PREFACE V 



As to the sequence of topics, I believe that since there is no end 

 to the subject, one point is as good for beginning as any other. For 

 practical administration of instruction it is of course desirable to have 

 a plan of some kind, and the experienced teacher will make up a 

 new plan each year only to find it desirable to make changes in it 

 before the year is over. There is no best sequence ; the order in this 

 book has been followed with classes that had to be taught according 

 to a syllabus with a totally different arrangement, and the material 

 can be studied quite as satisfactorily without following the chapters 

 and sections in order. It has been repeatedly demonstrated that 

 the material is usable for beginners in the subject, and that the 

 text is adjustable to a variety of syllabuses ; and these are the two 

 important considerations from the viewpoint of course of instruction. 



So far as possible, structural details are presented by means of 

 diagrams and pictures rather than by means of elaborate descriptions. 

 All the diagrams designed to clear up complex relations in space 

 or time have been drawn especially for this book, as well as all the 

 figures for which special credit has not been given. In this connec- 

 tion I wish to acknowledge my obligations to those who have been 

 good enough to lend me photographs and other illustrative material, 

 as well as to the artists who have so patiently collaborated in develop- 

 ing the drawings, — Mr. F. Schuyler Mathews, Mr. Frank M. Wheat, 

 Mr. Mateoto Nishimura, and Mr. Edward Taubele. 



In the course of my work I have had valuable assistance and criti- 

 cism from many colleagues and associates in the Commercial High 

 School (Brooklyn), the DeWitt Clinton High School and the Julia 

 Richman High School (New York), the American Museum of Natural 

 History, Columbia University, and other institutions. To mention any 

 of these would be to slight others ; and while some have given me 

 more time and more direct aid than others, I am too keenly aware of 

 the influence of even passing and casual suggestion to know where 

 to draw the line between those who have helped me and those 

 who have not. I am the lens through which is focused here and 

 now a fragmentary and fleeting view of the biological thoughts of 

 a hundred men and women; and it is this cross section of biology 

 that I offer to my fellow workers, with gratitude for what I have 

 myself received, and with the hope that it will be of help to them. 



B. C. G. 



