Vlll PREFACE 



evidence relied on is drawn largely from medical sources; 

 medical men form the largest body of scientific workers ; they 

 deal constantly with questions of Heredity, a knowledge of 

 which is of great importance to them ; but in a measure they 

 have neglected the systematic study of the subject. Little or 

 no direct instruction in it is given to medical students. There 

 does not exist even a text-book to which they may refer. But 

 a knowledge of Heredity is becoming essential to the educated 

 doctor. I have sought to supply the want. I hope, however, 

 the professional biologist and the general reader will not find 

 the work devoid of interest. I have tried throughout to give 

 prominence to broad generalizations rather than to isolated 

 facts, to found my conclusions wholly on verified evidence, 

 and to write in such a way that the reader's task will not 

 be unnecessarily toilsome. 



My hearty thanks are due to Dr. Gerald Leighton for 

 " going over " the first fifteen chapters, and to the Editors of 

 The Monthly Review and The Lancet for permission to adapt 

 for Chapter X. matter which had appeared in their pages. 



PREFACE TO THE SECOND EDITION 



THE first edition of this work has been exhausted with 

 such unexpected rapidity that I have been unable to make 

 alterations of importance in the body of the book. A 

 better definition, or description, of " spontaneous " varia- 

 tions has been introduced on page thirty-eight. Numerous 

 typographical errors have been corrected, for the discovery 

 of which my thanks are due to several correspondents. 

 The main addition to the first issue consists, however, in 

 two somewhat lengthy appendices. In the first, besides 

 the discussion of one or two allied questions, I have 

 endeavoured to make clearer the all-important distinction 

 between innate and acquired characters. In the second I 

 have dealt in some detail with the mutation theory of 

 evolution and the phenomena of alternative inheritance. 

 Several reviewers, notably that most indulgent of all critics, 

 Dr. C. W. Saleeby, have expressed the opinion that the 

 labours of Mendel and his followers have not been 

 adequately discussed by me. I have tried to rectify the 

 omission. 



Southsea, 4th Dec., 1905. 



