ity, though in some degree variable, rarely diminishes.

It must, however, be confessed that we cannot understand, excepting on
vague hypotheses, several facts with respect 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 having been
compounded into one.

It may seem fanciful, but I suspect that a similar {267} parallelism
extends to an allied yet very different class of facts. It is an old and
almost universal belief, founded, I think, on a considerable body of
evidence, 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, &c., from one soil or climate to
another, and back again. During the convalescence of animals, we plainly
see that great benefit is derived from almost any change in the habits of
life. Again, both with plants and animals, there is abundant evidence, that
a cross between very distinct individuals of the same species, that is
between members of different strains or sub-breeds, gives vigour and
fertility to the offspring. I believe, indeed, from the facts alluded to in
our fourth chapter, that a certain amount of crossing is indispensable even
with hermaphrodites; and that close interbreeding continued during several
generations between the nearest relations, especially if these be kept
under the same conditions of life, always induces weakness and sterility in
the progeny.

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 varied and become slightly different, give vigour and fertility
to the offspring. But we have seen that greater changes, or changes of a
particular nature, often render organic