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. 
Digitized by Microsoft® 
