viii Preface 



of life continues douun the ages to fnanifest itself through new 

 individuals that differ front one another and that form con- 

 stantly changing groups or species. 



Are the various species of plants and animals self-contained 

 realities, or are they mere abstractions of the human mind? 

 Have the species of living things existed unchanged from their 

 several beginnings, or have they become, in the course of time, 

 transformed into different species? The facts bearing upon 

 these and other common-sense questions, the facts that have 

 to be explained and reconciled, are presented objectively and 

 concretely, in the hope that readers of diverging views may 

 find them significant and helpful. The need for a general 

 theory and the more plausible special theories are then dis- 

 cussed. Finally, the practical importance of organic evolu- 

 tion is pointed out. There is no attempt to establish any gen- 

 eral doctrine of evolution as applying to stars and planets, to 

 atoms and molecules, to musical instruments and social in- 

 stitutions. 



I wish to express my sincere thanks to the many friends 

 who have helped me with criticisms and suggestions, and espe- 

 cially to Dr. R. A. Budington of Oberlin College; Dr. Otis 

 W. Caldwell of Teachers College, Columbia University; Dr. 

 Winterton C. Curtis of the University of Missouri; and Dr. 

 Caswell Grave of Washington University. I am under ob- 

 ligation also to the numerous scientists and publishers, and to 

 the American Museum of Natural History, for the use of 

 illustrations, which are individually acknowledged in the 

 book. 



