STRUCTURE AND ENVIRONMENT 23 



ment is propelled by the heart, which is an organ possessing definite 

 form and a certain type of activity. In the case of a complex animal 

 like the black bass, we might elaborate upon the relations of form and 

 structure to activity and function almost indefinitely. It is obvious 

 that the two features are related in the bass. When we consider animals 

 which possess less elaborate structure, the relations become less obvious 

 upon mere inspection because organs are less clearly differentiated, but 

 they are still more easily demonstrable through methods employed by 

 the biologist. 



In both the lower and the higher organisms, structure may be con- 

 trolled by activity. If one cuts off the posterior end or tail of a flat- 

 worm, a new tail is formed. Professor Child (37) found that if the 

 animals were permitted to crawl on the bottom of the containing vessels 

 while the new part was growing, the tail was pointed. If they were not 

 allowed to crawl, the tail was rounded. There are many other pieces 

 of experimental work which show that structure may be modified by 

 function. In but few cases, however, has the modified structure been 

 found to be inherited. 



At present the relations between function and structure have not 

 been investigated in many cases, but Child has made their relation 

 quite clear by comparing the organism to a river. " The relation between 

 structure and function in the organism is similar in character to the 

 relation between the river as an energetic process and its banks and 

 channel. From the moment that the river began to flow it began to 

 produce structural configurations in its environment, the products of 

 its activity accumulated in certain places and modified its flow." It 

 deposits and removes, and thus continually "moulds its banks and 

 bottom, forming here a bar, there an island, here a bay, there a point 

 of land, but still flowing on, though its course, its speed, its depth, the 

 character of the substances which it carries in suspension and in solution 

 all are altered by the structural conditions which it has built up by its 

 own past activity" (37a). Thus we see that function and structure are 

 mutually interactive and mutually interrelated, and, for the sake of 

 clearness only, we shall separate the two rather sharply in our discussion. 



III. The Basis for the Organization or Ecology 



We have already noted that ecology deals with animal life as lived in 

 nature, or, in other words, with the relations of animals to their environ- 

 ments. The question of what aspects of these relations are most im- 

 portant and best suited as a basis for the organization of ecology at 



