292 THE INFLUENCE OF INANIMATE SUEEOUNDINaS. 



these influences, and in various places expressly states that they 

 may sometimes have had precisely the same results "^ as 

 Natural Selection in its most limited acceptation. In short, if 

 I rightly understand Darwin, he applies this expression, not 

 exclusively to the struggle or combat between two individuals, 

 but conceives of it rather as the sum total of all the eiforts 

 which a newly constituted species must make to succeed in con- 

 quering all the hindrances to its development, and at the same 

 time to avail itself to the utmost of every favourable circum- 

 stance that offers. It must certainly be conceded that Darwin 

 generally applies the words ' ISTatm-al Selection ' to those cases 

 only of the most direct competition between two animals, of the 

 same or of closely allied species. This indeed is the obvious 

 inference from the fact that he considers it necessary to contrast 

 Natural and Sexual Selection, although the sole difference 

 between them properly consists in this ; that in the former the 

 struggle is for a dead object, in the latter for a living one, i.e. 

 the female. It may still further be conceded, as indeed Darwin 

 himself has admitted, that in ths first instance he somewhat 

 undervalued the selective influence exerted by the sixrrounding 

 and external conditions of life ; but to assert that he wholly 

 ignored them is far from the truth. On the contrary, these 

 influences constitute an essential part of his theory, though 

 Darwin himself assigns them but a small and undoubtedly too 

 limited jiart in it. Still Wagner's separation theory is not 

 thereby opposed to Darwin's, but, on the contrary, an integral 

 part of it ; and it is an indisputable fact that the various propo- 

 sitions which constitute the ' separation theory ' had long before, 

 if in a different form, been announced in the chapter on the 

 geographical distribution of animals in Darwin's work on the 

 Origin of Species. 



But while I must thus, in the most positive manner, dispute 

 the idea that Wagner's theory is in any way essentially opposed 

 to those of Darwin, I may on the other hand admit, once more, 

 that migration and the separation frequently occasioned by it, 

 as well as by currents, may exert a very decisive influence on 

 the formation of species. Such an influence is recognisable in 

 the fact that such land-snails as are difiicult to transport by 



