PREFACE vii 



minor matter in evolution, curious rather than otherwise, it is a 

 vital one in thremmatology, and we must discuss the subject the 

 best we are able, if only to learn how little we really know about 

 it and to point attention in the right direction. 



No attempt has been made to include exhaustive references. 

 On the other hand, they are confined for the most part to a few 

 standard books easy of access, and to save time the references 

 are mostly to definite pages. A general and more extended list 

 follows the summary of nearly every chapter, enabling the student 

 to pursue that particular subject further if desired ; but there is 

 no attempt at a complete bibliography. It was hoped that if the 

 list of references could be kept small the student and the breeder 

 would be the more likely to provide themselves with standard 

 literature bearing on the subject. I have made the freest use of 

 standard authors, giving full credit in all cases, generally in the 

 form of reference to text and page. This course has been dictated 

 by the desire to furnish the student with reliable facts rather 

 than a series of academic discussions upon disputed subjects. 



I desire to acknowledge the very great services of Dr. Rietz, 

 to whom I am indebted for much assistance in the more statistical 

 portions, and for the preparation of the appendix especially 

 directed to the mathematical student, not as a text but as an 

 introduction to further study in this special phase of science. 



I am also indebted to many of my colaborers in the Univer- 

 sity of Illinois and elsewhere, as well as to numerous breeders in 

 this and other states, who by their assistance have contributed 

 much to any success which this volume may meet. 



Its possible merits, therefore, I^^sJ share with others ; its 

 defects and shortcominsMre &^4l 



^BB E. DAVENPORT 



UNIVERSITY OF ILLINO 

 URBANA 



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