( xcviii ) 



I urged upon Mr. Marshall that the few recorded examples 

 of capture or observation in coitu were insufficient 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 diiferent forms are 

 produced in response to some stimulus which acts at a speci- 

 ally 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, stinicturally widely different, must 

 nevertheless be placed within the limits of a single species. 

 The investigation of the probable physiological causes of differ- 

 ence 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. 



III. Individual Modification : * — One of the most striking 

 developments of i-ecent 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 susceptible 

 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 

 realised only in the appropri ite environment — realised 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 facts which pointed in 

 its direction. Thus in Schmankewitsch's experiments 

 Crustacea of the species Arteviia salina were described as 

 gradually changing in the course of generations, as the result 

 of a progressive freshening of the water in which they were 

 kept, until they took on the characters of the genus Bran- 

 chipus. On this subject Darwin wrote to Karl Semper, 

 February 6, 1881 : — "When I read imperfectly some years 



* " A structural cliange wrought during the individual's lifetime (or 

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

 (or congenital)." Diet, of Phih and Psych., ed. by J. Mark Baldwin, 

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



