PREFACE 



This book is an attempt to present the fundamental fea- 

 tures of Genetics: those features of which every educated 

 person should have knowledge. Resemblances and differences 

 among organisms are due largely to diversities in the ma- 

 terials with which the different individuals begin life. This 

 fact forms the guiding principle in the presentation here 

 given. The distinction commonly made between heredity and 

 the mechanism of heredity is therefore abandoned; such a 

 distinction is out of date. Only through knowledge of the 

 materials on which heredity depends, and an understanding 

 of their methods of operation, is it possible to understand 

 the course taken by heredity and variation, so that study of 

 these matters forms the groundwork of Genetics. 



This groundwork involves detailed facts and relations 

 that must be thoroughly grasped; these are presented so 

 far as possible in sharply defined form; they are in many 

 cases condensed into formal numbered propositions. In the 

 chapters dealing with more general relations, and based on 

 the earlier chapters, the material is presented in the more 

 usual form of continuous discourse. References to sources 

 are collected into notes at the ends of the chapters. For the 

 better known matters, already gathered into books or mono- 

 graphs, reference is made to such collective accounts, from 

 which the original sources can be traced if desired. For more 

 recent knowledge, not yet unified, references are given to 

 original papers; such are more common in the later chapters. 



Johns Hopkins University, May 3, 1935 



