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.
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