40 HEREDITY AND ENVIRONMENT 



cells; cells which at an early stage were sensi- 

 tive to many kinds of stimuli give rise to 

 daughter cells which are especially sensitive to 

 one particular kind of stimulus, such as vi- 

 bration, light, or chemicals. 



Functions develop from a generalized to a 

 specialized condition by the process of "physi- 

 ological division of labor" which accompanies 

 morphological division of substance. But as 

 in the union of hydrogen and oxygen a new 

 substance, water, appears which was not 

 present before, and as in the development of 

 structures new parts, which were not present 

 in the germ, appear by a process of "creative 

 synthesis," so new functions appear in the 

 course of development, which are not merely 

 sorted out of the general functions present at 

 the beginning, but which are created by the 

 interaction and synthesis of parts and func- 

 tions previously present. For example, Lane 

 lias shown that young rats are quite insensible 

 to light until several days after birth although 

 the eye begins to form at a very early stage. 

 Doubtless every part of the eye is functioning 

 in a general way during the entire develop- 



