The Theory of Sexual Selection. 403 



on grounds of logic. If the theory in this respect is to 

 be attacked at all, it can only be on grounds otfact 

 namely, by arguing that the cause does not occur in 

 nature, or that, if it does, its importance has been exag- 

 gerated by the theory. Even, however, if the latter 

 proposition should ever be proved, we may now be 

 virtually certain that the only result would be the rele- 

 gation of all the residual phenomena of adaptation to 

 other causes of the physical order whether known or 

 unknown. Hence, as far as the matter of principle is 

 concerned, we may definitely conclude that the great 

 naturalistic movement of our century has already 

 brought all the phenomena of adaptation in organic 

 nature under precisely the same category of mecha- 

 nical .causation, as similar movements in previous 

 centuries have brought all the known phenomena of 

 inorganic nature : the only question that remains for 

 solution is the strictly scientific question touching the 

 particular causes of the mechanical order which have 

 been at work. 



So much, then, for the phenomena of adaptation. 

 Turning next to those of beauty, we have already seen 

 that the theory of sexual selection stands to these in 

 precisely the same relation as the theory of natural 

 selection does to those of adaptation. In other words, 

 it supplies a physical explanation of them ; because, 

 as far as our present purposes are concerned, it may 

 be taken for granted, or for the sake of argument, that 

 inasmuch as psychological elements enter into the 

 question the cerebral basis which they demand involves 

 a physical side. 



There is, moreover, this further point of resemblance 

 between the two theories : neither of them has any 

 D d 2 



