espect to the sterility of hybrids;
for instance, the unequal fertility of hybrids produced from reciprocal
crosses; or the increased sterility in those hybrids which occasionally
and exceptionally resemble closely either pure parent. Nor do I pretend
that the foregoing remarks go to the root of the matter: no explanation
is offered why an organism, when placed under unnatural conditions,
is rendered sterile. All that I have attempted to show is, that in two
cases, in some respects allied, sterility is the common result--in the
one case from the conditions of life having been disturbed, in the other
case from the organisation having been disturbed by two organisations
being compounded into one.

A similar parallelism holds good with an allied yet very different
class of facts. It is an old and almost universal belief, founded on a
considerable body of evidence, which I have elsewhere given, that slight
changes in the conditions of life are beneficial to all living things.
We see this acted on by farmers and gardeners in their frequent
exchanges of seed, tubers, etc., from one soil or climate to another,
and back again. During the convalescence of animals, great benefit is
derived from almost any change in their habits of life. Again, both with
plants and animals, there is the clearest evidence that a cross between
individuals of the same species, which differ to a certain extent, gives
vigour and fertility to the offspring; and that close interbreeding
continued during several generations between the nearest relations, if
these be kept under the same conditions of life, almost always leads to
decreased size, weakness, or sterility.

Hence it seems that, on the one hand, slight changes in the conditions
of life benefit all organic beings, and on the other hand, that slight
crosses, that is, crosses between the males and females of the same
species, which have been subjected to slightly different conditions, or
which have slightly varied, give vigour and fertility to the offspring.
But, as we have seen, organic beings long habituated to certain
uniform conditions under a state of nature, when subjected, as under
confinement, to a considerable change in their conditions, very
frequently are rendered more or less sterile; and we know that a cross
between two forms that have become widely or specifically different,
produce hybrids which are almost always in some degree sterile. I am
fully persuaded that this double parallel