Influence of Environment 223 



they become fertile females or queens; if fed on ordinary "bee 

 3read" they become infertile females or workers (Fig. 84). There 

 are marked structural differences between the workers and the 

 queens but the differences in their habits and instincts are even 

 more striking; 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 environmental stimuli on 

 development but not upon heredity. 



C. FUNCTIONAL ACTIVITY AS A FACTOR OF DEVELOPMENT 



Another factor of development which is partly intrinsic and part- 

 ly extrinsic is functional activity or use. Functional activity is re- 

 sponse to stimuli which may be external or internal in origin. The 

 entire process of development may be regarded as a series of such 

 responses on the part of the organism, whether germ cell, embryo 

 or adult. The nature of the response is determined by the nature 

 and state of the organism and by the character of the stimulus. 

 By the normal, or usual, series of stimuli certain parts are kept 

 active while other parts are kept inactive or are inhibited. 



Developmental Movements. Normal development is depend- 

 ent on the correlated activity of many parts of the organism. If 

 in any part stimuli and responses are lacking the development 

 of that part is arrested or inhibited. For example in the cleav- 

 age stages different substances are sorted and localized by pro- 

 toplasmic movements within cells and these substances are then 

 isolated by cell divisions and by the formation of partition walls 

 between cells; these protoplasmic movements occur in response 

 to stimuli and if these movements are stopped cleavage and dif- 

 ferentiation are arrested. In later stages the infolding of the 

 gastrula, or neural tube, or alimentary canal, and the foldings of 

 layers in general, which play so important a part in development, 

 are due to the movements of substances within cells and to the 

 movements of cells in the layers in which they lie, and if these 

 movements are inhibited normal development is prevented. 



