viii PREFACE 



posite view, a majority of students will accept the word of the 

 textbook. 



These facts have impressed themselves so strongly upon me 

 during my years of teaching that they have led to the production 

 of this book. Our treatment of evolutionary processes and our 

 methods of investigation have been at an impasse for many years. 

 If we continue to teach the old point of view, can we expect to 

 progress with reasonable rapidity beyond our old limits of knowl- 

 edge? We have taught with unwarranted emphasis upon some 

 of the factors in evolution, in spite of the fact that modern biology 

 shows very clearly that many things must enter into the evolu- 

 tion of organisms. Such emphasis can hardly lead to great dis- 

 coveries. 



In this volume I have attempted first to present the materials 

 of evolution in such a way that their true logical relationship is 

 clear to the student, second to give a concise account of the funda- 

 mental principles of genetics, and finally to sum up the theoretical 

 matter of the subject and to present a logical analysis of the 

 factors bearing upon evolutionary theory. Since nothing is so 

 interesting to man as himself, the bearing of all material upon the 

 human species has been treated as fully as seems warranted. 



The book has been written for students who desire a sound in- 

 troduction to the subject and not merely such an elementary ac- 

 count as is presented very adequately in most textbooks of biology. 

 The material contained in it has been used in my own classes for 

 students who have previously completed a course in general zoology 

 or biology and for a few students of marked ability without such 

 prior training. While the facts presented must often be unfamiliar 

 to such students, their significance should be evident with the 

 brief treatment given to them here. The work is not designed for 

 entertainment but for serious instruction in a difficult field of 

 biology, although I have yet to find a student to whom the sub- 

 ject is not intensely interesting. 



Acknowledgments for the use of illustrations are made where 

 the figures appear, but I wish to extend my thanks again to all 

 who have assisted in this way. My deepest gratitude is also due 

 to my wife, Winifred Wood Lindsey, for her intelligent criticism 

 of many scientific points, for assistance in the formulation of the 

 manuscript, and for invaluable aid in the laborious work of proof 

 reading. 



