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.
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