GENETICS AND ETHICS 473 



other hand those possibilities must remain 

 latent and undeveloped unless they are stimu- 

 lated into activity by the environment. 



Functional activity or use is one of the 

 most important factors of development. Func- 

 tional activity is response to stimuli, which 

 may be external or internal in origin. The 

 entire process of development may be regarded 

 as an almost endless series of such responses 

 on the part of the organism, whether germ 

 cell, embryo or adult, to external and internal 

 stimuli. It is a truism that use strengthens a 

 part and disuse weakens it; it is likewise a 

 truism that responses which are oft repeated 

 become more rapid and more perfect, and in 

 this way habits are formed. Practically all 

 education, whether of man or of lower ani- 

 mals, consists in habit formation, in establish- 

 ing constant relations between certain external 

 or internal stimuli and certain responses of the 

 organism. At first these stimuli are largely of 

 external origin; later the external stimuli may 

 be replaced more and more by internal ones; 

 but whatever the source of the stimulus the 

 response of the organism to these stimuli is 



