EVIDENCES OF EVOLUTION 59 



the chrysalid is affected by the colour of the 

 surroundings, which operates in some mys- 

 terious way through the skin. When the 

 pupation occurs on a light background the 

 chrysalids are lighter; when on a dark back- 

 ground the chrysalids are darker. This is 

 undoubtedly an advantageous response, for 

 it has been proved experimentally that in 

 natural conditions survival depends in great 

 part on the inconspicuousness of the pupae 

 in the place where they are fastened. 



In connection with experimentally induced 

 adaptive responses Professor T. H. Morgan 

 makes an important note: "It is remarkable 

 how rare are adaptive structural responses, 

 when we recall the fact that adaptation of 

 the organism to its surroundings is one of its 

 most characteristic properties. The poverty 

 of adaptive structural response does not 

 encourage one to look to external agents as 

 having brought about directly the structural 

 adaptation of organisms to external con- 

 ditions, 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 



