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, (6) 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- 
