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 toa 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 OF 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 
