CHAPTER III 
THE ANIMAL ENVIRONMENT: ITS GENERAL NATURE AND ITS 
CHARACTER IN THE AREA OF STUDY 
I. NATURE AND CLASSIFICATION OF ENVIRONMENTS (35¢, 55, 58)" 
The environment is a complex of many factors, each dependent 
upon another, or upon several others, in such a way that a change in any 
one effects changes in one or more others. The most important environ- 
mental factors are water, atmospheric moisture, light, temperature, 
pressure, oxygen, carbon dioxide, nitrogen, food, enemies, materials 
used in abodes, etc. In nature the combinations of these in proportions 
requisite for the abode of a considerable number of animals are called 
“environmental complexes” (55). It is our purpose to consider animals 
as inhabiting environmental complexes, rather than to isolate their 
responses to various single factors. The consideration of environmental 
complexes in any comprehensive way would consume much space and 
require extensive and special knowledge of many fields. Accordingly, 
we can present here only the briefest outline of some of the principles of 
classification, and the important features. 
If one is to understand the most elementary principle of the classi- 
fication of environments, he must recognize the distinction between 
local and (55, 58a) climatic environmental complexes. Local complexes 
are often referred to as secondary or minor conditions or as edaphic or 
soil conditions. The climate, and such features as types of vegetation 
covering large areas, e.g., steppe, deciduous forest, etc., are commonly 
regarded as climatic. Opposed to these, and lying within them, are the 
local conditions, such as streams, lakes, soils, exposure, etc, which are 
only indirectly dependent upon climate. The idea can be better illus- 
trated by the desert than by our own region. For example, in the 
Mohave Desert, the climatic conditions may be characterized as hot 
and arid. Within this desert are a few streams fed by mountain rain- 
fall. These streams are local conditions in themselves, and produce 
others, such as moist soil, and types of vegetation which do not belong 
to the desert. Within the area about Chicago are represented two 
geographic complexes, the savanna and the deciduous forest, and lying 
t Numbers in the text in parentheses refer to references in the Bibliography 
(pp. 325-36). 
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