PLANT AND ANIMAL IMPROVEMENT 215 



Among those plants which are largely self-pollinated by 

 nature, chance crossing, or, in some cases, systematic 

 hybridization, has originated new types. Self-pollination 

 has brought these types to uniformity, and by isolation 

 new varieties have been established. Among naturally 

 crossed plants genetical variations are continually being 

 produced and selection for certain of the more conspicu- 

 ous features has led to the creation of well-marked varie- 

 ties. Indian com is one of the best examples in this class. 

 There are many distinct types, and the less distinct but 

 fairly well recognized varieties are almost innumeral)lo, 

 adapting the plant to a range of conditions from the edge 

 of the Arctics to the Tropics, throughout the world. 



In eveiy locality where com is grown the usual habit 

 is to prevent inbreeding as much as possible. Many com 

 growers make a regular practice of bringing in seed from 

 other localities, and often two or more somewhat different 

 varieties are planted together and allowed to mix. The 

 reason why this practice is followed is easily apparent 

 from the controlled experiments on the effects of inbreed- 

 ing and cross-breeding upon this plant. But even keeping 

 in mind the injurious results of inbreeding, indiscriminate 

 crossing is not desirable. Many of the well-known varie- 

 ties in the Com Belt States, such as Eeid^s Yellow Dent, 

 Learning, and Boone County White, are the results of 

 long-continued selection for certain standards without 

 crossing with other varieties. Inbreeding, therefore, has 

 secured individuality for varieties of cultivated plants as 

 well as for breeds of animals. 



The value of inbreeding in plant and animal improve- 

 ment in the past may be summed up in the statement that 

 it is the greatest single agency in bringing about uni- 



