374 COOK 



have occasioned serious functional derangements or might have 

 too greatly increased the difficulties of existence, can be toler- 

 ated by the males without injury to the species. 



That secondary sexual characters are often so completely 

 without function, in the ordinary sense of the word, does not 

 mean that they are of no value to the organism. With refer- 

 ence to the environment they are often worse than useless, but 

 in the physiology of descent they may have an important func- 

 tion. The existence of two sexes doubles, as it were, the sym- 

 basic effect of cross-fertilization, by permitting the accumulation 

 of two sets of variations, a second reason for the more rapid 

 progress made by sexually diversified organisms. 



What has been called organic evolution has been thought of 

 too exclusively from the environmental side. Evolution has an 

 internal as well as an external function ; it has a bearing upon 

 the quality of organisms, as well as upon quantity. Species are 

 advantaged not only by characters which give them a wide 

 range and permit the propagation of large numbers, but it is of 

 equal importance that the vitality of the species be maintained 

 through the provision of adequate diversity of descent, as as- 

 sured by sexual specialization and by the access of new varia- 

 tions. 



The doctrine of sexual selection was invented by Darwin to 

 explain the so-called secondary characters, differences admit- 

 tedly useless from the environmental standpoint, the two sexes 

 of a species being subject, generally, to identical external con- 

 ditions. And yet there is everywhere manifest a tendency to 

 the further accentuation of sexual diversities, which are by no 

 means confined to man, or to the higher animals in which esthetic 

 instincts have been attained. 



Viewed as specializations of heterism, secondary sexual char- 

 acters have an obvious and general utility, though of an internal 

 nature. A species with two separated sexes is the stronger 

 because it can accumulate two lines of variations. Symbasic 

 interbreeding becomes, as it were, doubly effective, and the 

 stimulus of diversity can be utilized for a much longer period than 

 if the character were to spread to all the members of the species. 



If the present interpretation of the facts be correct, we have 



