ORGANIC ADAPTATION 317 



adaptability to environmental change — is at once the most strik- 

 ing and indispensable adaptation. 



1. Adaptations Essentially Functional 



Although the changes of the environment are almost incon- 

 ceivably complex — witness the kaleidoscopic series of events ex- 

 hibited in the hay infusion microcosm — there are certain general 

 conditions which every environment must supply, and without 

 which life cannot exist. These are food, including water and oxygen, 

 and certain limits of temperature and pressure. (Fig. 18.) 



Food. As we know, food represents the stream of matter and 

 energy which is demanded for the metabolic processes of living 

 matter. And each and every element which forms an integral 

 part of protoplasm must be available. Since all protoplasm con- 

 sists chiefly of a dozen chemical elements, these, of course, must 

 be present ; and further, since protoplasm is a colloidal complex in 

 which water plays a fundamental role, life processes without water 

 are impossible. But the old adage that what is food for one is 

 another's poison has a broader significance than is immediately 

 apparent. Although it is true there are general 'food-elements' 

 which all life demands, it is equally true that the combinations 

 in which these elements must be presented to the organism, in 

 order to be available for its metabolic processes, are subject to 

 the widest variation. 



We have emphasized and contrasted the nutrition of a typical 

 animal, green plant, and colorless plant, and have seen the re- 

 ciprocal part which they play in the circulation of the elements 

 in nature; so it is hardly necessary, with these facts in mind, to cite 

 special cases in order to illustrate the adaptation of organisms to 

 special nutritional conditions. Perhaps the demands of the Yeasts, 

 that affect human life from so many angles, will suffice. (Fig. 17.) 



The Yeasts include a host of microscopic colorless plants which 

 play an important part in the simplification of organic compounds. 

 An ounce of "brewers' yeast" contains about five billion cells. 

 Since they are devoid of chlorophyll, Yeast cells, of course, lack 

 photosynthetic powers, though like many other colorless plants 

 they are not dependent upon proteins for nitrogen but obtain it 

 in less complex form. But the essential fact of interest at present is 

 the chemical changes associated with Yeast metabolism — the 

 transformation of a large proportion of the sugar content of the 



