IjSITETSSIVE SE&EEC4ATI0]Sr. 331 



gical and morphological characters of the species, the chief, in- 

 dispensable condition being tbe prevention of interbreeding 

 between the diverging sections of tlie species. 



Sexual Selection is sometimes referred to as if it were the 

 influence of sexual instincts in giving cbaracter to the organs of 

 a given sex, first by the instincts of the same sex rousing the 

 organs to successful activity in securing propagation, the degree 

 of success depending on the degree of adaptation of the organ to 

 Hie purpose of the activity (as in the case of barnyard cocks 

 winning partners by the use of their spurs), and, second, by the 

 instincts of tlie opposite sex being roused to successful action 

 according as the endowments of the given sex are fitted to the 

 end (as in the case of peacocks winning partners by tlie disj)lay 

 of ornamentation). Starting, however, with this conception of 

 the nature of Sexual Selection, we shall find great difficulty in 

 obtaining from the principle any explanation of the origin of 

 species, or of divergent evolution of any kind. If divergent 

 instincts are the causes of divergent forms, colours, and qualities, 

 wdiat are the causes of the transformation of the instincts in 

 lines that are persistently divergent ? The problems of trans- 

 formation and divergence are as far from solution after the 

 application of the theory as before. 



If, on the other hand, we recognize Sexual Selection as the 

 harmonizing of the forms, colours, and qualities of a species with 

 its sexual instincts, and of the sexual instincts with its forms, 

 colours, and qualities, we shall not claim that either set of cha- 

 racters is directly and continuously the cause of transformation 

 in the other ; but rather that the two sets play upon each other 

 in such a way as to produce a state of unstable equilibrium in 

 both sets, the result of which is indefinite transformation in the 

 secondary sexual characters of each section of a species that 

 constitutes a separate intergenerant ; and that the Independent 

 Transformation inevitably results in Divergence. In Darwin's 

 presentation of the principle of Sexual Selection, the chief 

 endeavour is to show that differences in voice and ornamentation 

 between the males and females of the same species are jn-obably, 

 in a large degree, due to diversity in the action of Sexual 

 Selection upon the different sexes ; but this is a very different 

 result from differences in the same respects between those of the 

 same sex in closely allied varieties and species ; and no clear 



