56 THE PROBLEMS OF EVOLUTION 



basis the only admissible source of variations will 

 be recognized as these four factors. Their inter- 

 actions vary in results, but evidences of one or 

 another kind are everywhere available. If fluctu- 

 ations take place in the external environment, the 

 responses of individuals are modified. Such physi- 

 cal stimuli as intense sunlight and activity produce 

 demonstrable, usually useful, results which Conk- 

 lin has classified as follows : ^ 



Stimulus Beneficial Response 



Increased light Increased pigmentation 



Increased friction Increased thickness of epidermis 



Increased use Increased size or strength 



Unusual foods Appropriate digestive fluids 

 Unusual temperatures Acclimatization 



Poisons or toxins Toleration or antitoxins 



Injury Regulation or regeneration 



Changes in the organic environment are more 

 likely to lead to a modification of behavior than of 

 visible organization, although they may act upon 

 the organism in the form of a modification of food 

 supply. 



The organic results of such changes as these are 

 visible to us only in the individual, and have logi- 

 cally been called modifications. The term ac- 

 quired character is satisfactory but it has been so 

 sadly abused that it is better avoided. 



The possibility of changes occurring in the inter- 



6 Rice Institute Pamphlet, Vol. VIII, pp. 322-323, 1921. 



