PREFACE TO REVISED EDITION 



A book of this sort somewhat resembles a loose-leaf encyclopedia 

 in that it subjects itself very readily to revision and rearrangement 

 and thus may be kept abreast of the times. When certain sections 

 prove through actual class use to need revision or restatement or when 

 new discoveries necessitate a change in conclusions, appropriate cor- 

 rections may be made, new matter added, or whole chapters re- 

 written. When various topics have shown themselves to be either 

 logically or pedagogically in the wrong order, it is easy to rearrange 

 chapters, for the latter are to a large degree independent. When, 

 finally, any new chapter of superior excellence appears in a new pub- 

 lication, it is usually possible through the courtesy of author and pub- 

 lisher to add it to the worthy collection of excerpts already gathered 



together. 



The writer has been fortunate in that reviewers and colleagues both 

 in America and in Europe have offered many constructive criticisms 

 of the book and suggestions for its improvement. It is hoped that the 

 present edition will adequately reflect this expert advice. 



The order of presentation of the evidences of evolution has been 

 changed from one based on the degree of directness of the evidence 

 to one based on the logical succession of topics and their interde- 

 pendence. The chapters on "The Mutation Theory" and "The 

 Inheritance of Acquired Characters" have been placed near the end 

 of the book in order that they may be considered in the full light 

 of our present knowledge of genetics. The chapter on "Linkage 

 and Crossing-over" has been rewritten in a more elementary and cir- 

 cumstantial style in order to overcome, if possible, the difficulty that 

 students have always encountered in understanding the somewhat 

 condensed and technical account of Professor Castle. The discussion 

 of mutations has been modernized and considerably extended through 

 the addition of an article written especially for this book by Professor 

 R. Ruggles Gates and of a paper by Professor H. F. MuUer. The 

 section on eugenics has been strengthened by the addition of a lucid 



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