viii PREFACE 



tive, and whose precise manner of inheritance must 

 still be regarded as under discussion. 



The literature of the last two decades has recorded 

 many cases of Mendelian inheritance in man, as the 

 present book will, I think, testify. But it will also 

 be seen that even the pre-Mendelian literature 

 contained many scattered records of great interest 

 which are now seen to fall into the Mendelian categories 

 of explanation. I have made no attempt to search 

 the literature exhaustively, as the work is really only 

 a by-product of my own evolutionary interests, 

 but I hope that no records of first-rate importance 

 have been omitted, and I believe that several valuable 

 early papers are here for the first time brought into 

 orientation with the twentieth-century literature. 

 The book might easily have grown to considerably 

 greater proportions, but I have endeavoured to include 

 only the more essential subjects and discussions which 

 a book of this kind, written from the biological point of 

 view, ought to contain. Whenever experiments with 

 animals or plants bear directly on the topic in hand, 

 I have not hesitated to use them, and I trust this 

 will add to the value of the w^ork, both to the general 

 public interested in Eugenics, and the medical 

 profession, who should always be on the alert to 

 detect the inheritance element which so frequently 

 is present in the functional derangements with which 

 they have to deal. 



Many matters which are primarily of genetic 

 interest, but without at present any special bearing 

 on human heredity, have been omitted altogether. 

 If this work aids in the diffusion of an intelligent 

 interest and understanding of heredity in its bearing 

 on the welfare of future generations, its object will 

 have been achieved. The conceptions of heredity 

 are no longer vague and ill-defined, as in the writings 

 of a generation ago. They are clear and sharply 



