INTRODUCTION 5 



Will a given breed, variety, or family strain endure indefi- 

 nitely under proper conditions or will it inevitably "run out," 

 necessitating a constant return to foundation stock for new 

 combinations as the basis of improved strains ? 



What are the laws that determine the sex of offspring ? 



Do the same laws of breeding apply equally to animals and to 

 plants and to all species and varieties alike, or do different species 

 operate under somewhat different laws ? 



Is a given species, variety, or breed always subject to the same 

 laws ? that is, are identical variations always due to the same 

 causes and do given causes always produce the same effects ? 



How can results be secured with the -least wastage either in 

 time or numbers ? 



Upon the answers to these questions will depend the policies 

 of all breeding enterprises and the permanent value of particular 

 family strains. Upon some of these points there exists much 

 specific and reliable information ; upon others, unfortunately, the 

 evidence is yet scanty and uncertain. At the present rate of 

 progress, however, we will not have long to wait for much addi- 

 tional knowledge. In the meantime we must make the best use 

 possible of the information and experience at hand. 



These problems can best be answered not by directing atten- 

 tion to each separately, because they overlap, but rather by follow- 

 ing out what are known to be the characteristic lines of study in 

 the subject as a whole. The order pursued in this book is the one 

 believed to be most favorable both for this purpose and for the 

 most successful answering of these definite questions. 1 



1 A fair knowledge of general evolution is presumed on the part of the student 

 and reader. If this is not in his possession, he will do well to read at least 

 Darwin's Origin of Species for a broad though now somewhat old and incomplete 

 outlook upon the general field. If he desires to go further and enter the field of 

 controversy, he can do so most directly by reading Weismann's Essays on 

 Heredity and his Germ Plasm, together with Romanes' Examination of Weis- 

 mannism and his two volumes of Darwin and After Darwin. If this is done, it 

 Would be well to finish with Habit and Instinct by Morgan. 



