486 RECAPITULATION. 



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 further 

 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 favor 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 second- 

 ary 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 varie- 

 ties 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 a larger genera apparently 

 have restricted ranges, and in their aflSnities they are 

 clustered in little groups round other species — iu both 

 respects resembling varieties. These are strange relations 

 on the view that each species was independently created, 

 but are intelligible if each existed first as a variety. 



As each species tends by its geometrical rate of repro- 

 duction to increase inordinately in number; and as the 

 modified descendants of each species will be enabled to 

 increase by as much as they become more diversified in 

 habits and structure, so as to be able to seize on many and 

 widely different places in the economy of nature, there 

 will be a constant tendency in natural selection to preserve 

 the most divergent offspring of any one species. Hence, 

 during a long- continued course of modification, the 

 slight differences characteristic of varieties of the same 

 species, tend to be augmented into the greater differences 



