wagnee's theoet. 291 



viduals, by antipathy and sympathy, by incompatible differences 

 of structural character, &c. Actual separation, which is often 

 but not exclusively the result of migration, may no doubt 

 sometimes prove a stronger means of preventing free crossing 

 than such physical or structural peculiarities, but it cannot 

 possibly be disputed that these, in many cases, certainly suffice to 

 effect the same result as, in other cases, is brought about by 

 Wagner's favourite means, local separation. Hence, Wagner's 

 theory lays far too much stress on migration as a factor, so far 

 as regards the indispensable prevention of crossing, and alto- 

 gether ignores others which, under some circumstances, are of 

 quite equal eflEiciency. Consequently migration must be put in 

 the same category with all the other causes which, according to 

 Darwin, may interfere to prevent cross-breeding ; and so 

 Wagner's theory forms in fact a subsidiary to Darwin's. 



The second apparent difference between their views seems to 

 lie in the method by which the selection between the different 

 varieties takes place. Darwin says that it is ' the struggle for 

 existence,' while Wagner vehemently quarrels with this expres- 

 sion and regards the influence of external surroundings as the sole 

 efficient means — the influence, that is, of the conditions of exis- 

 tence. At the first glance this might appear to be a funda- 

 mental difference ; but the difference in the expressions used is 

 altogether superficial and may have arisen merely from a mis- 

 understanding of the word used by Darwin. Wagner ex 

 pressly says, in his latest work, in terms that cannot be misun- 

 derstood, that he is of opinion that the words ' Struggle for 

 Existence ' * are used by Darwin to denote exclusively that direct 

 combat between two individuals of the same species in their 

 efforts to possess themselves of the same prey or of the same 

 female.'" This, however, seems to me a quite erroneous inter- 

 pretation of Darwin's expression. For although Darwin him- 

 self frequently explains that in his opinion the personal strug- 

 gle between two individuals of the same species exerts a far 

 greater selective power than the surrounding conditions can 

 effect with all their sudden changes, he by no means ignores 



* ' Not verr happily rendered into German,' says Dr. Semper him- 

 self, ' by the words, Ka/mpf ums Dasdn.' 



