VI PREFACE 



brief survey of the phenomena of sex and reproduction 

 in the chapter on the Perpetuation of Life. No reference 

 is made in the text to the much discussed problems of sex 

 hygiene. It was thought desirable to leave this topic to the 

 discretion of the teacher to be handled in whatever way 

 he or she considers to be most effective. An attempt, 

 however, has been made to supply the students with a 

 general basis of knowledge of the facts of reproduction 

 and development so that a little added instruction of the 

 right sort will aid them in avoiding the dangers and pit- 

 falls into which ignorance is continually leading so many 

 of our youth. Sex hygiene should be associated not merely 

 with considerations of personal welfare, but with the wel- 

 fare of future generations. It is highly important that the 

 youth of to-day who are to be the parents of to-morrow 

 should be imbued with a sense of their obligations as 

 fathers and mothers of children. They need to be made 

 aware that it is a matter of great moment what kind of 

 people are supplying the larger part of our future popula- 

 tion. And an effort has accordingly been made in the 

 discussion of heredity and eugenics to prepare them for an 

 appreciation of the importance of a knowledge of the forces 

 that are working toward the improvement or the deteriora- 

 tion of the inborn qualities of the race. 



In the preparation of this book I have profited by the 

 criticism of several of my colleagues. I am indebted to 

 Dr. W. W. Cort for reading the entire manuscript, to Dr. 

 J. Grinnell for reading the chapters on birds and mammals, 

 to Mr. Tracy Storer for reading the chapters on reptiles 

 and amphibians, to Dr. J. F. Daniel for reading the chap- 

 ter on fishes, and to Dr. E. C. Van Dyke for reading the 



