RESULTS OF NATURAL SELECTION. 89 



competitors it will be exterminated. Unless favorable vari- 

 ations be inherited by some at least of the offspring, nothing 

 can be effected by natural selection. The tendency to re- 

 version 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, notwith- 

 standing 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 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 vari- 

 eties. 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 con- 

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

 With hermaphrodite organisms which cross only occasion- 

 ally, and likewise for animals which unite for each birth, but 

 which wander little and can increase at a rapid rate, a new 

 and improved variety might be quickly formed on any one 

 spot, and might there maintain itself in a body and after- 

 ward spread, so that the individuals of the new variety 

 would chiefly cross together. On this principle nurserymen 

 always prefer saving seed from a large body of plants, as the 

 chance of intercrossing is thus lessened. 



Even with animals which unite for each birth, and which 

 do not propagate rapidly, we must not assume that free 

 intercrossing would always eliminate the effects of natural 

 selection; for I can bring forward a considerable body of 



