AGENCIES, HABITAT, AND GROWTH 93 



This characteristic vital action is the visible expres- 

 sion of a reciprocal exchange; certain materials and 

 forces from the other world, entering the protoplasm, 

 usually through specific doors or passageways, and 

 ultimately reaching every living part. Certain other 

 things are discharged from these parts into the outer 

 world, through their own specific exits. Life endures 

 only so long as this cooperative exchange endures. On 

 the whole, it is a profitable one, that is, more is re- 

 ceived and retained than is given off; otherwise growth, 

 the chief manifestation of living things, could not take 

 place. 



All vital processes, therefore, require for their con- 

 tinued being definite materials and forces from the 

 outer world. Every living point, however minute and 

 remote that point may be, must receive within a de- 

 finite period its due share of food, air, and stimuli; 

 and harmful agents must be excluded, or removed, else 

 it will cease to live or to grow. 



Thus all living things, and all their organs, by 

 means of this process of self-insulation and assimilation, 

 tend to acquire a definite localization, and structural 

 individuality. That is, they tend to grow in their own 

 likeness through their power to select and add to them- 

 selves other things like themselves; they are protected 

 against contamination, or dilution, or perversion, by 

 their power to reject and exclude. 



This process of growth tends to go on indefinitely, 

 subject to the new demands created by growth. 



4. Checks and Releases to Growth. But just there 

 a new factor inevitably arises ; for growth cannot take 

 place without changing the original conditions under 

 which growth was initiated. This change is brought 



