X PREFACE TO THE SECOND EDITION 



avoided them but have attempted always to distinguish clearly between fact 

 and theory. In some cases I have pointed out what seems to me to be the 

 conclusion justified by the evidence; in other cases I have indicated the wis- 

 dom of suspended judgment, awaiting further evidence. In the interest of 

 clarity and of economy of the reader's time I have, in discussion of a given 

 subject, included most readily understandable or most telling examples and 

 supporting facts, without any pretense of exhaustive cataloguing. Teachers 

 will, of course, augment my discussions as they think best. 



As with the first edition, the main body of the first part of the book is 

 concerned with the facts of evolution. This part is preceded (Chapter 2) 

 by a brief discussion of the theory of how evolution occurs, and followed 

 (Chapter 15) by a more complete summary of this subject. Chapter 15 is 

 designed to serve (a) as a concluding chapter for those who desire but a 

 brief summary of theory, and (b) as an introduction to the succeeding 

 chapters in which theory is discussed in more detail for those who desire it. 

 These later chapters have been completely reorganized and largely rewritten 

 in the second edition. In response to many suggestions by users of the first 

 edition a discussion of the pririciples of Mendelian inheritance and their 

 chromosomal basis has been added (Chapter 17) to augment the discussion 

 of population genetics (Chapter 19) and provide genetical foundation for 

 understanding other causative factors in evolution. The second edition in- 

 cludes expanded discussions of some subjects included in the first edition 

 (e.g., hybridization, speciation, and selective mating) as well as discussions 

 of some subjects absent from the first edition (e.g., industrial melanism, 

 mimicry, genetic assimilation and the Baldwin effect, and genetic homeo- 

 stasis). 



In each of the earlier chapters of the book I have attempted to bring 

 the facts and interpretations up to date in the light of recent research. The 

 discussion of the relationship between recapitulation and the origin of the 

 Metazoa (Chapter 4) has been completely rewritten, as has the chapter on 

 the evolution of man, a field in which discoveries of new fossils and more 

 complete analyses of old ones lead to almost monthly changes in our knowl- 

 edge and ideas. Some subjects have been augmented, for example, the evo- 

 lution of the horse (by discussion of the brain and the springing mechanism 

 of the foot), and the discussions of classification and geographic distribu- 

 tion (clines, rassenkreise, and zoogeographic "rules"). 



In a very real sense it is impossible to acknowledge adequately assistance 

 given by others in preparation of this book. I am a debtor to countless 

 scientists known and unknown to me. Science is "team play" at its best. It 

 is a pleasure, however, to mention the names of certain individuals who 



