248 COOK 



Heterism becomes specialized when there are permanently 

 established differences among the members of the species, as in 

 the familiar phenomenon of sex. There is also a series of many 

 gradations between unspecialized heterism of merely individual 

 differences, and the fully established sex-differentiation. The 

 separate sexes of the higher animals are so familiar a phe- 

 nomenon that we have been satisfied to consider them merely as 

 incidental to the process of reproduction, and have thus over- 

 looked the additional physiological value of sexual differences 

 as specializations of heterism, to insure diversity of descent. 



In man himself and the higher mammals and birds the prin- 

 ciple of sexual selection enunciated by Darwin may have had an 

 influence in the further accentuation of sexual differences such 

 as beards, wattles, combs, tail-feathers and other means of 

 rendering one sex or the other conspicuous and thus attracting 

 their mates, but secondary sexual differences are not confined 

 to the higher groups or even to animals. Many plants are 

 unisexual and the two sexes often have differences other 

 than those of the essential organs. As the two sexes of plants 

 neither see nor come near each other, the pollen being carried 

 by the wind or by insects, there can be no question of sexual 

 selection here. Even types as lowly as the mosses and liver- 

 worts often have the sexes separate and very unlike. Nature 

 furnishes, indeed, hundreds and thousands of instances of inde- 

 pendently acquired sexual diversity without use either in environ- 

 mental relations or in reproductive processes. 



The use lies, we may believe, not in the particular differences 

 but in the diversity of descent which the species is enabled to 

 maintain. Diversity is of value to a species not only to enable 

 it to exist under a variety of conditions, but also because diver- 

 sity in descent is an important factor in maintaining the organic 

 strength or vital efficiency of the individual organisms. We may 

 still believe that all character differences have their uses, but the 

 use is not confined to environmental or selective considerations. 

 More fundamental than these is the use of the diversity to the 

 organisms themselves. 



Sexual differences contribute, in other words, to the increased 

 effectiveness of sexual reproduction, that is, they intensify the 



