EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 371 



normal litter of either of the varieties that were crossed (five to 

 six young per litter). Since crossing did not restore the normal 

 fertility of the individuals, it would seem as if there must have 

 been a strong tendency to sterihty in each of the strains crossed. 

 If such were the case, continued inbreeding, apparently without 

 selection, would bring out this latent character and intensify it. 



It seems rather remarkable that, of the many writers who have 

 cited the results of the above series of experiments as proof that 

 close inbreeding lessens fertihty, not one, to my knowledge, has 

 emphasized the fact that all of these experiments were made with 

 hybrids and not with a pure strain. Hybridization in itself, as 

 many investigators have noted, often produces a most marked 

 effect on fertility. Some hybrids are equal, or even superior 

 to the parent stock in fertility others are completely sterile; 

 and among the hybrid offspring from various crosses all grades of 

 productiveness from normal to complete sterility have been found. 

 When hybridization increases fertihty, its most marked effect is 

 usually found in the animals of the Fi and Fo generations, and in 

 later generations productiveness, as a rule, tends to decrease. 



In connection with another problem, I have for several years 

 been breeding the Fi hybrids between the wild Norway and the 

 albino rat, and I have also inbred various strains of 'extracted' 

 rats, brother and sister, for several generations. Careful records 

 have been kept of the litter production in all of these strains. 

 While the great majority of the Fi hybrid females are fertile, at 

 least 25 per cent of the Fo females are completely sterile, and 

 about 10 per cent of those that do breed have only one or two 

 litters. None of the 'extracted' strains that I have studied 

 have even been as fertile as the inbred Albinos. The increase of 

 sterility and the diminution in litter size with continued in- 

 breeding has been very marked in some of these strains, but this 

 lessened productiveness has been due, I believe, to hybridization, 

 and it has not been influenced by inbreeding save in as far as in- 

 breeding has intensified the tendencies which acted unfavorably 

 upon productiveness. By rigid selection of only the most fertile 

 individuals for breeding, from a large potential breeding stock, it 

 might be possible to eUminate from the 'extracted' strains of rats 



