PREFACE vii 



cooperation of his wife, Margaret Mitchell Woodruff, must 

 not remain unmentioned, though it cannot be adequately 

 expressed. 



The original illustrations as well as those from other sources 

 which have been modified or merely redrawn are, with a 

 few exceptions, the work of Mr. R. E. Harrison, Yale, 1923. 

 In most cases these figures have been selected because of 

 their proved pedagogic value. Acknowledgments are due to 

 the authors and publishers of the following works, from which 

 illustrations have been reproduced by permission: Coulter, 

 Barnes, and Cowles' Textbook of Botany, Coulter's Plant 

 Life and Plant Uses (American Book Co.) ; Kellicott's Social 

 Direction of Human Evolution, Jordan and Kellogg's Evolu- 

 tion and Animal Life, Darwin's Life and Selected Letters, 

 Huxley's Life and Letters (D. Appleton & Co.); Folsom's 

 Entomology, Gager's Fundamentals of Botany (P. Blakis- 

 ton's Sons & Co.) ; Jennings' Behavior of the Lower Organ- 

 isms (Columbia University Press); Bergen's Foundations 

 of Botany, Bergen and Caldwell's Practical Botany, Bergen 

 and Davis' Principles of Botany, Densmore's General 

 Botany, Hough and Sedgwick's The Human Mechanism, 

 Linville and Kelly's General Zoology (Ginn & Co.); Kelli- 

 cott's General Embryology, Sedgwick and Wilson's General 

 Biology (Henry Holt & Co.); Morgan's Physical Basis of 

 Heredity (J. P. Lippincott & Co.); Romanes' Darwin and 

 After Darwin (Open Court Publishing Co.); Conklin's 

 Heredity and Environment in the Development of Men 

 (Princeton University Press); Conn and Budington's Physi- 

 ology and Hygiene (Silver, Burdett & Co.) ; Coulter's Evolu- 

 tion of Sex in Plants (University of Chicago Press) ; Abbott's 

 General Biology, Buchanan and Buchanan's Bacteriology, 

 Campbell's University Textbook of Botany, Ganong's Text- 

 book of Botany for Colleges, Hegner's College Zoology, and 



