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.
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