y of these creatures are perfectly
vigorous and live very long.

With our true domestic animals, on the other hand, fertility is perfect,
and is very little affected by changed conditions. Thus, we see the
common fowl, a native of tropical India, living and multiplying in
almost every part of the world; and the same is the case with our
cattle, sheep, and goats, our dogs and horses, and especially with
domestic pigeons. 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. Allowing for all the circumstances which are
known to prevent the production of seed, such as too great luxuriance of
foliage, too little or too much heat, or the absence of insects to
cross-fertilise the flowers, Mr. Darwin shows that many species which
grow and flower with us, apparently in perfect health, yet never produce
seed. Other plants are affected by very slight changes of conditions,
producing seed freely in one soil and not in another, though apparently
growing equally well in both; while, in some cases, a difference of
position even in the same garden produces a similar result.[51]

_Reciprocal Crosses._

Another indication of the extreme delicacy of the adjustment between the
sexes, which is necessary to produce fertility, is afforded by the
behaviour of many species and varieties when reciprocally crossed.