( 30.) 
I urged upon Mr. Marshall that the few recorded examples 
of capture or observation 77 coitw 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 different 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, structurally 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. 
IU. Individual Modijication : *—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 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 appropriate 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 Artemia 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 change wrought during the individual's lifetime (or 
acquired), in contradistinction from variation, which is of germinal origin 
(or congenital).” Dict. of Phil. and Psych., ed. by J. Mark Baldwin, 
New York and London, vol. ii, 1902, p. 94. 
