NATURAL SELECTION 93 



of nature, if any one species does not become modified 

 and improved in a corresponding degree with its com- 

 petitors, it will soon be exterminated. 



In man's methodical selection, a breeder selects for 

 some definite object, and free intercrossing will wholly 

 stop his work. But when many men, without intend- 

 ing to alter the breed, have a nearly common standard 

 of perfection, and all try to get and breed from the 

 best animals, much improvement and modification 

 surely but slowly follow from this unconscious process 

 of selection, notwithstanding a large amount of 

 crossing with inferior animals. Thus it will be in 

 nature ; for within a confined area, with some place in 

 its polity not so perfectly occupied as might be, natural 

 selection will always tend to preserve all the individuals 

 varying in the right direction, though in different 

 degrees, so as better to fill up the unoccupied place. 

 But if the area be large, its several districts will almost 

 certainly present different conditions of life ; and then 

 if natural selection be modifying and improving a 

 species in the several districts, there will be inter- 

 crossing with the other individuals of the same species 

 on the confines of each. And in this case the effects 

 of intercrossing can hardly be counterbalanced by 

 natural selection always tending to modify all the 

 individuals in each district in exactly the same manner 

 to the conditions of each ; for in a continuous area, the 

 physical conditions at least will generally graduate 

 away insensibly from one district to another. The 

 intercrossing will most affect those animals which unite 

 for each birth, which wander much, and which do not 

 breed at a very quick rate. Hence in animals of this 

 nature, for instance in birds, varieties will generally 

 be confined to separated countries ; and this I believe 

 to be the case. In hermaphrodite organisms which 

 cross only occasionally, and likewise in animals which 

 unite for each birth, but which wander little and which 

 can increase at a very rapid rate, a new and improved 

 variety might be quickly formed on any one spot, and 

 might there maintain itself in a body, so that whatever 



