Vlll PREFACE TO THE THIRD EDITION 



presented early, in order to give the student a skeleton or 

 dimensions, so to speak in which he shall later insert the par- 

 ticulars which he discovers. He must have this in order to 

 unify his own results in the brief time at his disposal. The 

 lack of this unifying result is the ground of the just complaint 

 concerning much of the unorganized and unrelated laboratory 

 instruction in the secondary schools and early college classes. 



9. While it is necessary to bring our materials from various 

 departments of Zoology and is desirable that the student should 

 be able to recognize whether a given problem is primarily one 

 of structure or function or relation, the total result of an ele- 

 mentary course of Zoology should be a sense of unity, of con- 

 tinuity, and of interdependence. The final view of the student 

 should be of life and organic progress, and not of a disjointed 

 science, dissected in the house of its friends. 



10. The teacher should have some latitude in the choice of 

 matter and emphasis, in order that both may be properly suited 

 to his equipment and locality. It should be impossible for the 

 teacher or the class to use a text-book in a slavish, or parasitic 

 fashion. Therefore a text-book should contain and suggest 

 much more than one teacher or one class can use in the time 

 allowed. This not only gives the teacher a chance (and makes 

 it necessary for him) to mould his own course, but causes the 

 student -to realize that he is a mere beginner when he has com- 

 pleted his first course. 



In attempting to apply these principles to the present book 

 the author has made use of the following devices : 



i. The book is divided into three portions: (i) a general 

 part dealing largely with broad biological problems and princi- 

 ples, which constitute the foundations of the science and are 

 felt to be for the most part, beyond even the verification of the 

 elementary student (Chapters I-VIII) ; (2) a special part (Chap- 

 ters IX-XXV), in which the various principal phyla of animals 

 are taken up in succession, beginning with the lowest. The 

 purpose has been to make this part particularly illustrative of 

 the principles laid down in the general portion; and (3) a group 

 of synthetic chapters (XXVI-XXIX) to induce the student to 

 gather up the details of his course by a new reorganization of 

 the materials. 



