OCR Text |
Show 96 CIROUMSTANCES FAVOURABLE [CHAP. IV. modified and improved in a corresponding degroe with its competitors, it will soon be exterminated. In man's methodical selection, a breeder selects for some definite object, and free intercrossing will wholly stop his work. But when many men, without intending to alter the breed, have a nearly common standard of perfection, and all try to get and breed from the best animals, much improvement and modification surely but slowly follows from this unconscious process of selection, notwithstanding a large amount of crossing with inferior animals. Thus it will be in nature ; for within a confined area, 'vith some place in its polity not so perfectly occupied as might be, natural selection will always tend to preserve all the individuals varying in the right direction, though in different degrees, so as better to fill up the unoccupied place. But if the area be large, its several districts will almost certainly present different conditions of life ; and then if natural selection be modifying and improvin~ a species in the several districts, there will be intercross1ng with the other individuals of the same species on the confines of eaeh. .And in this case the effects of intercrossing can hardly be counterbalanced by natural selection always tending to modify all the individuals in each district in exactly the same manner to the conditions of each; for in a continuous area, the conditions will generally graduate away insensibly from one district to another. The intercrossing will most affect those animals which unite for each birth, which wander much, and which do not breed at a very quick rate. l-Ienee in animals of this nature, for instance in birds, varieties will generally be confined to separated countries; and this I believe to be the case. In hermaphrodite organisms which cross only occasionally, and likewise in animals which unite for each birth, but which wander little and which can increase at a very rapid rate, a new and improved variety might be quickly formed on any one spot, and might there rnaintain itself in a body, so that whatever intercrossing took place would be chiefly between the individuals of the same new variety. .A local variety when once thus formed might subsequently slowly ipread to other districts. On CHAP. IV.] TO NATURAL SELECTION. 97 the above principle, nurserymen always prefer getting seed from a lars-e body ?f pla~ts of the sa1ne variety, as the chance of Intercrossing With other varieties is thus lessened. Even i~ the case of slow-breeding animals, which unite for eac~ birth, ":'e must not overrate the effects of intercros~ es In retarding natural selection ; for I can brin a considerable c~ta!ogue of facts, showing that within fhe s~m.e area, varietie~ of t~e same animal can long remajn d~stinct, f~om haunting different stations, from breeding at S~Ightly diff~rent seasons, or from varieties of the same lnnd preferru:~g to pair together. I~tercross~ng .P~ays a very important part in nature in keeping t?e Individuals of . the sa:me species, or of the sam.e variety, true and uniform In character. It will ob':"Iously_ thus act far more efficiently with those anhnals whwh unite for each birth; but I have already attempted ~o show that we have reason to believe that occasional Interc::osses take place with all animals and with all plants. E_ven 1f these take place only at long intervals, I am conyinc~ d that the young thus produced will gain so much In ':Igour and fertility over the offspring from Jon _ continued self-f~r~ilisation, that they will have a bett~· chanc~ of surviving and propagating their kind; and thus, In ~he long run, the influence of intercrosses even at. rare In~ervals, will be great. If there exist 0 ; anic bmngs wh1?h never intercross, uniformity of char~cter can ~e retain~d amongst them, as long as their conditions ?f hfe remain the san1e, only through the principle of I~.entance, and through natural selection destroyin~ any 'f }?Z dehpart from the proper type; but if their conditions o I e c ange and they undergo modification uniformit of character can be ~iven to their modified off's rin y sol~ly _by natural selection preserving the same favoJrabf~ vanations. f Isolation, als?, is an hnportant element in the process o natu:al selection. _In a co:r;:fined or isolated area, if not v~rf large, the oi:ganic and Inorganic conditions of life Wil gene:ally ~e In a great degree uniform; so that natura selection will tend to modify all the individuals of a 5 . |