112 DEVELOPMENT AND HEREDITY. 



by the forces of the environment acting upon the 

 organism. This development is also an adaptation, 

 inasmuch as it enables the organism to perform 

 more readily those actions which the environment 

 makes necessary for its existence. 



It may be objected that the stimuli are too few 

 in number and too constantly the same to produce 

 any such variety of form and habit as is found even 

 among many unicellular animals. But if we con- 

 sider the matter more closely, we shall see that 

 there is an inevitable gradual change in the influence 

 of the stimuli, and that this change induces new 

 effects, and opens the way for new stimuli to an 

 almost infinite degree. Let us follow our hypo- 

 thetical organism a step farther. By the assimila- 

 tion of new particles of matter the bulk is doubled, 

 and the mass is increased more rapidly than the 

 surface. The amount of oxygen necessary for the 

 life of the protoplasm increases as the mass in- 

 creases ; but the amount of oxygen that can be 

 absorbed from the surrounding medium increases 

 only as the surface of the protoplasm increases. 

 Therefore in the process of growth the mass would 

 at some time become too great for the amount of 

 oxygen that can be taken in by the surface. The 

 result would be the asphy.xiation of the interior 

 part of the mass of protoplasm. In the movements 



