n of life; and it would appear that the germ, in a somewhat
analogous manner, is more effectually stimulated by the male element, when
taken from a distinct individual, and therefore slightly modified in
nature, than when taken from a male having the same identical constitution.
On the other hand, numerous facts have been given, showing that when
animals are first subjected to captivity, even in their native land, and
although allowed much liberty, their reproductive functions are often
greatly impaired or quite annulled. Some groups of animals are more
affected than others, but with apparently capricious exceptions in every
group. Some animals never or rarely couple: some couple freely, but never
or rarely conceive. The secondary male characters, the maternal functions
and instincts, are occasionally affected. With plants, when first subjected
to cultivation, analogous facts have been observed. We probably owe our
double flowers, rich seedless {177} fruits, and in some cases greatly
developed tubers, &c., to incipient sterility of the above nature combined
with a copious supply of nutriment. Animals which have long been
domesticated, and plants which have long been cultivated, can generally
withstand with unimpaired fertility great changes in their conditions of
life; though both are sometimes slightly affected. With animals the
somewhat rare capacity of breeding freely under confinement has mainly
determined, together with their utility, the kinds which have been
domesticated.

We can in no case precisely say what is the cause of the diminished
fertility of an animal when first captured, or of a plant when first
cultivated; we can only infer that it is caused by a change of some kind in
the natural conditions of life. The remarkable susceptibility of the
reproductive system to such changes,--a susceptibility not common to any
other organ,--apparently has an important bearing on Variability, as we
shall see in a future chapter.

It is impossible not to be struck with the double parallelism between the
two classes of facts just alluded to. On the one hand, slight changes in
the conditions of life, and crosses between slightly modified forms or
varieties, are beneficial as far as prolificness and constitutional vigour
are concerned. On the other hand, changes in the conditions greater in
degree, or of a different nature, and crosses between forms which have been
slowly and greatly modified by natural means,--in other wor