have ever lived,
have come to diametrically opposite conclusions in ranking forms by
this test. The sterility is innately variable in individuals of the
same species, and is eminently susceptible of favourable and
unfavourable conditions. The degree of sterility does not strictly
follow systematic affinity, but is governed by several curious and
complex laws. It is generally different, and sometimes widely
different, in reciprocal crosses between the same two species. It is
not always equal in degree in a first cross and in the hybrid produced
from this cross.
In the same manner as in grafting trees, the capacity of one species or
variety to take on another, is incidental on generally unknown
differences in their vegetative systems, so in crossing, the greater or
less facility of one species to unite with another, is incidental on
unknown differences in their reproductive systems. There is no more
reason to think that species have been specially endowed with various
degrees of sterility to prevent them crossing and blending in nature,
than to think that trees have been specially endowed with various and
somewhat analogous degrees of difficulty in being grafted together in
order to prevent them becoming inarched in our forests.
The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems
to depend on several circumstances; in some cases largely on the early
death of the embryo. The sterility of hybrids, which have their
reproductive systems imperfect, and which have had this system and
their whole organisation disturbed by being compounded of two distinct
species, seems closely allied to that sterility which so frequently
affects pure species, when their natural conditions of life have been
disturbed. This view is supported by a parallelism of another
kind;—namely, that the crossing of forms only slightly different is
favourable to the vigour and fertility of their offspring; and that
slight changes in the conditions of life are apparently favourable to
the vigour and fertility of all organic beings. It is not surprising
that the degree of difficulty in uniting two species, and the degree of
sterility of their hybrid-offspring should generally correspond, though
due to distinct causes; for both depend on the amount of difference of
some kind between the species which are crossed. Nor is it surprising
that the facility of effecting a first cross, the fertility of the
hybrids produced, and the capacity of being grafted together—though
this latter capacity evidently depends on widely different
circumstances—should all run, to a certain extent, parallel with the
systematic affinity of the forms which are subjected to experiment; for
systematic affinity attempts to express all kinds of resemblance
between all species.
First crosses between forms known to be varieties, or sufficiently
alike to be considered as varieties, and their mongrel offspring, are
very generally, but not quite 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.