36 ENVIRONMENTAL RELATIONS 



disappear from the locality with changes in environment produced 

 either by themselves or by physiographic or climatic changes (57, 58). 

 The general growth or evolution of environmental conditions and 

 the communities which belong to them, are included under succession. 

 The word succession is used in three distinct senses. We speak of 

 (a) geological succession, (b) seasonal succession, and (c) ecological 

 succession. 



a) Geological succession is primarily a succession of species through- 

 out a period or periods of geological time. It is due mainly to the 

 dying-out of one set of species and the evolution of others which take 

 their places, or in some cases to migration. 



b) Seasonal succession is the succession of species or stages in the life 

 histories of species over a given locality, due to hereditary and environic 

 differences in the life histories (time of appearance) of species living there. 



c) Ecological succession of animals is succession of mores over a given 

 locality as conditions change. If species have relatively fixed mores we 

 have succession of species. When mores are flexible we may have the 

 same species remaining throughout, with changes in mores. It is on the 

 basis of ecological succession that we arrange the data presented in chaps, 

 iv to xiv and proceed with discussion. The response of the organism 

 to the condition of the environment is only occasionally or partially 

 dependent upon ecological succession, but this is the only notable 

 phenomenon about which habitats and animal communities can be 

 arranged into a natural order. 



2. CLASSIFICATION OF COMMUNITIES 



Ecological classification of animals must be based upon community 

 or similarity of physiological makeup, behavior, and mode of life. Those 

 natural groups of animals which possess likenesses are the communities 

 which we must recognize. One community ends and another begins 

 where we find a general more or less striking difference in the larger mores 

 characters of the organisms concerned. These communities usually 

 occupy relatively uniform environments (58a). 



a) Ecological terminology (13). — Terminology in ecology is still 

 unsettled and changing. Groupings have thus far been based upon 

 similarity of habitat. Habitat likenesses have in general been based 

 upon general resemblances. General resemblances have not always been 

 accompanied by similar physical conditions. In general there has 

 been an agreement in the recognition of strata, of associations as com- 



