538 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



different kinds of vegetation but also in different climates, and with 

 different physiographic conditions. As Cowles (1911) has shown, 

 there are several cycles or series of successions of vegetation. Many 

 of these changes are dependent upon physical conditions which are 

 equally potent in their influence upon animals. Thus physical and 

 vegetational changes in combination influence animals directly and 

 indirectly, and in the conditions due to this fact we find the basis for 

 the important control which vegetation exerts upon animals. 



Animals themselves form an important part of their own environ- 

 ment, not only in their relation to their own kind, as mates or as prog- 

 eny, but also as members of an animal community whose members 

 must adjust their activities to one another through symbiotic, com- 

 petitive, or predatory relations. If any animal becomes abnormally 

 abundant, that is, more numerous than the conditions can support, 

 this number in itself becomes a weakness, through the positive attrac- 

 tion of the organisms (plant and animal) which are able to prey upon 

 it, and soon the normal abundance is restored. For example, in a 

 coniferous forest, bark beetles {Scolytoidea) may increase to such an 

 extent that the forest is largely destroyed, and a succession is pro- 

 duced in the vegetation as the conifers are replaced by a growth of 

 aspen and birch. As a result of this destruction of the kind of food 

 and habitat essential for the next generation of beetles, a proper 

 habitat is lacking, and the restoration of the normal number of 

 beetles is hastened. This same example also shows how one kind of 

 animal may influence the character of a whole community by its 

 control over the vegetation. 



The influence of man must be looked on from the same standpoint 

 as one views the activity of any other animal ; as that of a member of 

 an animal community. He hastens and retards the changes in his 

 environment as do other animals. In general his early methods are 

 predatory; he reaps where he does not sow; but later the milder 

 competitive and symbiotic relations and the constructive or produc- 

 tive aspect become more prominent. Civilization is an attempt to 

 make the environment " to order," but as yet man has not learned 

 how to produce a permanent " optimum " along the lines of an eco- 

 logical community. As has already been said, to understand man 

 we should view him as an integral part of an ecological community, 

 as one member of a biotic community of plants and animals, or at 

 least of an animal community which includes all animals that are 

 influenced by man — and not consider him, as some students do, as a 

 distinct entity with little regard to his animal and plant associates. 



The main features of the preceding discussion may be summarized 

 as in the following table : 



