SEASONAL FORMS AND DIAGNOSIS 73 



where the widest difference in colour and pattern exists, 

 in combination with others which are far more deep- 

 seated, I urged upon Mr. Marshall that the few recorded 

 examples of capture or observation in coitu were insuf- 

 ficient evidence of specific identity, and that nothing- 

 short of Epigony would suffice. 



In seasonal dimorphism, in the dimorphism of social 

 insects, and doubtless in a large proportion of other 

 examples, it is probable, indeed often certain, that the 

 different forms are produced in response to some stimulus 

 which acts at a specially susceptible period of the life- 

 history ; but from the point of view of the systematist 

 the mature individuals can only be known as forms 

 which, structurally widely different, must nevertheless 

 be placed within the limits of a single species. The 

 investigation of the probable physiological causes of 

 difference is, however, of the utmost importance from 

 other points of view. Altogether apart from its bearing 

 upon dimorphism, the effect of individual susceptibility to 

 stimulus requires treatment in a separate category. 



c. Individual Modification 1 : — One of the most striking 

 developments of recent years has been the growth in the 

 number of these very cases in which an individual animal 

 or plant has been rendered by Natural Selection sus- 

 ceptible to some stimulus associated with each one of its 

 possible normal environments. Every individual of such 

 species comes into the world with two or more very 

 distinct and very different possibilities before it, each 

 of which will be realized only in the appropriate environ- 

 ment — realized as the response to some stimulus provided 

 by the environment itself. We can see clearly that this 

 idea was in Darwin's mind, although there were then but 

 few observations which pointed in its direction. Thus in 

 Schmankewitsch's experiments Crustacea of the species 

 Artemia salina were described as gradually changing in 

 the course of generations, as the result of a progressive 



1 'A structural change wrought during the individual's lifetime (or 

 acquired), in contradistinction from variation, which is of germinal origin 

 (or congenital).' Did. of Phil and Psych., ed. by J. Mark Baldwin, New 

 York and London, vol. ii. 1902, p. 94. 



