HYBRID VIGOR OR HETEROSIS 159 



work of Phillips ^''^ on crosses between the large French 

 Rouen and the small domestic Mallard duck, and by the 

 work of Punnett ^^^ on crosses betwen the Silver Sebright 

 bantam and Gold-pencilled Hamburgh breeds of poultry. 

 There is an increase over the average of the two parents, 

 but the Fi's do not reach the size of the larger parent 

 race. Part of the reason for the comparatively small 

 sizes of the F^ 's in these crosses, however, is due to the 

 fact that the crosses were always made on the small hens 

 allowing the hybrid birds to get their start in life with 

 only the nutriment stored ia the smaller eggs. 



The greatest amount of data on this subject, just as 

 there is the greatest amount of interest, has been obtained 

 from the mammals. In the meat breeds of cattle, swine 

 and sheep, as ia poultry, it is a common practice to cross 

 distinct races and sell the progeny. The increase in size 

 and the rapidity with which this size is obtained are so 

 general a phenomenon that it bids fair partially to replace 

 the older method of pure line breeding. Not only are 

 varietal crosses thus characterized, but specific crosses. 

 We have already mentioned the mule. With the disad- 

 vantage attached to sterility, the mule certainly would^not 

 have held its own throughout the past forty centuries 

 were it not for its tremendous capacity for work and its 

 remarkable resistance to disease. Crosses between the 

 ass and the zebra, and between the cow and the zebu also 

 give animals of considerable merit, and one can hardly 

 refrain from thinking that within a few years some con- 

 siderable use will be made of them. 



For precise data on the effect of crossing different 

 races, however, we must turn to the small mammals used 

 so constantly in experimental work, the mouse, the rat, the 



