272 Darwin, and after Darwin. 



too slight to serve any other purpose, may act as 

 " recognition marks," whereby the opposite sexes are 

 enabled at once to distinguish between members of 

 their own and of closely resembling species. Of 

 course this hypothesis can only apply to the higher 

 animals; but the point here is that, supposing it to 

 hold for them. Mr. Wallace proceeds to argue thus : 

 Recognition marks "have in all probability been 

 acquired in the process of differentiation for the 

 purpose of checking the intercrossing of allied forms," 

 because " one of the first needs of a new species would 

 be to keep separate from its nearest allies, and this 

 could be more readily done by some easily seen 

 external mark 1 ." Now, it is clearly not so much 

 as logically possible that these recognition-marks 

 (supposing them to be such) can have been acquired 

 by natural selection, "for the purpose of checking 

 intercrossing of allied forms." For the theory of 

 natural selection, from its own essential nature as a 

 theory, is logically exclusive of the supposition that 

 survival of the fittest ever provides changes in antici- 

 pation of future uses. Or, otherwise stated, it involves 

 a contradiction of the theory itself to say that the 

 colour-changes in question were originated by natural 

 selection, in order to meet " one of the first needs of a 

 new species," or for the purpose of subsequently 

 preventing intercrossing with allied forms. If it had 

 been said that these colour-differentiations were 

 originated by some cause other than natural selection 

 (or, if by natural selection, still with regard to some 

 previous, instead of prophetic, " purpose "), and, when so 

 " acquired/' then began to serve the " purpose" assigned, 

 1 Darwinism, pp. ai8 and 227. 



