PREFACE 



Mode in Evolution, in 1950 by Stebbins' Variation and Evolution in Plants, 

 in 1953 by Simpson's Major Features of Evolution, and in 1957 by Darling- 

 ton's Zoogeography. In addition to these books, an enormous amount of 

 valuable evolutionary research has been published in a host of technical 

 journals. 



The present book was first published in 1952 in an attempt to make this 

 flood of evolutionary scholarship available to students. I am indebted to 

 those many professors and students who used the book and encouraged 

 me to believe that it served its purpose well. In the eight years which have 

 intervened since its publication, however, the flood of new research on 

 evolution has continued and expanded. Many new data must be presented. 

 Old theories and interpretations must be reassessed, and new ones must 

 be tentatively put forward. Hence this second edition is mandatory, and 

 I offer it with the hope that it will prove even more useful than its prede- 

 cessor. 



The general organization of the first edition has been retained. Part I 

 includes seven chapters, summarizing the contents of the older books on 

 evolution, and serving primarily to define the subject. These chapters 

 present the major evidences for evolution. A new chapter, on evidence 

 from comparative physiology and comparative biochemistry, has been 

 added. Part II, Phylogeny, deals with the evolution of the higher cate- 

 gories, and attempts to trace the main lines of evolution in the Plant and 

 Animal Kingdoms, including the probable lineage of man. Part III, the 

 Origin of Variation, deals with the mode of origin of those hereditary 

 variations which form the necessary substrate for the action of natural 

 selection. Part IV, the Origin of Species, deals with those factors which 

 sort out the varying arrays of organisms into species, genera, and higher 

 groups. A new chapter on quantitative aspects of evolution has been 

 added here. Lastly, Part V, Retrospect and Prospect, summarizes briefly 

 what has gone before, attempts to put it in perspective, and brings 

 together some predictions of bolder scientists as to what future evolution 

 may bring. 



The title of Huxley's book, Evolution: the Modern Synthesis, is par- 

 ticularly apt, for the modem study of evolution requires a synthesis of 

 material from all fields of biology, together with some material from a 

 good many other sciences. It has now been just over a century since 

 Johannes Muller allegedly committed suicide because of despondency 

 resulting from the realization that it would not be possible to master the 

 whole domain of science. The difficulty of mastering all pliases of modern 

 evolutionary science is quite comparable, and the present work no doubt 

 shows many deficiencies. Yet the task of writing a text which can intro- 

 duce students to this important and fascinating field must ])e undertaken. 

 The kind reception which was accorded the first edition of this book 

 leads me to believe that the present edition, which has been improved in 

 many respects, should fill even more successfully the lugent need for such 

 a book. 



I am indebted to many persons lor the successful completion ol this 

 revision, and 1 am happy to acknowledge their help. The illustrations of 



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