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Show 458 GENERAL RESULrrs. CHAP. XII. say why the individuals of certain species profit greatly, and others very little by being crossed .. ~lhcrc arc s01ne few species which have boon self-fertl~rscu for a vast nu1nbcr of generations, an~ yet arc vrg.orous cnol~gh to cornpete successfully wrth a. h_ost of su.rrounch~lg plants. Highly self-fertile vanetrcs so~10~1~ cs anso among plants which hav~ . boon s?lf-fertrhsou and grown under uniform condrtrons dunng. several gonerations. We can form no conceptron why the advantage from a cross is sometimes dir~ctcu exclusively to the vegetative system, and so1nctnncs to t~e reproductive system, but com~o~1l! to both. It Is equally inconceivable -vvhy sorne 1nd.rvrcluals of the sa~e speci s should be sterile, whilst others are. fully fertile with their own pollen; why a change of clnnatc should either lessen or increase the sterility of self-sterile species; and why the individuals of some ~p~cics sho~ld be even 1nore fertile with pollen from a dist1nct speCies than with their own pollen. And so it is with many other facts, which are so bscure that we stand in awe b fore the mystery of life. Under a practical point of view, agriculturists and horticulturists may learn something fro In the conclusions at which we have arrived. Firstly, we see that the injury from th close breeding of animals a~cl from the self-fertilisation of plants, does not necossanly depend on any t ndcncy to dis ")ase or weakness of constitution common to the r latccl parents, and only indirectly on their relationship, in so far as. they .ar~ apt to resmnble each other in all respects, rncludmg their sexual nature. And, secondly, that the advantages of cross-fertilisation d pen l on tho sexual eleme~ts of the parents having becom - in some degree di:ffore~tiatecl by the exposure of th ir prog nr·t ors t o d1 f ferent CHAP. XII. GENERAL RESULT . 459 c. on .e li .t ions ' or f.r 'orn th e·n , h avr· ng r· ntercrosse ·l with ~ndrvrd?-als thus exposed, or, lastly, fro~ what w n.ll rn our ~gnorancc spontaneous variation. He th r foro who wrshes to pair closely related animals ought to keep them under conditions as difC rent as possibl . Some fc:V breeders, guided by their keen powers of observat~on, have acted on this principle, and have kept stocks of the sarne animals at two or n1orc distant an l diff~re~tl.y situated farms. They have then coupled the rnchvrduals from these farms with excellent results:* Thjs same plan is also unconsciously followed whenever t~e 1nales, 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. The effects of close interbreeding on animals, judging again from plants, would be deterioration in general vigour, including fertility, with no necessary loss of excellence of form. ; and this seems to be the usual result. It is a common practice with horticulturists to obtain seeds from another place having a very different soil, so as to avoid raising plants for a long succession of generations under the same conditions; but with all the species which freely intercross by the aid of insects or the wind, it would be an incomparably better plan to obtain seeds of the required variety, which had been raised for some generations under as different conditions as possible, and sow them in alternate rows with seeds matured in the old garden. The two stocks would then intercross, with a thorough blending of their whole organisations, and with no loss of purity to the variety; and this would yield far more * ' Variation of Animals and xvii. 2nd edit. vol. ii. pp. 98, 105. Plants under Domestication,' ch. |
