94 CIRCUMSTANCES FAVORABLE TO THE 



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 between distinct individuals 

 is a very general, if not universal, law of nature. 



CIRCUMSTAi>rCES FAVORABLE FOR THE PRODUCTIOIS" OF 

 NEW FORMS THROUGH NATURAL SELECTION. 



This is an extremely intricate subject. A great amount 

 of variability, under which term individual differences are 

 always included, will evidently be favorable. A large num- 

 ber of individuals, by giving a better chance within any 

 giveJi 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 indefi- 

 nite 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 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 pro- 

 cure and breed from the best animals, improvement surely 

 but slowly follows from this unconscious process of selec- 

 tion, notwithstanding that there is no separation of selected 

 individuals. Thus it v/ill 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 pre- 

 served. But if the area be large, its several districts will 

 almost certainly present different conditians of life; and 



