ANIMALS AND THEIR ENVIRONMENTS — ADAMS. 527 



habitats, and parasitic, herbivorous, and predaceous habits. The 

 study of ecology, upon the basis of such a unit, may be called aggre- 

 gate ecology. Still another unit is available, based upon the animals 

 which live together in a given combination of environmental condi- 

 tions, as in a pond, on the shore of the sea, in a cave, within the bodies 

 of animals, on the floor of tho forest, or in the tree tops, etc. The 

 animals found living together in such conditions form an animal 

 association or a social community, and the study of the responses of 

 such a community is the province of assoclational ecology. 



8. THE ANIMAL ASSOCIATION. 



In the study of the animal association as a unit, we consider it as 

 an agent, whose modes of activity, or responses, are of primary inter- 

 est. We desire to know the kinds of animals which compose the 

 community, the optimum and limiting influences which control its 

 activity, the character of its responses, and the orderly sequence of 

 changes in the environment to which it is responding. 



The maintenance of an association depends upon the maintenance 

 of the individual members which compose it, just as the maintenance 

 of the entire animal depends upon the activities of the cells. There 

 is the same basis for speaking of the responses of the association as 

 there is for speaking of the responses of the individual. The asso- 

 ciation can continue to exist indefinitely only in such environments 

 as possess, in available form, substance and energy for its individual 

 members. The activities of the individuals transform energy and 

 substance, producing growth, development, multiplication, and be- 

 havior. The persistence of an association in a given habitat brings 

 about the formation of certain waste products, which if not changed 

 or transformed at a certain rate, or transported from the environ- 

 ment in some way, tend to limit the optimum activity of the indi- 

 viduals and of the association."- In the association, as in the individ- 

 ual, there must be an internal relative balance before there can be 

 such a surplus of energy that the association can radiate or exert 

 outward stress or pressure. An association which is only maintain- 

 ing itself is not at an optimum, for in this latter condition there is 

 a surplus of energy, and ; the activity, rate of multiplication, and 

 favorable development under normal conditions are favorable to the 

 extension of the association. The pressure which such an association 

 exerts is shown by the progressive extension of its range of influence. 

 By the active movements of the animals, by the activity of the en- 

 vironment, or by both together, they tend to invade other habitats 

 and areas, and in such of these as afford favorable conditions they 

 tend to survive and extend the area of the association. From the 



