2 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



are wholly wanting in inorganic matter. Practically none of 

 the other distinctions usually given will stand close scrutiny 

 and critical analysis. 



A recent estimate by a reliable naturalist of the number of 

 known kinds of animals puts this number at 522,400. It is 

 quite certain that there are as many more not yet known. Of 

 these million living animal kinds each of us knows but few, 

 some of us very few indeed. And these few we usually know 

 most superficially; usually only their general external appear- 

 ance and a little about their habits. Yet this little that we 

 know about animals is sufficient to serve as an introduction to 

 the science of zoology, if we will think seriously of it and try 

 to arrange it in some orderly or classified way. That indeed 

 is what the whole science of zoology is; an orderly arrangement 

 of all the known facts about animals. 



This arrangement usually begins by a grouping of the facts 

 under five principal heads. These are animal classification, or 

 systematic zoology; animal morphology, or structural zoology; 

 animal physiology, or functional zoology; animal embryology 

 and life-history, or developmental zoology; and animal rela- 

 tions to their environment, or ecological zoology. Animal 

 psychology or behavior is also sometimes made a special head 

 in the classifying of zoological facts, but it may better be 

 included in the subject of animal physiology. 



No one elementary text-book can deal in a comprehensive 

 way with all of these phases of the study of animals. And yet 

 no one phase can be satisfactorily studied wholly by itself. 

 The classifying of animals into related groups depends upon a 

 knowledge of animal structure and development. To under- 

 stand animal structure one must know something of animal 

 physiology, and vice versa. Finally, to understand the relations 

 of animals to the world they live in, to the plants that serve 

 them as food and protection, to the other animals that they 

 associate with as friends or enemies, and to man, whose rela- 

 tions with them are much more complex and important than 

 we may, at first, think, it is absolutely necessary to know 

 something about all the other subjects of animal study. 



This book, therefore, which is intended to guide students 



