EXTERNAL INFLUENCES AS CAUSES OF VARIATION 229 



able to extract the materials for their activities often from the 

 most forbidding sources and under the most discouraging cir- 

 cumstances. Indeed, many forms of animal activity are depend-, 

 ent for support not so much upon the materials of the food as 

 upon its energy. This applies more especially to mature animals 

 and their functional activity, but we also remember that in the 

 business of body building, materials are required in large excess 

 of the actual amounts retained in the body. 



Lower forms of life, however, seem often greatly dependent 

 upon their culture medium. Thus many bacteria grow quite dif- 

 ferently upon the potato and in agar or in beef broth, and a germ 

 disease often presents in one species symptoms substantially 

 different from those it presents in another. 



The animal and the plant (among the higher forms) are 

 nourished upon fundamentally distinct plans. Both require 

 that food be in the soluble form before it can be useful, and 

 each makes use only of such available materials as it may need. 

 But the animal is provided with an elaborate excretory system 

 by which it frees itself of all residues, both of undigested food and 

 of broken-down tissues, and also of digested food in excess of 

 requirements. By this means the animal is promptly freed from 

 all redundant food material. 



It is not so with the plant. It takes in food by absorption at 

 its roots, and the water carries with it anything and everything 

 that may happen to be dissolved. If nothing poisonous enters, 

 the plant will live, but it will be loaded with residues, because it 

 has no excretory system. The water passes off by evaporation 

 at the leaf surface, leaving at that point large quantities of what- 

 ever was dissolved in the juices of the plant. We are not sur- 

 prised, therefore, that vegetation of the same species differs 

 very widely in composition in different localities, especially with 

 respect to mineral content, depending upon the character of the 

 soil in which it was grown. This difference may be, as in the 

 case just cited, quite independent of vital processes, and due to 

 nothing more than accident. 



Plants, and the simpler organisms generally, are of necessity 

 far more dependent upon their environment, and especially their 

 food, than are the higher animals, that have to a large extent 



