It therefore seems probable, that this facility for
breeding under changed conditions was an original property of the
species which man has domesticated--a property which, more than any
other, enabled him to domesticate them. Yet, even with these, there is
evidence that great changes of conditions affect the fertility. In the
hot valleys of the Andes sheep are less fertile; while geese taken to
the high plateau of Bogota were at first almost sterile, but after some
generations recovered their fertility. These and many other facts seem
to show that, with the majority of animals, even a slight change of
conditions may produce infertility or sterility; and also that after a
time, when the animal has become thoroughly acclimatised, as it were, to
the new conditions, the infertility is in some cases diminished or
altogether ceases. It is stated by Bechstein that the canary was long
infertile, and it is only of late years that good breeding birds have
become common; but in this case no doubt selection has aided the change.
As showing that these phenomena depend on deep-seated causes and are of
a very general nature, it is interesting to note that they occur also
in the vegetable kingdom.
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