ENVIRONAL AND SEXUAL SELECTION. 201 



petuating their perverted instincts. If my view is correct, the change 

 producing divergent sexual characteristics may be cither in the 

 instinct or in the characters with which the instinct is correlated. It 

 seems probable that in the vast majority of cases the more strongly 

 divergent forms have been reached by a multitude of deviations alter- 

 nating between the psychical and the physiological and morphological 

 characters of the species, the chief, indispensable condition being the 

 prevention of interbreeding between the diverging sections of the 

 species. 



Sexual selection is sometimes referred to as if it were the influence 

 of sexual instincts in giving character to the organs of a given sex, 

 first by the instincts of the given sex rousing the individuals of that 

 sex to successful activity in securing propagation, the degree of suc- 

 cess depending on the degree of adaptation of the organs of the indi- 

 vidual to the purpose of the activity (as in the case of barnyard cocks 

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

 stincts of the 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 the display of ornamentation). 

 Starting, however, with this conception of the nature of sexual selec- 

 tion, 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, 

 colors and qualities, what are the causes of the transformation of the 

 instincts in lines that are persistently divergent ? The problems of 

 transformation and divergence are as far from solution after the appli- 

 cation of the theory as before. 



If, on the other hand, we recognize sexual selection as the har- 

 monizing of the forms, colors, and qualities of a species with its sexual 

 instincts and of the sexual instincts with its forms, colors, and quali- 

 ties, we shall not claim that either set of characters 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 indefi- 

 nite 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 

 endeavor is to show that differences in voic/? and ornamentation 

 between males and females of the same species are probably, 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 



