15 



the methods which are useful. Because the anunal body has been 

 found to be composed of a single cell or a multitude of cells, a com- 

 mon belief has grown up that the cell is the natural unit for study. 

 This opinion seems to be due to overlooking the fact that there is just 

 as much reason for considering the ivholc animal as the unit. The 

 unicellular animals are whole animals as truly as they are cells, and 

 in multicellular animals the activity of single cells means little inde- 

 pendently of the animal as a whole. It thus seems that ecologically at 

 least the smallest valuable unit for study is the individual animal. The 

 responses of the individual, as a kind of animal, to its condition of 

 existence form the basis for what may be called individual ecology. 

 Animals which are related by descent from common ancestors, as a 

 community of social animals (e. g., an ant colony), or taxonomic 

 units, such as genera, families, orders, etc. (e. g., fish, birds, catfishes, 

 and salamanders), are also units which may be studied ecologically. 

 Some of these hereditary units are, ecologically, fairly homogeneous, 

 as, for instance, when a taxonomic unit is equally distinct ecologically : 

 e. g., the woodpeckers with their arboreal habits. In other cases the 

 taxonomic unit contains animals of great ecological diversity, as in the 

 case of beetles, which possess almost unlimited ecological diversity, in- 

 cluding littoral, aquatic, subterranean, and arboreal habitats, and para- 

 sitic, herbivorous, and predaceous habits. The study of ecology, upon 

 the basis of such a unit, may be called aggregate ecology. Still another 

 unit is available, based upon the animals which live together in a given 

 combination of environmental conditions, as in a pond, on the shore 

 of the sea, in a cave, within the bodies of animals, on the floor of the 

 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 

 associational ecology. 



8. thf; 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 com- 

 munity, 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 



