ASPECTS OF KINETIC EVOLUTION 235 



CLASSIFICATION OF INTRASPECIFIC DIFFERENCES. 



Intraspecific differences may be classified by reference to 

 three considerations ; the nature of the diversity, its origin or 

 occurrence, and its relation to environmental fitness. Such a 

 classification is open to the objection that it requires an advance 

 decision upon the evolutionary bearings of the facts which are 

 being classified for evolutionary purposes. This objection also 

 applies, however, to all preceding efforts at classifying vari- 

 ations. Such classifications have no value, of course, as the 

 basis of arguments. Their use is purely that of permitting an 

 orderly arrangement of materials and of illustrating distinctions. 

 They aid in discrimination, not in demonstration. 



The utility of the proposed arrangement may be best appreci- 

 ated by thinking of it, not as a classification, but as affording 

 points of view or avenues of approach to the study of the intricate 

 complexities of evolutionary problems. The purpose of physio- 

 logical study is not classification, but the comprehension of 

 causal relations. 



Differences oj Growth Stages. — Changes of size, form, 

 structure, and function shown in the life-history of normal mem- 

 bers of the species, including metamorphosis and alternation of 

 generations and structural phases. The forms of diversity 

 grouped under this head would not be called variations except 

 in the most general sense of the term, but they must be taken 

 into account in making a complete outline of intraspecific dif- 

 ferences. 



Differences of Normal Descent (Heterisni). — Individual and 

 other differences, including those of sex and polymorphism, 

 which appear among the members of the species under normal 

 conditions of interbreeding in the same environment, and even 

 among the simultaneous offspring of the same parents. 



Differences of heterism have no relation to accommodational 

 fitness, though they may assist in the evolution of adaptive 

 characters. They have sometimes been called fortuitous or 

 fluctuating variations because they had no apparent utility, the 

 organic advantage of diversity of descent not having been 

 recognized. 



Differences of Accommodation to Environment (Art ism). — 



