174 MODES OF RESEARCH IN GENETICS 



domestic animals such a thing as a new breed is 

 brought forward by the breeder only on very rare 

 occasions. Almost all of our existing breeds of 

 horses, cattle, sheep, and swine have long histories 

 as "pure breeds," and no new ones are being added 

 now. With smaller animals such as poultry the 

 case is of course somewhat different. There we 

 have no registered pedigrees and, with some dif- 

 ficulty, new breeds may be launched. 



The plant breeder, on the other hand, makes 

 nearly all of his improvements by the production 

 of new varieties. This he does either by hybridi- 

 zation, actually building up a new type, or by 

 isolation of superior pure-breeding forms from 

 already existing mixtures. He is not hampered 

 by a body of tradition that only the "pure-bred " 

 is of any particular value. Almost if not quite 

 every one of the most valuable strains of agricul- 

 tural plants to-day carries the "bar sinister." To 

 the animal breeder they would be "grades" or 

 "crosses" however gametically pure, and only with 

 the greatest difficulty would ever have gained a 

 chance to show their worth. 



No one would deny that the systems of registry 

 for live stock and the exploitation of the "pure- 

 bred" have been of great value in the develop- 

 ment of the animal industry of the world. They 

 certainly have; and every day the economic 

 importance of the system becomes greater, for 

 obvious reasons. All systems of pedigree registra- 



