INTRODUCTION 3 



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, 1 

 but it has served only as a background, and in comparing them we 

 have relied upon comparison of (a) habitats and (b) 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. 



