INFLUENCE OF ENVIRONMENT 337 



marked structural differences between the 

 workers and the queens but the differences in 

 their habits and instincts are even more strik- 

 ing; all of these differences whether in bodily 

 structure or in instincts are determined by the 

 character of the food and not by heredity. 

 Innumerable cases of a similar sort could be 

 named which show the great effect of environ- 

 mental stimuli on development. 



C. FUNCTIONAL ACTIVITY AS A FACTOR 

 OF DEVELOPMENT 



Another factor of development which is 

 partly intrinsic and partly extrinsic is func- 

 tional activity or use. Functional activity is 

 response to stimuli which may be external or 

 internal in origin. The entire process of de- 

 velopment may be regarded as a series of such 

 responses on the part of the organism, whether 

 germ cell, embryo or adult, to such stimuli. 

 The nature of the response is determined by 

 the nature and state of the organism at the 

 time and by the character of the stimulus. By 

 the normal or usual series of stimuli certain 



