80 CIRCUMSTANCES FAVOURABLE TO THE [Chap. IV. 



Circumstances favourable for the production of new forms 

 through Natural Selection. 



This is an extremely intricate subject. A great amount of varia- 

 bility, under wbicb term individual differences are always included, 

 -will evidently be favourable. A large number of individuals, by 

 giving a better chance within any given period for the appearance 

 of profitable variations, will compensate for a lesser amount of 

 variability in each individual, and is, I believe, a highly important 

 element of success. Though Nature grants long periods of time for 

 the work of natural selection, she does not grant an indefinite 

 period ; for as all organic beings are striving to seize on each place 

 in the economy of nature, if any one species does not become modi- 

 fied and improved in a corresponding degree with its competitors, it 

 will be exterminated. Unless favourable 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 undergoes modifi- 

 cation 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. 

 Intercrossing will chiefly affect those animals which unite 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 separated countries ; and this 

 I find to be the case. With hermaphrodite organisms which cross 

 only occasionally, and likewise with animals which unite for each 



