of genera; and
all this diversity appears to have arisen for a purpose which has been
obtained by many other, and apparently simpler, changes of structure or
of function, in other plants. This seems to show us, in the first place,
that variations in the mutual relations of the reproductive organs of
different individuals must be as frequent as structural variations have
been shown to be; and, also, that sterility in itself can be no test of
specific distinctness. But this point will be better considered when we
have further illustrated and discussed the complex phenomena of
hybridity.

_Cases of the Fertility of Hybrids, and of the Infertility of Mongrels._

I now propose to adduce a few cases in which it has been proved, by
experiment, that hybrids between two distinct species are fertile _inter
se_; and then to consider why it is that such cases are so few in
number.

The common domestic goose (Anser ferns) and the Chinese goose (A.
cygnoides) are very distinct species, so distinct that some naturalists
have placed them in different genera; yet they have bred together, and
Mr. Eyton raised from a pair of these hybrids a brood of eight. This
fact was confirmed by Mr. Darwin himself, who raised several fine birds
from a pair of hybrids which were sent him.[53] In India, according to
Mr. Blyth and Captain Hutton, whole flocks of these hybrid geese are
kept in various parts of the country where neither of the pure parent
species exists, and as they are kept for profit they must certainly be
fully fertile.

Another equally striking case is that of the Indian humped and the
common cattle, species which differ osteologically, and also in habits,
form, voice, and constitution, so that they are by no means closely
allied; yet Mr. Darwin assures us that he has received decisive
evidence that the hybrids between these are perfectly fertile _inter
se_.

Dogs have been frequently crossed with wolves and with jackals, and
their hybrid offspring have been found to be fertile _inter se_ to the
third or fourth generation, and then usually to show some signs of
sterility or of deterioration. The wolf and dog may be originally the
same species, but the jackal is certainly distinct; and the appearance
of infertility or of weakness is probably due to the fact that, in
almost all these experiments, the offspring of a single pair--themselves
usually from the same litter--- were bred in-and-in, and this alone
sometimes produces the