INTRODUCTION 
Just at the beginning of the present century, there seems to have 
been a revival of interest in plants and animals in relation to their 
environments, and various workers have turned from the study of 
anatomy and classification in the laboratory to the study of organisms in 
nature. In this, the botanists have preceded the zoGlogists, in success 
if not in time. In 1901 Dr. H. C. Cowles published a bulletin on the 
Plant Societies of the Chicago Area. ‘This was one of the first attempts of 
an American biologist to treat all the plants of a given area in a strictly 
ecological manner. This study of all the organisms of an area, from the 
point of view of their relations to each other and to their environment, 
is still a new or at least a renewed idea. Zodlogists have devoted most 
of their attention to the study of animals from the standpoint of a single 
individual and of single species. Practically all of the more general 
study has been comparative. We have comparative anatomy, compara- 
tive embryology, comparative physiology, and comparative psychology. 
These are comparisons of the structure or physiology of one species, or 
group of species, with that of another species or group of species. 
Our point of view is very different. We shall deal with many species 
from the standpoint of their dependence upon each other and their 
relations to their environments. We shall attempt to present what has 
been learned upon this subject during several years of investigation and 
field teaching. In the spring of 1903, the writer made his first field 
excursion in the Chicago area, and from that time has been engaged in 
further study of the subject. 
The study of organisms in relation to environment is entitled ecology. 
The definition of ecology, like that of any growing science, is a thing to 
be modified as the science itself is modified, crystallized, and limited. At 
present, ecology is that branch of general physiology which deals with the 
organism as a whole, with its general life processes, as distinguished from 
the more special physiology of organs (51), and which also considers the 
organism with particular reference to its usual environment. 
Undertaking such a study from the point of view of many organisms 
involves matters of both ecological and taxonomic classification. Classi- 
fication of animals is difficult because animals are so exceedingly numer- 
ous. There are probably from 10,000 to 20,000 species of animals which 
the naturalist may encounter in the area which we are treating, while 
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