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 F; 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 
qualities, 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, 
