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NATURAL SELECTION 149 



favorable variations be inherited by some at least of the 

 offspring, nothing can be effected by natural selection. 

 The tendency to reversion may often check or prevent 

 the work; but as this tendency has not prevented man 

 from forming by selection numerous domestic races, why 

 should it prevail against natural selection ? 



In the case of methodical selection, a breeder selects 

 for some definite object, and if the individuals be allowed 

 freely to intercross, his work will completely fail. But 

 when many men, without intending to alter the breed, 

 have a nearly common standard of perfection, and all try 

 to procure and breed from the best animals, improvement 

 surely but slowly follows from this unconscious process 

 of selection, notwithstanding that there is no separation 

 of selected individuals. Thus it will be under nature; ! 

 for within a confined area, with some place in the natural | 

 polity not perfectly occupied, all the individuals varying 

 in the right direction, though in different degrees, will | 

 tend to be preserved. But if the area be large, its 

 several districts will almost certainly present different 

 conditions of life; and then, if the same species under- 

 goes modification in different districts, the newly-formed 

 varieties will intercross on the confines of each. But we 

 shall see in the sixth chapter that intermediate varieties, 

 inhabiting intermediate districts, will in the long run 

 generally be supplanted by one of the adjoining varieties. I 

 Intercrossing will chiefly affect those animals which unite I 

 for each birth and wander much, and which do not breed \ 

 at a very quick rate. Hence with animals of this nature, 

 for instance, birds, varieties will generally be confined to.L 

 separated countries; and this I find to be the case. With | 

 hermaphrodite organisms which cross only occasionally, xj/ 



