6 Studies in the Theory of Descent. 



substances, which might induce different deposits 

 of colour in the wings of the perfect insect. This 

 latter hypothesis was readily confuted by the fact, 

 that the most strongly marked of the dimorphic 

 species, A. Levana, fed exclusively on Urtica major. 

 The caterpillar of this species certainly exhibits a 

 well defined dimorphism, but it is not seasonal 

 dimorphism : the two forms do not alternate with 

 each other, but appear mixed in every brood. 



I have repeatedly reared the rarer golden-brown 

 variety of the caterpillar separately, but precisely 

 the same forms of butterfly were developed as 

 from black caterpillars bred at the same time under 

 similar external conditions. The same experiment 

 was performed, with a similar result, in the 

 last century by Rb'sel, the celebrated miniature 

 painter and observer of nature, and author of the 

 well-known " Insect Diversions" a work in use 

 up to the present day. 



The question next arises, as to whether the 



causes originating the phenomena are not the same 



as those to which we ascribe the change of winter 



and summer covering in so many mammalia and 



birds whether the change of colour and marking 



\ does not depend, in this as in the other cases, upon 



! the indirect action of external conditions of life, 



i. e., on adaptation through natural selection. We 



are certainly correct in ascribing white coloration 



to adaptation 5 as with the ptarmigan, which is 



6 [In 1860 Andrew Murray directed attention to the dis- 



