60 THE PROBLEMS OF EVOLUTION 



characters are a result of the interacting factors, 

 heritage and environment, and it is probable that 

 all are potentially useful in relation to conceivable 

 environmental conditions, save possibly during 

 periods of development and atrophy. A structure 

 may become an adaptation to conditions which 

 have had no part in its production, and conditions 

 may bring about the development of structures 

 which do not adapt the individual to respond to 

 them. But regardless of these facts, the things 

 which enable the organism to meet conditions of 

 environment are adaptations. 



The changes which individuals undergo, on the 

 other hand, are in part evidences of the process of 

 adaptation. Each is provided at birth with a 

 hereditary equipment of adaptations, but as it 

 encounters fluctuating conditions of environment 

 it must adjust itself by the modification of its 

 previous condition. The deposition of pigment in 

 the skin is an obvious adaptive response, since it 

 provides protection against the ultra-violet rays 

 which bring it about. That individual responses 

 are not always adaptive is amply shown by Sum- 

 ner's experiments with white mice (see Ch. VI). 

 Increased size of appendages appears in no way to 

 be of value in relation to the higher temperature 

 which brought it about. 



It is, of course, difiicult to suggest the causes of 

 existing organs without risk of serious error. The 



