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 @ priéort considerations, which thus 
show that natural selection mst have been concerned 
to some extent in the process of evolution, we find in 
organic nature evidence @ posterior? of the extent to 
which this principle Zas 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 a// 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 opcration 
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 sclection—which, as now 
so fully explained, is nothing more than a theory 
of cumulative modifications by means of selective 
breeding. 
