On the Origin of Species by Means of Natural Selection (1859)
by Charles Darwin
CHAPTER VIII.
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.
HE 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 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 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 Hon. and Rev. W.
Herbert. He is as emphatic in his conclusion that some hybrids are
perfectly fertile—as fertile as the pure parent-species—as
are Kölreuter and Gärtner that some degree of sterility
between distinct species is a universal law of nature. He
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, &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 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 its 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, &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.
As Gärtner found that there was sometimes an innate difference in
different individuals of the same two species in crossing; so
Sagaret believes this to be the case with different individuals of the
same two species in being grafted together. As in reciprocal crosses,
the facility of effecting an union is often very far from equal, so it
sometimes is in grafting; the common gooseberry, for instance, cannot
be grafted on the currant, whereas the currant will take, though with
difficulty, on the gooseberry.
We have seen that the sterility of hybrids, which have their
reproductive organs in an imperfect condition, is a very different
case from the difficulty of uniting two pure species, which have their
reproductive organs perfect; yet these two distinct cases run to a
certain extent parallel. Something analogous occurs in grafting; for
Thouin found that three species of Robinia, which seeded freely on
their own roots, and which could be grafted with no great difficulty
on another species, when thus grafted were rendered barren. On the
other hand, certain species of Sorbus, when grafted on other species,
yielded twice as much fruit as when on their own roots. We are
reminded by this latter fact of the extraordinary case of Hippeastrum,
Lobelia, &c., which seeded much more freely when fertilised with
the pollen of distinct species, than when self-fertilised with their
own pollen.
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, 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.
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, &c., from one soil or
climate to another, and back again. During the convalescence of
animals, we plainly see that great benefit is derived from almost any
change in the habits of life. Again, both with plants and animals,
there is abundant evidence, that a cross between very distinct
individuals of the same species, that is between members of different
strains or sub-breeds, gives vigour and fertility to the offspring. I
believe, indeed, from the facts alluded to in our fourth chapter, that
a certain amount of crossing is indispensable even with
hermaphrodites; and that close interbreeding continued during several
generations between the nearest relations, especially if these be kept
under the same conditions of life, always induces weakness and
sterility in the progeny.
Hence it seems that, on the one hand, slight changes in the conditions
of life benefit all organic beings, and on the other hand, that slight
crosses, that is crosses between the males and females of the same
species which have varied and become slightly different, give vigour
and fertility to the offspring. But we have seen that greater changes,
or changes of a particular nature, often render organic beings in some
degree sterile; and that greater crosses, that is crosses between
males and females which have become widely or specifically different,
produce hybrids which are generally sterile in some degree. I cannot
persuade myself that this parallelism is an accident or an
illusion. Both series of facts seem to be connected together by some
common but unknown bond, which is essentially related to the principle
of life.
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
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 difference 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 organization, 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 fertilized 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
fertilization 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
fertilized 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 harmonizes 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.
[ Charles Darwin,
On
the Origin Of Species: A Facsimile of the First Edition,
Cambridge, Massachusetts: Harvard University Press, 1964, pp. 245-278. ]
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