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 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,
etc., 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 beings in some
degree sterile; and that greater crosses, that is crosses between males
and females which have become widely or specifically different, produce
hybrids which are generally sterile in some degree. I cannot persuade
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