v i PREFACE 



call attention to approved methods of study, and to indicate lines 

 of research most likely to furnish valuable information in the not 



distant future. 



It is necessary to introduce a considerable amount of mathe- 

 matical work in the later chapters. No excuse is offered for this 

 introduction, and it is earnestly desired that the reader give 

 special attention to this portion of the text, whether easy or diffi- 

 cult of following, because it is by this road that many new princi- 

 ples will arrive and that many of our future operations must be 

 ordered ; for nothing is clearer than that the successful breeder 

 of the future will be a bookkeeper and a statistician. For the 

 convenience of the non-mathematical reader general formulae are 

 placed in footnotes, and some of the more abstract matter is 

 placed in the form of an appendix for the benefit of the more 

 mathematically inclined. 



The writer has taught this subject for fifteen years and is fully 

 aware of the pedagogic difficulties involved as well as of the 

 limitations of knowledge. He has tried many different outlines 

 and many different methods of presentation, and has chosen the 

 one here employed because in experience it seems the most 

 favorable for the presentation of the subject-matter involved and 

 at the same time for putting the student in a frame of mind 

 favorable for the undertaking of economic breeding operations 

 and for the reception of new truths as they shall be discovered. 



Variation rather than heredity has been chosen for the initial 

 and leading thought because better calculated, as experience has 

 shown, to afford a favorable outlook and to develop such con- 

 ceptions of evolution as ^Mnost useful later on. 



The evolutionist who I H^ancjfe^can these pages would 

 be struck by the absence orsomeB Be cardinal features of 

 evolution, as he would also note the S feing prominence given 

 to certain other questions of seeming n^^r importance. Herein 

 exists the difference between thremmatology and evolution, and 

 this very matter has given the author more difficulty than all 

 others, viz. to rearrange values and to determine proper relations 

 of old questions in a new field. 



We must discuss the causes of variation even though we are 

 told by the best students that such attempts are premature. A 



