ASPECTS OF KINETIC EVOLUTION 245 



structure there must be an interweaving of lines of descent 

 among diverse individuals. This requirement is most conspicu- 

 ously met by the familiar phenomena of sex-differentiation, but 

 can be traced upward through all the intermediate stages from 

 simple heterism, or mere individual diversity. 



As manifestations of heterism are to be included all stages of 

 intraspecific diversity, from individual differences to the extreme 

 specializations of the sexes and polymorphic forms of the higher 

 plants and animals. The function of heterism is to afford diver- 

 sity of descent, under conditions of symbasic interbreeding. 

 Narrow segregation or selective inbreeding tends to eliminate 

 heterism, but with the inevitable result of degeneration. Heteric 

 characters are highly heritable and though sometimes affected 

 by environmental conditions are in no way dependent upon them 

 or caused by them. 



Purity of stock and uniformity of characters are not syn- 

 onymous terms, as commonly supposed. A very "pure " inbred 

 strain may degenerate and become inconstant through mutation, 

 or there may be the diversity of dimorphism or polymorphism 

 in a species or variety which has not been crossed with any alien 

 blood. 



Heterism, in its most general and unspecialized sense, is what 

 has been called by some authors individual variation or fluc- 

 tuating variation. It includes the regular and normal individual 

 diversity of the memhers of a species which is not induced by 

 differences of external conditions. Some writers do not admit 

 that there is any such diversity, not caused by external conditions. 

 It is very difficult, of course, to say that any given character 

 or difference may not be connected with an environmental 

 change, but it is very easy to ascertain with reference to most 

 of the so-called individual differences, that the environmental 

 relation, if any, is not at all constant, and not to be established 

 on the basis of any form of scientific observation yet suggested. 

 We are perfectly aware that the children of the same parents, 

 born and raised under the same roof are often very unlike, while 

 on the other hand, close family likeness may persist between 

 children born and bred in remote parts of the earth involving 

 the completest possible change of climate, food, and other con- 

 ditions of existence. 



