CHAPTER IV 

 THE MECHANISM OF HEEEDITY 



,The scientific era in the investigation of heredity be- 

 gan in the latter half of the nineteenth century with the 

 vrork of Galton and of Mendel. Both enthusiastic and 

 competent investigators, their efforts made with differ- 

 ent material and from diverse points of view, did not 

 fare the same. Galton measured the inheritance of groups 

 of individuals by their resemblance to their progenitors 

 and failed because his method could not take into account 

 the true relationship between the germinal constitution 

 and the body characters of an individual ; Mendel deter- 

 mined the inheritance of a single organism by making the 

 characters of its progeny the criterion and succeeded. 

 Without knowledge of the cell mechanism of gameto- 

 genesis and fertilization, Mendel described the results of 

 his hybridization experiments in terms which agreed pre- 

 cisely with these later discoveries in the field of cytology. 

 Mendelian heredity has proved to be the heredity of sex- 

 ual reproduction: the heredity of sexual reproduction 

 is Mendelian. 



Progress in the study of heredity through investiga- 

 tions patterned after Mendel's model has been so great 

 that the subject now forms an important sub-division of 

 general biology — Genetics. The details of the subject 

 have outgrown the limits of a single volume, and a knowl- 

 edge of the generalities is no longer confined to the pro- 

 fessional biologist. For such reasons we propose to 



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