The Theory of Sexual Selection. 403 
on grounds of /egic. If the theory in this respect is to 
be attacked at all, it can only be on grounds of fact— 
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 sczen¢zfic 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 
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