240 THE TREND OF THE RACE 



was a general decrease of productivity in successive generations. 

 When two deteriorated inbred strains were crossed the yield 

 was generally markedly increased. In order to insure the great- 

 est production in corn it is necessary to use seed that results from 

 the crossing of different strains. 



In tobaccos which are commonly self-pollinated the effects of 

 crossing are much more variable. In the cross between Nicotiana 

 tabacum and N. sylvestris East and Hayes found that the Fi hy- 

 brids were superior to the parents in height, vigor and profusion 

 of flowers, although they were sterile. Crosses between some 

 tobaccos resulted in small, weak plants, and crosses between 

 others were entirely without result. In fact the tobaccos present 

 almost every gradation between negative results and a greatly 

 enhanced vigor of progeny. 



There are many plants, such as our garden peas and beans, in 

 which the opportunity for self-pollination is normally excluded, 

 which propagate indefinitely without deterioration. Others re- 

 produce parthenogenetically or propagate by purely vegetative 

 methods without any apparent loss of vigor. In such species 

 crosses may produce plants of increased size and sometimes 

 greater fertility, or the reverse, according to the particular kinds 

 used. While it is a very general fact that crossing of related 

 varieties produces superior types, the rule is very far from being 

 a universal law. 



Most breeders of animals have held that close inbreeding, 

 while of value for the preservation or the enhancement of desired 

 quahties, tends to produce a deterioration of the stock. The 

 experiments of Crampe, Ritzima Bos, Weismann, von Guaita 

 and Fabre-Domengue afforded support to the commonly accepted 

 opinions of the practical breeder. These results, however, should 

 be accepted with caution In the light of more recent investigations. 



The work of Castle and his pupils on the fruit fly Drosophila 

 showed that brother and sister matings could be carried on for 

 59 generations without loss of fertility, although the crossing of 

 two inbred strains produced a more fertile progeny. Moenk- 

 haus found that within a closely inbred strain of Drosophila, 



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