208 Heredity and Environment 



possibilities would forever remain germ cells were it not for 

 environmental stimuli. The realization of germinal possibilities 

 is dependent upon the responses of the germ to environmental 

 stimuli, and although heredity is a relatively constant factor while 

 environment is a more variable one, nevertheless the two are in- 

 dispensable to development. Only by experiment can the relative 

 importance of heredity and environment in development be de- 

 termined. Extensive experiments have been made within recent 

 years on developing animals and plants in order to discover the 

 factors involved in development, and the modifications which may 

 thus be produced are very striking. 



B. EXPERIMENTAL MODIFICATION OF DEVELOPMENT 



The study of development under experimental conditions has 

 given rise to a new branch of biology, viz., experimental embry- 

 ology or the physiology of development. By changes in environ- 

 mental conditions notable modifications may be produced in adult 

 organisms, but these modifications are much greater when the 

 changed environment acts on the organism during the period of its 

 development. 



I. Developmental Stimuli 



It is by no means easy to define such general terms as "environ- 

 ment," "stimulus," and "response." In its common use "environ- 

 ment" means all that lies outside the individual, if it is defined 

 from the standpoint of the entire organism. But from the stand- 

 point of an organ or cell it is the surrounding organs, cells or 

 fluids of the body ; the latter may be defined as "internal envi- 

 ronment." If developmental stimuli arise outside the organism 

 they are plainly extrinsic or environmental, but if they arise 

 within the organism they are said to be intrinsic though they 

 may be due to changes in the "internal environment." 



Stimuli are chiefly energy changes of a physical or chemical 

 nature. A stimulus to which an adult organism responds by 



