PREFACE TO THE THIRD EDITION XI 



2. Each chapter of the general part cohtains the following 

 elements: — (i) the general statement of principles or facts; 

 (2) interspersed with this are such practical exercises for labo- 

 ratory, field, or library, as have been found practicable for 

 elementary classes. These are intended to compensate for the 

 enforced brevity and abstractness of definitions and description, 

 by causing the student to find concrete illustration of the princi- 

 ples; (3) an analytic summary of the most important general 

 truths of the chapter in outline, at the close of the chapter; 

 and finally (4), a list of supplementary topics for individual 

 laboratory or library investigation and report. These topics 

 supplement and illustrate the text, and enrich the review by 

 introducing a new viewpoint and new matter. It is not intended 

 that all of these topics shall be demanded of the whole class. 

 The writer has got best results in interest and knowledge by 

 allowing each member of the class to select some topic in which 

 he is interested and to make a brief report of his investigation 

 before the whole group. 



3. In the chapters of the special part each phylum is intro- 

 duced by field and laboratory work on some representatives 

 taken as types. This is purposely made brief and suggestive 

 in order that the teacher and class may build up their own 

 detailed program. This is followed, corrected, and enlarged 

 by a brief discussion of the typical condition of the organs and 

 functions in the group as a whole. This serves to unify the 

 isolated and local observations of the student. Next foUows 

 a brief statement of the most important facts of classification, 

 together with ecological and economic suggestions. Finally, 

 each chapter concludes with a list of supplementary questions 

 calling for field, laboratory, and library work in review, and as 

 a brief view of new material. 



4. The figures are carefully selected, — the majority of them 

 being specially made for this book. With each figure of special 

 moment is a brief list of queries designed to assist the student 

 in the study of the figure. It is a common complaint among 

 teachers that it is difficult to get students to appreciate and to 

 use illustrations intelligently, and to relate them to the text. 

 A sane emphasis on these questions will solve this problem. 



