INTRODUCTION 2 
appear in either the lists and tables or in the descriptions of figures. No 
attempt has been made to include the same animals in the text, tables, 
and illustrations, as the only aim has been to make each part as useful 
as possible. 
While the amount of work that might have been done along the lines 
here represented is infinite, this work represents only a general survey. 
The data are incomplete, but we believe them to be adequate for the 
purpose of illustrating the principles involved. Considerable experi- 
mental work has been conducted with reference to animal communities,’ 
but it has served only as a background, and in comparing them we 
have relied upon comparison of (a) habitats and (0) species. The latter 
is fraught with many dangers, for it assumes, in the absence of evidence 
to the contrary, that the physiological character of a species is the same 
in the different situations in which it is taken. Observation has shown 
this to be true for most species within rather uncertain limits. There 
are, however, many well-known exceptions to this, some of which are cited 
in the text. Such use of species is certainly to be avoided in the study 
of the extensive or geographic distribution of animals, and it remains 
to be seen how far it may be employed locally. Certainly ecology cannot 
reach its best development if it relies upon such a method. Whatever 
further investigation may prove on this point, it is hoped at least that 
we may be ablo to suggest problems which may be attacked from new 
points of view. Should this object be accomplished, the work will have 
served its purpose. 
«The term community, as used here, refers to all the animals living in the same 
surroundings. 
