INTRODUCTION 301 



the American Breeders Association changed its name to the American 

 Genetic Association. With its policy of unrestricted membership from 

 the beginning this organization has done great service in fostering the 

 common interests of geneticists and practical breeders. There are 

 state associations of plant breeders in New York, Wisconsin, Minnesota, 

 Illinois, Pennsylvania, Ohio, Nebraska, and Georgia. In certain other 

 states the agricultural and horticultural societies have fostered plant- 

 breeding work to a greater or less extent. The meetings held with their 

 addresses and discussions, the exhibits of new introductions and occasional 

 demonstrations in plant improvement by the experiment station or other 

 agencies, have aided in bringing to the seed growers and farmers of the 

 United States the knowledge of superior plants and their practical value. 

 The Canadian Seed Growers' Association has fulfilled a similar mission. 

 Summary. Starting with the sporadic efforts of a century or more 

 ago to find some better varieties of fruits and grains, there has been a 

 gradual broadening of the great movement to increase agricultural 

 output and raise the quality of raw materials by means of plant improve- 

 ment. Throughout the later stages of this development scientific 

 knowledge has become increasingly important until now the specialist 

 on a particular crop plant may invoke the aid of every branch of agri- 

 cultural science in selecting his material for breeding operations, making 

 the desired crosses and selecting the progeny. All this has been done 

 without much, if any, definite knowledge concerning the heredity of the 

 plant in question. Within a decade the science of genetics has developed 

 to a stage where it is capable not only of furnishing a rational explana- 

 tion for the phenomena of variation and heredity which in the past 

 seemed obscure and contradictory, but also of guiding the breeder 

 who will familiarize himself with the established principles of the science, 

 so that he may reach his goal with greater speed and economy. It is 

 the purpose of the following chapters to set forth these principles in 

 as clear and practical a manner as possible. It will be assumed, of 

 course, that the reader is familiar with the fundamental treatment of 

 the preceding chapters. 



