Evidences of Theory of Natural Selection. 315 



natural selection as a principle which must be, at any 

 rate, one of the factors of organic evolution, supposing 

 such evolution to have taken place. Next, when we 

 turn from these a priori considerations, which thus 

 show that natural selection must have been concerned 

 to some extent in the process of evolution, we find in 

 organic nature evidence a posteriori of the extent to 

 which this principle has been thus concerned. For we 

 find that among all the countless millions of adaptive 

 structures which are to be met with in organic nature, 

 it is an invariable rule that they exist in relation to the 

 needs of the particular species which present them : 

 they never have any primary reference to the needs of 

 other species. And as this extraordinarily large and 

 general fact is exactly what the theory of natural 

 selection would expect, the theory is verified by the 

 fact in an extraordinarily cogent manner. In other 

 words, the fact goes to prove that in all cases where 

 adaptive structures or instincts are concerned, natural 

 selection must have been either the sole cause at work, 

 or, at the least, an influence controlling the operation 

 of all other causes. 



Lastly, an actually experimental verification of the 

 theory has been furnished on a gigantic scale by the 

 operations of breeders, fanciers, and horticulturists. 

 For these men, by their process of selective accumula- 

 tion, have empirically proved what immense changes 

 of type may thus be brought about; and so have 

 verified by anticipation, and in a most striking man- 

 ner, the theory of natural selection which, as now 

 so fully explained, is nothing more than a theory 

 of cumulative modifications by means of selective 

 breeding. 



