80 THE ORIGIN OF SPECIES 



From these several considerations and from the many special 

 facts which I have collected, but which I am unable here to give, it 

 appears that with animals and plants an occasional intercross be- 

 tween distinct individuals is a very general, if not universal, law 

 of nature. 



CIRCUMSTANCES FAVORABLE FOR THE PRODUCTION OF NEW FORMS 

 THROUGH NATURAL SELECTION 



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

 ability, under which term individual differences are always in- 

 cluded, will evidently be favorable. A large number of individuals, 

 by giving a better chance within any given period for the appear- 

 ance of profitable variations, will compensate for a lesser amount 

 of variability in each individual, and is, I believe, a highly im- 

 portant element of success. Though nature grants long periods of 

 time for the work of natural selection, she does not grant an in- 

 definite 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 modified and improved in a corresponding degree with its 

 competitors it will be exterminated. Unless 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 inter- 

 cross, 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 proc- 

 ess 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 modi- 

 fication in different districts, the newly formed varieties will inter- 

 cross 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 



