222 INBREEDING AND OUTBREEDING 



The greatest increases occurred in the crosses between 

 flint and dent varieties, and often there was a really note- 

 worthy hastening of the time of ripening, which is of con- 

 siderable importance in those regions where early fall 

 frosts are a limiting factor. 



The greatest improvement to be made in this way 

 comes from crossing varieties which have previously been 

 put through a process of self-pollination. "When certain 

 inbred strains are crossed the increase in growth is re- 

 markable, as previously noted. This comes partly from 

 the fact that following inbreeding the maximum effect of 

 hybrid vigor is obtained while in ordinary varieties seg- 

 regation brings about partial homozygosity in many 

 plants. It is also due to the elimination of many unde- 

 sirable characters during the process of inbreeding. The 

 crossed plants are remarkably uniform. One plant is a 

 replica of another. Given proper conditions they all pro- 

 duce good ears which form a remarkable contrast to ordi- 

 nary varieties in their similarity to each other. There 

 are no barren stalks, and the abnormalities and mon- 

 strosities which commonly occur in every field of com are 

 almost entirely absent. In those cases in which one or 

 both of the parent strains is resistant to parasitic infec- 

 tion, such as smut, the cross is also resistant and this is a 

 factor for greater production. 



There are, on the other hand, certain serious disad- 

 vantages in the practical utilization of first generation 

 crosses between inbred strains. In the first place the 

 yields of the inbred plants are low, which makes the cost 

 of the crossed seed high. What is more serious, the seeds 

 produced on inbred plants are small and less well devel- 

 oped than seeds of ordinary com, and the seedlings com- 



