EVIDENCES OF EVOLUTION 61 



encourage one to look to external agents as 

 having brought about directly the structural 

 adaptation of organisms to external conditions, 

 even if it could be shown that such influences 

 are inherited." 



Many naturalists have experimented with 

 the pupae of butterflies and moths, subjecting 

 them, for instance, to unusual conditions of 

 temperature, and many very interesting re- 

 sults have been reached. In cases where there 

 are distinct summer and winter adult forms, 

 the pupa which should give rise to the former 

 may be made to give rise to the latter, or the 

 pupa may be affected by cold or by heat so 

 that what emerges resembles not the local 

 form of the species, but a northern or southern 

 variety. Perhaps the most important general 

 result from our present point of view is that 

 " the differences effected by changes in the 

 environment have been shown in some cases 

 to resemble the kind of differences that 

 separate species from each other." This is 

 suggestive and important, though it does not 

 by any means prove that species have arisen 

 in this way. 



Mr. J. T. Cunningham put very young 

 flounders in an aquarium lighted from below, 

 and observed that as they underwent their 

 peculiar metamorphosis the pigment first 



