xiv PREFACE 



and are handed on by heredity to succeeding generations. 

 This essential idea of Lamarckism was refined and extended 

 by Herbert Spencer, by Darwin himself, by Cope and many 

 others; but it has thus far failed of the crucial test of observa- 

 tion and experiment, and has far fewer adherents to-day than 

 it had forty years ago. 



We now perceive that Darwin's original thought turned 

 to the opposite idea, namely, to sudden changes in the heredity- 

 germ itself^ as giving rise spontaneously to more or less adap- 

 tive changes of body form and function which, if faA'orable to 

 survival, might be preserved and accumulated through natural 

 selection. This pure Darwinism has been refined and extended 

 by Wallace, Weismann, and especially of late by de Vries, 

 whose "mutation theory" is pure Darwinism in a new guise. 



Weismann's great contribution to thought has been to 

 point out the very sharp distinction which undoubtedly exists 

 between the hereditary forces and predispositions in the hered- 

 ity-germ and the visible expression of these forces in the or- 

 ganism. It is in the "germ-plasm," as Weismann terms it — 

 in this volume termed the '"heredity-chromatin" — that the real 

 evolution of all predispositions to form and function is taking 

 place, and the problem of causes of evolution has become an 

 infinitely more difficult one since Weismann has compelled us 

 to realize that the essential question is the causes of germinal 

 evolution rather than the causes of bodily evolution or of en- 

 vironmental evolution. 



Again, despite the powerful advocacy of pure Darwinism 

 by Weismann and de Vries in the new turn that has been 

 given to our search for causes by the rediscovery of the law of 

 Mendel and the heredity doctrines which group under Men- 



* Osborn, H. F., "Darwin's Theory of Evolution by the Selection of Minor Saltations," 

 The Amer. Naturalist, February, 191 2, pp. 76-82. 



