Chap. IV.] EESULTS OF NATURAL SELECTION. 125 



variations, will compensate for a lesser amount of va- 

 riability 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 modified 

 and improved in a corresponding degree with its com- 

 petitors, 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, improve- 

 ment 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 



