Firstly,
we see that the injury from the close breeding of animals and from the
self-fertilisation of plants, does not necessarily depend on any
tendency to disease or weakness of constitution common to the related
parents, and only indirectly on their relationship, in so far as they
are apt to resemble each other in all respects, including their sexual
nature. And, secondly, that the advantages of cross-fertilisation depend
on the sexual elements of the parents having become in some degree
differentiated by the exposure of their progenitors to different
conditions, or from their having intercrossed with individuals thus
exposed, or, lastly, from what we call in our ignorance spontaneous
variation. He therefore who wishes to pair closely related animals ought
to keep them under conditions as different as possible. Some few
breeders, guided by their keen powers of observation, have acted on this
principle, and have kept stocks of the same animals at two or more
distant and differently situated farms. They have then coupled the
individuals from these farms with excellent results. (12/17. 'Variation
of Animals and Plants under Domestication' chapter 17 2nd edition volume
2 pages 98, 105.) This same plan is also unconsciously followed whenever
the males, reared in one place, are let out for propagation to breeders
in other places. As some kinds of plants suffer much more from
self-fertilisation than do others, so it probably is with animals from
too close interbreeding.
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