80 CONTROLLED NATURAL SELECTION 



one sex; this conception is easier. Such a 

 variation would be correlated with the sexual 

 determinant ; variations not so correlated or 

 bound to a sexual determinant cannot, one 

 would conceive, be made to do so by Natural 

 Selection : for instance, favour their fixation 

 in the male, and their eradication in the 

 female. 



Sexual Selection, if it explains the origin 

 of extra sexual characters, does not explain 

 their use, yet, as has just been shown, they 

 must be useful, or otherwise Natural Selec- 

 tion would remove them. 



Beside this argument, there are many facts 

 that sexual selection fails to correlate and 

 which it ought to correlate. They mostly 

 come under the following headings : 



1. Sex selection accounts for the brilliant 

 colours of males, but fails to account for 

 equally brilliant colours which are frequently 

 found in both sexes. 



2. Sexual selection, if it has been observed, 

 has not been observed to be especially asso- 

 ciated with animals which show the greatest 

 sexual differences in characters. Birds, for 

 instance, in which the sexes are similar, do 

 not show less selection by the female of 

 the male, than birds whose males are of 



