592 VICTOR E. SHELFORD 
B. THE NATURE OF THE ENVIRONMENT 
The animal environment is a complex of many factors. Each 
is dependent upon another or several others, in such a way that 
any change in one factor affects several. Some of the most im- 
portant environmental factors are water (atmospheric moisture), 
oxygen, carbon-dioxide, nitrogen, temperature, pressure, currents, 
excretory products, food, enemies, and materials for abode (soil, 
vegetation, etc.). In nature, such combinations of these and other 
factors, in the proportion requisite for the maintenance of the 
life of a considerable number of animal species, are cal ed environ- 
mental complexes (Davenport ’03). 
C. ENVIRONMENTAL RELATIONS OF ANIMALS 
The only features which space will permit us to discuss are the 
physiological and ecological relations. In this field we must 
confine ourselves to a comparison of plants and animals and the 
bearing of the important environmental re’ations on geographic 
distribution. 
1. Comparison of the environmental phenomena of plants and 
animals 
An organism is a system of inter-dependent and definitely 
related processes (i.e., a system in dynamic equil brium). The 
definite relations of the inter-dependent processes of the organism 
(dynamic equilibrium) may be disturbed by changes in the exter- 
nal forces which surround the organism and to which the processes 
are adjusted (Jennings, 06). Such a disturbance is what we ordi- 
narily call stimulation. 
With this idea as a background, we give in parallel columns a 
comparison of the more obvious relations of plants and animals 
to their environments, as shown by experimental work. The 
column on the right is written by Dr. H. C. Cowles, Associate 
Professor of Plant Ecology in the University of Chicago. 
