f 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 wh