Preface vii 



probably far exceed any limits we can yet perceive. Among 

 them we must foresee not merely advances in the art of 

 breeding animals and plants, but a control over the destiny 

 of our own species. These things are spoken of in their 

 place. To prevent disappointment, however, it must be at 

 once admitted that for fanciers Mendelism can as yet do 

 comparatively little. "Fancying" provides the chief interest 

 in life for thousands of persons in this country. It is an 

 occupation with which the scientific naturalist should have 

 more sympathy than he has commonly evinced. If the 

 scientific world had kept in touch with the operations of the 

 " fancy " much nonsense which has passed into scientific 

 orthodoxy would never have been written. The study of 

 Mendelian phenomena will do something to bring about a 

 fruitful interchange of experience. But for the " fancy " 

 our work can as yet do two things only. First, in the 

 study of the workings of the Mendelian system it will 

 provide a most fascinating pursuit, which if followed with 

 assiduous care may at any moment lead to some consider- 

 able advance in scientific knowledge. Secondly, the prin- 

 ciples already ascertained will be found of practical assistance 

 in the formation of new breeds and may save many mistakes 

 and waste of time. But applied to the business of breeding 

 winners in established breeds they cannot materially help, 

 for almost always the points which tell are too fine to be 

 dealt with in our analysis. 



In a work of this kind an author must necessarily speak 

 of various subjects on which his knowledge can be super- 

 ficial only, and I trust that if inaccuracies have been intro- 

 duced, readers will be good enough to send me corrections. 



Much and varied assistance has been given me by 

 many persons. Such help on special points has been 

 acknowledged in the text, but a fuller and more prominent 

 acknowledgment is due to my colleagues. Without their 

 cooperation there would have been, so far as Cambridge is 



