PREFACE 

 TO THE SECOND EDITION 



Organic evolution is the greatest general principle in biology. Its impli- 

 cations extend far beyond the confines of that science, ramifying into all 

 phases of human hfe and activity. Accordingly, understanding of evolution 

 should be part of the intellectual equipment of all educated persons. 



As its name implies, this book is intended to form an introduction to or- 

 ganic evolution for readers who have not previously "met" the subject, and 

 who have little or no background knowledge of biology in general. In writ- 

 ing I have constantly striven to render each subject understandable to such 

 readers and to make my discussions elementary in the best sense of the 

 word. 



For the general reader and beginning student I distinguish sharply be- 

 tween textbooks and reference books. Reference books and advanced text- 

 books, with their encyclopedic treatment, exhaustive discussions, and 

 detailed citations of literature, are essential tools for the scientist and the 

 advanced student. But they are not suitable introductions to a branch of 

 science. This function is served by the elementary textbook or "teaching 

 book." The present volume is of this variety. In it the author serves as guide 

 to a traveler in a strange country. On the basis of previous experience and 

 greater familiarity with the terrain, the author guides his reader through the 

 jungle of confusing facts and conflicting theories. The guide, not being all- 

 wise, may make mistakes of judgment in selecting the path, but at least the 

 reader will be more likely to reach his destination than he would have been 

 without any guide at all. Having once mastered a route the reader will then 

 be competent to explore other paths than those selected for him by his 

 original guide. He will then, in other words, be in position to profit from 

 reference books and research treatises. In the hope that readers will be 

 stimulated to read more deeply in subjects which interest them I have in- 

 cluded in the second edition more extensive lists of references than the first 

 edition possessed. 



Controversial subjects form an integral part of our discussion. I have not 



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