ASPECTS OF KINETIC EVOLUTION 253 



ticular conditions. Where the alternatives are sharply defined 

 as in the two sexes of man and the higher animals this theory 

 might appear to be applicable, but where, as in many plants, 

 there are, even in the same species, all stages of sexual differ- 

 entiation, or many distinct castes or forms, with or without 

 reference to the sexes, the theory of determinants becomes im- 

 practicably complex. 



In the experiments of Standfuss with butterflies it has been 

 found possible, by changes in the temperatures in which the 

 pupae are kept, to influence the colors of the adults so as to 

 approximate those of a different geographical variety or seasonal 

 form. It has been inferred as a consequence that temperature 

 is a direct evolutionary factor in causing one species to change 

 into another. In reality, however, this is but one of the many 

 instances in which failure to distinguish between the taxonomic 

 and the evolutionary standpoints has permitted confusion to 

 enter. Some of these seasonal and geographical forms of but- 

 terflies have been named as distinct species, but if it be found 

 that the supposedly distinctive characters are merely artisms or 

 accommodations to temperature, the proper step is to revise our 

 classification before attempting to use it as a basis of evolu- 

 tionary inferences. The largest possibility suggested in the 

 present instance is that abnormal temperatures may induce in one 

 part of a species a character which another part has reached by 

 normal evolutionary process. The fact that the different geo- 

 graphical color races may have been described and named as 

 species and varieties cannot be made to prove that temperature 

 is a cause of species-formation. 



This power of accommodation to the environment, specific 

 elasticity or artism, may be thought of for evolutionary purposes 

 as a general character of the species, but like other characters it 

 is possessed in different degrees by different individuals, and this 

 difference of degree is as heritable as any other feature. Some 

 individuals and strains of a species may have greater range of 

 elasticity on both ends of the series, while others have greater 

 freedom of change in one direction than in the other, for example, 

 they can become very hairy, but not very smooth. Still again, 

 we find mutative variations toward a restriction of the normal 



