CHAPTER IX - HYBRIDISM
Fertility of Varieties when Crossed, and of their Mongrel Offspring,
not universal
It may be urged, as an overwhelming argument, that there must be
some essential distinction between species and varieties, inasmuch
as the latter, however much they may differ from each other in
external appearance, cross with perfect facility, and yield
perfectly fertile offspring. With some exceptions, presently to be
given, I fully admit that this is the rule. But the subject is
surrounded by difficulties, for, looking to varieties produced under
nature, if two forms hitherto reputed to be 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,
which are considered by most botanists as varieties, are said by
Gartner to be quite sterile when crossed, and he subsequently 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 some doubt.
For when it is stated, for instance, that certain South American
indigenous domestic dogs do not readily unite with European dogs,
the explanation which will occur to every one, and probably the true
one, is that they are descended from aboriginally distinct species.
Nevertheless the perfect fertility of so many domestic races,
differing widely from each other in appearance, for instance those
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. In the first place, it may be
observed that the amount of external difference between two species is
no sure guide to their degree of mutual sterility, so that similar
differences in the case of varieties would be no sure guide. It is
certain that with species the cause lies exclusively in differences in
their sexual constitution. Now the varying conditions to which
domesticated animals and cultivated plants have been subjected, have
had so little tendency towards modifying the reproductive system in
a manner leading to mutual sterility, that we have good grounds for
admitting the directly opposite doctrine of Pallas, namely, that
such conditions generally eliminate this tendency; so that the
domesticated descendants of species, which in their natural state
probably would have been in some degree sterile when crossed, become
perfectly fertile together. With plants, so far is cultivation from
giving a tendency towards sterility between distinct species, that
in several well-authenticated cases already alluded to, certain plants
have been affected in an opposite manner, for they have become
self-impotent whilst still retaining the capacity of fertilising,
and being fertilised by, other species. If the Pallasian doctrine of
the elimination of sterility through long-continued domestication be
admitted, and it can hardly be rejected, it becomes in the highest
degree improbable that similar conditions long-continued should
likewise induce this tendency; though in certain cases, with species
having a peculiar constitution, sterility might occasionally be thus
caused. Thus, as I believe, we can understand why with domesticated
animals varieties have not been produced which are mutually sterile;
and why with plants only a few such cases, immediately to be given,
have been observed.
The real difficulty in our present subject is not, as it appears
to me, why domestic varieties have not become mutually infertile
when crossed, but why this has so generally occurred with natural
varieties, as soon as they have been permanently modified in a
sufficient degree to take rank as species. We are far from precisely
knowing the cause; nor is this surprising, seeing how profoundly
ignorant we are in regard to the normal and abnormal action of the
reproductive system. But we can see that species, owing to their
struggle for existence with numerous competitors, will have been
exposed during long periods of time to more uniform conditions, than
have domestic varieties; and this may well make a wide difference in
the result. For we know how commonly wild animals and plants, when
taken from their natural conditions and subjected to captivity, are
rendered sterile; and the reproductive functions of organic beings
which have always lived under natural conditions would probably in
like manner be eminently sensitive to the influence of an unnatural
cross. Domesticated productions, on the other hand, which, as shown by
the mere fact of their domestication, were not originally highly
sensitive to changes in their conditions of life, and which can now
generally resist with undiminished fertility repeated changes of
conditions, might be expected to produce varieties, which would be
little liable to have their reproductive powers injuriously affected
by the act of crossing with other varieties which had originated in
a like manner.
I have not as yet spoken as if the varieties of the same species
were invariably fertile when intercrossed. But it is impossible to
resist the evidence of the existence of a certain amount of
sterility in the few following cases, which I will briefly abstract.
The evidence is at least as good as that from which we believe in
the sterility of a multitude of species. The evidence is, also,
derived from hostile witnesses, who in all other cases consider
fertility and sterility as safe criterions of specific distinction.
Gartner kept during several years a dwarf kind of maize with yellow
seeds, and a tall variety with red seeds growing near each other in
his garden; and although these plants have separated sexes, they never
naturally crossed. He then fertilised thirteen flowers of the one kind
with pollen of the other; but only a single head produced any seed,
and this one head produced only five grains. Manipulation in this case
could not have been injurious, as the plants have separated sexes.
No one, I believe, has suspected that these varieties of maize are
distinct species; and it is important to notice that the hybrid plants
thus raised were themselves perfectly fertile; so that even Gartner
did not venture to consider the two varieties as specifically
distinct.
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 experimented on are ranked by Sageret, who mainly founds his
classification by the test of infertility, as varieties, and Naudin
has come to the same conclusion.
The following case is far more remarkable, and seems at first
incredible; but it is the result of an astonishing number of
experiments made during many years on nine species of Verbascum, by so
good an observer and so hostile a witness as Gartner: namely, that the
yellow and white varieties when crossed produce less seed than the
similarly coloured varieties of the same species. 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 similarly coloured flowers, than
between those which are differently coloured. Mr. Scott also has
experimented on the species and varieties of Verbascum; and although
unable to confirm Gartner's results on the crossing of the distinct
species, he finds that the dissimilarly coloured varieties of the same
species yield fewer seeds in the proportion of 86 to 100, than the
similarly coloured varieties. Yet these varieties differ in no respect
except in the colour of their flowers; and one variety can sometimes
be raised from the seed of another.
Kolreuter, whose accuracy has been confirmed by every subsequent
observer, has proved the remarkable fact, that one particular
variety of the common tobacco was more fertile than the other
varieties, when crossed with a widely distinct species. He
experimented 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 the 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 it can no longer be maintained that varieties
when crossed are invariably quite fertile. From the great difficulty
of ascertaining the infertility of varieties in a state of nature, for
a supposed variety, if proved to be infertile in any degree, would
almost universally be ranked as a species;- from man attending only to
external characters in his domestic varieties, and from such varieties
not having been exposed for very long periods to uniform conditions of
life;- from these several considerations we may conclude that
fertility does not constitute a fundamental distinction between
varieties and species when crossed. The general sterility of crossed
species may safely be looked at, not as a special acquirement or
endowment, but as incidental on changes of an unknown nature in
their sexual elements.
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