-- 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, Kolreuter and Gartner, who
almost devoted their lives to this subject, without being deeply impressed
with the high generality of some degree of sterility. Kolreuter makes the
rule universal; but then he cuts the knot, for in ten cases in which he
found two forms, considered by most authors as distinct species, quite
fertile together, he unhesitatingly ranks them as varieties. Gartner,
also, makes the rule equally universal; and he disputes the entire
fertility of Kolreuter's ten cases. But in these and in many other cases,
Gartner is obliged carefully to count the seeds, in order to show that
there is any degree of sterility. He always compares the maximum number of
seeds produced by two species when crossed and by their hybrid offspring,
with the average number produced by both pure parent-species in a state of
nature. But a serious cause of error seems to me to be here introduced: a
plant to be hybridised must be castrated, and, what is often more
important, must be secluded in order to prevent pollen being brought to it
by insects from other plants. Nearly all the plants experimentised on by
Gartner were potted, and apparently were kept in a chamber in his house.
That these processes are often injurious to the fertility of a plant cannot
be doubted; for Gartner gives in his table about a score of cases of plants
which he castrated, and artificially fertilised with their own pollen, and
(excluding all cases such as the Leguminosae, in which there is an
acknowledged difficulty in the manipulation) half of these twenty plants
had their fertility in some degree impaired. Moreover, as Gartner during
several years repeatedly crossed the primrose and cowslip, which we have
such good reason to believe to be varieties, and only once or twice
succeeded in getting fertile seed; as he found the common red and blue
pimpernels (Anagallis arvensis and coerulea), which the best botanists rank
as varieties, absolutely sterile together; and as he came to the same
conclusion in several other analogous cases; it seems to me that we may
well be permitted to doubt whether many other species are really so
sterile, when intercrossed, as Gartner believes.
It is certain, on the one hand, that the sterility of various species when
crossed is so different in degree and graduates away so insensibly, and, on
the other hand, that the fertility of pure species is so easily affected by
various circumstances, that for all practical purposes it is most difficult
to say where perfect fertility ends and sterility begins. I think no
better evidence of this can be required than that the two most experienced
observers who have ever lived, namely, Kolreuter and Gartner, should have
arrived at diametrically opposite conclusions in regard to the very same
species. It is also most instructive to compare--but I have not space here
to enter on details--the evidence advanced by our best botanists on the
question whether certain doubtful forms should be ranked as species or
varieties, with the evidence from fertility adduced by different
hybridisers, or by the same author, from experiments made during different
years. It can thus be shown that neither sterility nor fertility affords
any clear distinction between species and varieties; but that the evidence
from this source graduates away, and is doubtful in the same degree as is
the evidence derived from other constitutional and structural differences.
In regard to the sterility of hybrids in successive generations; though
Gartner was enabled to rear some hybrids, carefully guarding them from a
cross with either pure parent, for six or seven, and in one case for ten
generations, yet he asserts positively that their fertility never
increased, but generally greatly decreased. I do not doubt that this is
usually the case, and that the fertility often suddenly decreases in the
first few generations. Nevertheless I believe that in all these
experiments the fertility has been diminished by an independent cause,
namely, from close interbreeding. I have collected so large a body of
facts, showing that close interbreeding lessens fertility, and, on the
other hand, that an occasional cross with a distinct individual or variety
increases fertility, that I cannot doubt the correctness of this almost
universal belief amongst breeders. Hybrids are seldom raised by
experimentalists in great numbers; and as the parent-species, or other
allied hybrids, generally grow in the same garden, the visits of insects
must be carefully prevented during the flowering season: henc
e hybrids
will generally be fertilised during each generation by their own individual
pollen; and I am convinced that this would be injurious to their fertility,
already lessened by their hybrid origin. I am strengthened in this
conviction by a remarkable statement repeatedly made by Gartner, namely,
that if even the less fertile hybrids be artificially fertilised with
hybrid pollen of the same kind, their fertility, notwithstanding the
frequent ill effects of manipulation, sometimes decidedly increases, and
goes on increasing. Now, in artificial fertilisation pollen is as often
taken by chance (as I know from my own experience) from the anthers of
another flower, as from the anthers of the flower itself which is to be
fertilised; so that a cross between two flowers, though probably on the
same plant, would be thus effected. Moreover, whenever complicated
experiments are in progress, so careful an observer as Gartner would have
castrated his hybrids, and this would have insured in each generation a
cross with the pollen from a distinct flower, either from the same plant or
from another plant of the same hybrid nature. And thus, the strange fact
of the increase of fertility in the successive generations of artificially
fertilised hybrids may, I believe, be accounted for by close interbreeding
having been avoided.
Now let us turn to the results arrived at by the third most experienced
hybridiser, namely, the Hon. and Rev. W. Herbert. He is as emphatic in his
conclusion that some hybrids are perfectly fertile--as fertile as the pure
parent-species--as are Kolreuter and Gartner that some degree of sterility
between distinct species is a universal law of nature. He experimentised
on some of the very same species as did Gartner. The difference in their
results may, I think, be in part accounted for by Herbert's great
horticultural skill, and by his having hothouses at his command. Of his
many important statements I will here give only a single one as an example,
namely, that 'every ovule in a pod of Crinum capense fertilised by C.
revolutum produced a plant, which (he says) I never saw to occur in a case
of its natural fecundation.' So that we here have perfect, or even more
than commonly perfect, fertility in a first cross between two distinct
species.
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 Gartner
believes to be the case, the fact would have been notorious to nurserymen.
Horticulturists raise large beds of the same hybrids, and such alone are
fairly treated, for by insect agency the several individuals of the same
hybrid variety are allowed to freely cross with each other, and the
injurious influence of close interbreeding is thus prevented. Any one may
readily convince himself of the efficiency of insect-agency by examining
the flowers of the more sterile kinds of hybrid rhododendrons, which
produce no pollen, for he will find on their stigmas plenty of pollen
brought from other flowers.
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 kn
ow 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