EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 373 



offspring of the fourteenth generation of a closely inbred race, 

 pairs of individuals showing very different degrees of productive- 

 ness and then inbreeding their descendants. Moenkhaus con- 

 tinued some of his lines for seventy-five generations and found 

 that close inbreeding (brother and sister) was not deleterious 

 either to fertility or to vigor. Hyde ('14) has found also that in 

 certain strains of Drosophila sterility is an inherited character 

 that is not influenced by inbreeding, and that ''selection is an 

 effective agent in controlling it." 



In the present series of inbreeding experiments on the rat, the 

 productiveness of the strain was decreased by malnutrition during 

 the early generations, but normal fertility was restored as soon as 

 the animals were adequately nourished. In later generations the 

 fertility in the inbred animals was greater than that in the series 

 of stock controls reared under similar environmental conditions. 

 Thus even after a high degree of sterility had been introduced 

 into the strain it was not retained in spite of the fact that close 

 inbreeding was continued. In the later generations any tend- 

 ency to sterility that appeared was evidently suppressed by 

 selection. In the rat, as in Drosophila, selection seems a more 

 potent factor for good than inbreeding is for evil. 



During the past few years it has been shown, by a series of 

 brilliant experiments, that characters tend to be inherited in 

 groups and that this grouping depends upon the fact that the 

 genetic factors involved are not segregated independently in 

 gametogenesis, but tend to be linked together (Morgan et al., '15). 

 In these experiments with the rat it has been found that animals 

 tliat are large and vigorous when young tend to mature early, 

 to be very productive, and to live to an advanced age. While 

 all of these characters are influenced to a considerable degree by 

 environmental conditions, it is evident that they must all depend 

 to some extent upon heritable genes, since they are transmitted 

 from generation to generation. A selection of breeding animals 

 on the basis of size and early maturity has meant also selection 

 for high fecundity and for characters that represent superior vigor 

 of constitution, it would seem as if the genetic factors involved 

 must tend to be inherited together, although they are probably 

 not linked as are many of the genes in Drosophila. 



