PREFACE TO THE THIRD EDITION 



With the exception of perhaps a dozen pages in which only 

 minor changes have been made, the preparation of the present 

 edition has entailed the complete rewriting of the text of the 

 preceding edition. This has been necessary for the accomplish- 

 ment of desired changes in emphasis and in the order and arrange- 

 ment of subject matter. Some new material on the frog has 

 been added with the idea of correlating the subject matter of 

 the text more closely with that of the laboratory program and, 

 as a result, the frog assumes a position of importance in the 

 early chapters of the book. This emphasis on the frog is in 

 keeping with the common practice of using the frog as the sub- 

 ject of laboratory study at the very beginning of the introductory 

 course in zoology. The primary reasons for selecting the frog 

 for this purpose are that it is a common animal, large enough for 

 the inexperienced student to study with profit, and also that it 

 displays easily recognizable resemblances to the human body 

 in both structure and function. In addition, the frog and allied 

 forms have played an important part in the hands of investiga- 

 tors in the development of the science of biology. 



Another departure from the preceding editions consists in the 

 attempt to avoid a cleavage between structure and function in 

 the descriptions of individual organs and organ systems, by 

 combining the discussion of structure with that of function, as 

 far as possible. The aim, of course, is to emphasize the essential 

 unity of structure and function. Thus metabolism is discussed 

 in connection with the structure of the individual organs con- 

 cerned rather than pigeonholed in a separate chapter. Similarly, 

 examples of different tissues are described as they are encountered 

 in the study of gross anatomy rather than under a separate head- 

 ing at the beginning of the book, where they have but little 

 meaning for the student. 



The chapter on endocrines has been completely reorganized 

 and brought up to date. A more logical order has been followed 

 in the treatment of the topics of heredity, evolution, and adapta- 



