HYBRID VIGOR OR HETEROSIS 161 



smaller than the first generation hybrids from birth on, 

 showing that some of the growth impetus produced by the 

 hybridization had not been retained. But the growth 

 curve of the second generation hybrids rises rapidly at 

 first, showing the healthy start in life they obtained from 

 their vigorous F l mothers. 



Perhaps no such increase in vigor as that shown in the 

 species cross just described is usually found when dif- 

 ferent sub-races are crossed. It would not be expected, 

 for ordinary races of mammals are continually being 

 crossed within the variety and, therefore, hybridization 

 would not be expected to increase heterozygosis to any 

 marked degree. But results similar to those obtained in 

 plants may be expected if the genetic conditions are sim- 

 ilar. This is proved by the data Wright obtained when he 

 crossed guinea-pigs born of unrelated inbred mothers and 

 fathers. The cross-breds were distinctly superior to their 

 inbred relatives in nearly all characters connected with 

 vigor. In spite of the fact that their inbred mothers were 

 small and somewhat deficient in vigor, a slightly larger 

 per cent, of cross-breds than of inbreds were born alive, 

 and a distinctly larger per cent, of those born alive were 

 raised. They were somewhat heavier at birth in a given 

 size of litter and gained in weight much more rapidly 

 between birth and weaning. They matured earlier and 

 produced larger litters and produced them more regularly 

 than the inbreds. 



Thus the results with animals are comparable to those 

 obtained with plants in all essential features. Briefly, in 

 crosses which are fertile the effects are such as to con- 

 tribute to a greatly increased reproductive ability, making 

 11 



