ASPECTS OF KINETIC EVOLUTION 375 



in the familiar phenomenon of sex an example of a fundamental 

 evolutionary principle which has thus far escaped formal recog- 

 nition. Heterism is a concrete property or requirement for con- 

 structive evolution, though left quite out of account in theories 

 which have thought to explain organic development by external 

 influences of environment, or by internal "mechanisms of 

 heredity." 



Sex specialization in species corresponds to paragamy in 

 cells ; the sustained diversity of the associated sexes is curiously 

 analogous to the prolonged separation of the parental chromo- 

 somes. Sexuality supplements paragamy, and both serve the 

 same purpose of increasing the vitality of the individual organ- 

 isms and the coherence of the specific networks of descent. 



Superscxual Species. — A species consisting of organisms of 

 two sexes, but with one or both sexes again subdivided into 

 two or more kinds of individuals. 



That the uses of the diversities of the sexes are not limited 

 merely to the reproductive functions, is well shown by the fact 

 that specializations of heterism are sometimes carried beyond the 

 stage of definite sexuality. Thus there are, among the sexually 

 differentiated higher animals and birds, numerous instances of 

 the existence of two color-forms, indifferently intermingled, but 

 not intergraded. It has been found, for example, that there 

 are in eastern North America two kinds of screech-owls, red and 

 gray, which are not separated geographically or in breeding. 



The following reference to the occurrence of leopards of two 

 colors in the Malay region may serve as a sample of many 

 similar observations among the mammals. 



" Man}" of the hunters I have met, and some of the authors I 

 have read, appear to consider the black leopard a distinct 

 species, but it is simply a freak of the ordinary spotted leopard, 

 just as the silver and the black fox are freaks from the common 

 red. In a litter from a red vixen I have seen a silver among 

 red pups ; and I met a man in the jungle where lower Siam 

 meets the Malay Peninsula who had found a black among the 

 spotted leopard's cubs, upon which, however, the spots, of course, 

 are not very clearly defined until they become older." 



..." I noticed after I got its pelt off, that in the sun it had 



