CHAPTER VIII 



CONCEPTIONS AS TO THE CAUSE OF HYBRID 

 VIGOR 



THE early plant hybridizers, although they frequently 

 discussed the increased size and vigor of their crosses, 

 seldom commented on the effect of inbreeding, and made 

 no speculations as to the cause of either. The animal 

 breeders of the period were more imaginative. Ac- 

 quainted with both phenomena, but more familiar with the 

 results of inbreeding, they unhesitatingly linked the two 

 the first as an antidote for the second. They attributed 

 most of the injurious effects which appeared in their herds 

 to the concentration of undesirable traits. If unfavor- 

 able characters and tendencies to disease were present, 

 mating similar animals brought out these undesirables 

 more pronouncedly; whereas, if healthy animals from un- 

 related herds were brought in, such tendencies were 

 checked, the defects disappeared, and the health and vigor 

 of the herds returned. 



Darwin, however, refused to ascribe any large part 

 of the effects of inbreeding to this cause. He knew of 

 many cases in which weakened animals from different in- 

 bred herds had been mated together, and gave progeny 

 of full health and vigor and of increased size. The unde- 

 sirable features induced in both herds by inbreeding dis- 

 appeared when animals of the different herds were mated. 

 Instead of a concentration of the less favorable traits of 

 the two parental lines the reverse seemed to have oc- 

 curred. Similar cases in plants were familiar to him, and 



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