CHAPTER IX - HYBRIDISM
THE view commonly entertained by naturalists is that species, when
intercrossed, have been specially endowed with sterility, in order
to prevent their confusion. This view certainly seems at first
highly probable, for species living together could hardly have been
kept distinct had they been capable of freely crossing. The subject is
in many ways important for us, more especially as the sterility of
species when first crossed, and that of their hybrid offspring, cannot
have been acquired, as I shall show, by the preservation of successive
profitable degrees of sterility. It is an incidental result of
differences in the reproductive systems of the parent-species.
In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded; namely, the
sterility of species when first crossed, and the sterility of the
hybrids produced from them.
Pure species have of course their organs of reproduction in a
perfect condition, yet when intercrossed they produce either few or no
offspring. Hybrids, on the other hand, have their reproductive
organs functionally impotent, as may be clearly seen in the state of
the male element in both plants and animals; though the formative
organs themselves are perfect in structure, as far as the microscope
reveals. In the first case the two sexual elements which go to form
the embryo are perfect; in the second case they are either not at
all developed, or are imperfectly developed. This distinction is
important, when the cause of the sterility, which is common to the two
cases, has to be considered. The distinction probably has been slurred
over, owing to the sterility in both cases being looked on as a
special endowment, beyond the province of our reasoning powers.
The fertility of varieties, that is of the forms known or believed
to be descended from common parents, when crossed, and likewise the
fertility of their mongrel offspring, is, with reference to my theory,
of equal importance with the sterility of species; for it seems to
make a broad and clear distinction between varieties and species.
Degrees of Sterility.- First, for the sterility of species when
crossed and of their hybrid offspring. It is impossible to study the
several memoirs and works of those two conscientious and admirable
observers, Kolreuter and Gartner, who almost devoted their lives to
this subject, without being deeply impressed with the high
generality of some degree of sterility. Kolreuter makes the rule
universal; but then he cuts the knot, for in ten cases in which he
found two forms, considered by most authors as distinct species, quite
fertile together, he unhesitatingly ranks them as varieties.
Gartner, also, makes the rule equally universal; and he disputes the
entire fertility of Kolreuter's ten cases. But in these and in many
other cases, Gartner is obliged carefully to count the seeds, in order
to show that there is any degree of sterility. He always compares
the maximum number of seeds produced by two species when first
crossed, and the maximum produced by their hybrid offspring, with
the average number produced by both pure parent-species in a state
of nature. But causes of serious error here intervene: a plant, to
be hybridised, must be castrated, and, what is often more important,
must be secluded in order to prevent pollen being brought to it by
insects from other plants. Nearly all the plants experimented on by
Gartner were potted, and were kept in a chamber in his house. That
these processes are often injurious to the fertility of a plant cannot
be doubted; for Gartner gives in his table about a score of cases of
plants which he castrated, and artificially fertilised with their
own pollen, and (excluding all cases such as the Leguminosae, in which
there is an acknowledged difficulty in the manipulation) half of these
twenty plants had their fertility in some degree impaired. Moreover,
as Gartner repeatedly crossed some forms, such as the common red and
blue pimpernels (Anagallis arvensis and caerulea), which the best
botanists rank as varieties, and found them absolutely sterile, we may
doubt whether many species are really so sterile, when intercrossed,
as he believed.
It is certain, on the one hand, that the sterility of various
species when crossed is so different in degree and graduates away so
insensibly, and, on the other hand, that the fertility of pure species
is so easily affected by various circumstances, that for all practical
purposes it is most difficult to say where perfect fertility ends
and sterility begins. I think no better evidence of this can be
required than that the two most experienced observers who have ever
lived, namely Kolreuter and Gartner, arrived at diametrically opposite
conclusions in regard to some of the very same forms. It is also
most instructive to compare- but I have not space here to enter on
details- the evidence advanced by our best botanists on the question
whether certain doubtful forms should be ranked as species or
varieties, with the evidence from fertility adduced by different
hybridisers, or by the same observer from experiments made during
different years. It can thus be shown that neither sterility nor
fertility affords any certain distinction between species and
varieties. The evidence from this source graduates away, and is
doubtful in the same degree as is the evidence derived from other
constitutional and structural differences.
In regard to the sterility of hybrids in successive generations:
though Gartner was enabled to rear some hybrids, carefully guarding
them from a cross with either pure parent, for six or seven, and in
one case for ten generations, yet he asserts positively that their
fertility never increases, but generally decreases greatly and
suddenly. With respect to this decrease, it may first be noticed
that when any deviation in structure or constitution is common to both
parents, this is often transmitted in an augmented degree to the
offspring; and both sexual elements in hybrid plants are already
affected in some degree. But I believe that their fertility has been
diminished in nearly all these cases by an independent cause,
namely, by too close interbreeding. I have made so many experiments
and collected so many facts, showing on the one hand that an
occasional cross with a distinct individual or variety increases the
vigour and fertility of the offspring, and on the other hand that very
close interbreeding lessens their vigour and fertility, that I
cannot doubt the correctness of this conclusion. Hybrids are seldom
raised by experimentalists in great numbers; and as the
parent-species, or other allied hybrids, generally grow in the same
garden, the visits of insects must be carefully prevented during the
flowering season: hence hybrids, if left to themselves, will generally
be fertilised during each generation by pollen from the same flower;
and this would probably be injurious to their fertility, already
lessened by their hybrid origin. I am strengthened in this
conviction by a remarkable statement repeatedly made by Gartner,
namely, that if even the less fertile hybrids be artificially
fertilised with hybrid pollen of the same kind, their fertility,
notwithstanding the frequent ill effects from manipulation,
sometimes decidedly increases, and goes on increasing. Now, in the
process of artificial fertilisation, pollen is as often taken by
chance (as I know from my own experience) from the anthers of
another flower, as from the anthers of the flower itself which is to
be fertilised; so that a cross between two flowers, though probably
often on the same plant, would be thus effected. Moreover, whenever
complicated experiments are in progress, so careful an observer as
Gartner would have castrated his hybrids, and this would have
ensured in each generation a cross with pollen from a distinct flower,
either from the same plant or from another plant of the same hybrid
nature. And thus, the strange fact of an increase of fertility in
the successive generations of artificially fertilised hybrids, in
contrast with those spontaneously self-fertilised, may, as I
believe, be accounted for by too close interbreeding having been
avoided.
Now let us turn to the results arrived at by a third most
experienced hybridiser, namely, the Hon. and Rev. W. Herbert. He is as
emphatic in his conclusion that some hybrids are perfectly fertile- as
fertile as the pure parent-species- as are Kolreuter and Gartner
that some degree of sterility between distinct species is a
universal law of nature. He experimented on some of the very same
species as did Gartner. The difference in their results may, I
think, be in part accounted for by Herbert's great horticultural
skill, and by his having hot-houses at his command. Of his many
important statements I will here give only a single one as an example,
namely, that "every ovule in a pod of Crinum capense fertilised by
C. revolutum produced a plant, which I never saw to occur in a case of
its natural fecundation." So that here we have perfect or even more
than commonly perfect fertility, in a first cross between two distinct
species.
This case of the Crinum leads me to refer to a singular fact,
namely, that individual plants of certain species of Lobelia,
Verbascum and Passiflora, can easily be fertilised by pollen from a
distinct species, but not by pollen from the same plant, though this
pollen can be proved to be perfectly sound by fertilising other plants
or species. In the genus Hippeastrum, in Corydalis as shown by
Professor Hildebrand, in various orchids as shown by Mr. Scott and
Fritz Muller, all the individuals are in this peculiar condition. So
that with some species, certain abnormal individuals, and in other
species all the individuals, can actually be hybridised much more
readily than they can be fertilised by pollen from the same individual
plant! To give one instance, a bulb of Hippeastrum aulicum produced
four flowers; three were fertilised by Herbert with their own
pollen, and the fourth was subsequently fertilised by the pollen of
a compound hybrid descended from three distinct species: the result
was that "the ovaries of the three first flowers soon ceased to
grow, and after a few days perished entirely, whereas the pod
impregnated by the pollen of the hybrid made vigorous growth and rapid
progress to maturity, and bore good seed, which vegetated freely." Mr.
Herbert tried similar experiments during many years, and always with
the same result. These cases serve to show on what slight and
mysterious causes the lesser or greater fertility of a species
sometimes depends.
The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, &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, "reproduces itself as perfectly as if
it had been a natural species from the mountains of Chili." I have
taken some pains to ascertain the degree of fertility of some of the
complex crosses of rhododendrons, and I am assured that many of them
are perfectly fertile. Mr. C. Noble, for instance, informs me that
he raises stocks for grafting from a hybrid between Rhod. ponticum and
catawbiense, and that this hybrid "seeds as freely as it is possible
to imagine." Had hybrids when fairly treated, always gone on
decreasing in fertility in each successive generation, as Gartner
believed to be the case, the fact would have been notorious to
nurserymen. Horticulturists raise large beds of the same hybrid, and
such alone are fairly treated, for by insect agency the several
individuals are allowed to cross freely with each other, and the
injurious influence of close interbreeding is thus prevented. Any
one may readily convince himself of the efficiency of insect-agency by
examining the flowers of the more sterile kinds of hybrid
rhododendrons, which produce no pollen for he will find on their
stigmas plenty of pollen brought from other flowers.
In regard to animals, much fewer experiments have been carefully
tried than with plants. If our systematic arrangements can be trusted,
that is, if the genera of animals are as distinct from each other as
are the genera of plants, then we may infer that animals more widely
distinct in the scale of nature can be crossed more easily than in the
case of plants; but the hybrids themselves are, I think, more sterile.
It should, however, be borne in mind that, owing to few animals
breeding freely under confinement, few experiments have been fairly
tried: for instance, the canary-bird has been crossed with nine
distinct species of finches, but, as not one of these breeds freely in
confinement, we have no right to expect that the first crosses between
them and the canary, or that their hybrids, should be perfectly
fertile. Again, with respect to the fertility in successive
generations of the more fertile hybrid animals, I hardly know of an
instance in which two families of the same hybrid have been raised
at the same time from different parents, so as to avoid the ill
effects of close interbreeding. On the contrary, brothers and
sisters have usually been crossed in each successive generation, in
opposition to the constantly repeated admonition of every breeder. And
in this case, it is not at all surprising that the inherent
sterility in the hybrids should have gone on increasing.
Although I know of hardly any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have reason to believe that the
hybrids from Cervulus vaginalis and reevesii, and from Phasianus
colchicus with P. torquatus, are perfectly fertile. M. Quatrefages
states that the hybrids from two moths (Bombyx cynthia and arrindia)
were proved in Paris to be fertile inter se for eight generations.
It has lately been asserted that two such distinct species as the hare
and rabbit, when they can be got to breed together, produce
offspring which are highly fertile when crossed with one of the
parent-species. The hybrids from the common and Chinese geese (A.
cygnoides), species which are so different that they are generally
ranked in distinct genera, have often bred in this country with either
pure parent, and in one single instance they have bred inter se.
This was effected by Mr. Eyton, who raised two hybrids from the same
parents, but from different hatches; and from these two birds he
raised no less than eight hybrids (grandchildren of the pure geese)
from one nest. In India, however, these cross-bred geese must be far
more fertile; for I am assured by two eminently capable judges, namely
Mr. Blyth and 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 or perfectly fertile.
With our domesticated animals, the various races when crossed
together are quite fertile; yet in many cases they are descended
from two or more wild species. From this fact we must conclude
either that the aboriginal parent-species at first produced
perfectly fertile hybrids, or that the hybrids subsequently reared
under domestication became quite fertile. This latter alternative,
which was first propounded by Pallas, seems by far the most
probable, and can, indeed, hardly be doubted. It is, for instance,
almost certain that our dogs are descended from several wild stocks;
yet, with perhaps the exception of certain indigenous domestic dogs of
South America, all are quite fertile together; but analogy makes me
greatly doubt whether the several aboriginal species would at first
have freely bred together and have produced quite fertile hybrids.
So again I have lately acquired decisive evidence that the crossed
offspring from the Indian humped and common cattle are inter se
perfectly fertile; and from the observations by Rutimeyer on their
important osteological differences, as well as from those by Mr. Blyth
on their differences in habits, voice, constitution, &c., these two
forms must be regarded as good and distinct species. The same
remarks may be extended to the two chief races of the pig. We must,
therefore, either give up the belief of the universal sterility of
species when crossed; or we must look at this sterility in animals,
not as an indelible characteristic, but as one capable of being
removed by domestication.
Finally, considering all the ascertained facts on the
intercrossing of plants and animals, it may be concluded that some
degree of sterility, both in first crosses and in hybrids, is an
extremely general result; but that it cannot, under our present
state of knowledge, be considered as absolutely universal.
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