RECAPITULATION AND CONCLUSION 509 



under changing and complex conditions of life, should not 

 variations useful to nature's living products often arise, and 

 be preserved or selected ? What limit can be put to this power, 

 acting during long ages and rigidly scrutinising the whole 

 constitution, structure, and habits of each creature, — favour- 

 ing the good and rejecting the bad? I can see no limit to 

 this power, in slowly and beautifully adapting each form to 

 the most complex relations of life. The theory of natural 

 selection, even if we look no farther than this, seems to be 

 in the highest degree probable. I have already recapitulated, 

 as fairly as I could, the opposed difficulties and objections: 

 now let us turn to the special facts and arguments in favour 

 of the theory. 



On the view that species are only strongly marked and 

 permanent varieties, and that each species first existed as a 

 variety, we can see why it is that no line of demarcation can 

 be drawn between species, commonly supposed to have been 

 produced by special acts of creation, and varieties which are 

 acknowledged to have been produced by secondary laws. On 

 this same view we can understand how it is that in a region 

 where many species of a genus have been produced, and 

 where they now flourish, these same species should present 

 many varieties ; for where the manufactory of species has 

 been active, we might expect, as a general rule, to find it still 

 in action; and this is the case if varieties be incipient species. 

 Moreover, the species of the larger genera, which afford the 

 greater number of varieties or incipient species, retain to a 

 certain degree the character of varieties; for they differ from 

 each other by a less amount of difference than do the species 

 of smaller genera. The closely allied species also of the 

 larger genera apparently have restricted ranges, and in their 

 affinities they are clustered in little groups round other 

 species — in both respects resembling varieties. These are 

 strange relations on the view that each species was inde- 

 pendently created, but are intelligible if each existed first as 

 a variety. 



As each species tends by its geometrical rate of reproduc- 

 tion to increase inordinately in number; and as the modified 

 descendants of each species will be enabled to increase by as 



