vi Preface 



obviously in a subject fast extending under the influence 

 of many workers there can be no finality, yet at the present 

 time our knowledge of the main facts has reached a 

 definite stage, and a useful and relatively permanent pre- 

 sentation of the phenomena can be attempted. 



The range and diversity of facts, zoological and botani- 

 cal, from which the material is drawn are so wide that it 

 has been difficult to present them adequately within a 

 moderate compass. Many of the types studied might 

 singly provide the subject of a treatise, and the temptation 

 to annotative excursion has been very great ; but the course 

 which seemed most useful was to admit only such detail as 

 had a clear significance in the exposition of principle, or as 

 a suggestion for further inquiry. The reader therefore will 

 understand that if he turns to the original records specified 

 he will almost always find information, perhaps important, 

 which is omitted here. 



In the original plan of the book it was intended to 

 discuss somewhat fully the bearing of the new facts on the 

 great problems of Biology, but it is perhaps more fitting 

 that these theoretical considerations should be detached 

 from a presentation of the concrete phenomena. In 1907 

 I had the honour of delivering the Silliman Lectures in 

 Yale University, and I then took these wider aspects of 

 Genetics as my theme, showing the bearing of the new 

 knowledge on current theory, especially on that of Evolu- 

 tion, and the nature of Variation. The substance of these 

 lectures I propose to publish separately with amplifications, 

 and on the present occasion allusion to these matters has 

 been restricted to the briefest possible indication of the lines 

 of thought which Mendelism inevitably suggests. 



A chapter dealing with practical applications of Mendelian 

 principles has been introduced. Such applications will 

 probably far exceed any limits we can yet perceive. Among 

 them we must foresee not merely advances in the art of 



