48 HELEN DEAN KING 



marked deterioration in the stodv. Castle ('16 a) inbred rats 

 within narrow hnes.of selection for seventeen successive gen- 

 erations and was able to maintain races 'with fair vigor and 

 fecundity.' Inbreeding experiments with Drosophila, carried on 

 for many generations by Castle et al. ('06) and by Moenkhaus 

 ('11), have shown that in this form, also, races of large size and 

 vigor can be maintained under the closest inbreeding by simply 

 selecting the most vigorous parents for breeding. 



Undoubtedly various species of plants and animals react dif- 

 ferently under inbreeding. In tobacco, inbreeding is ''beneficial 

 and offers an effective means of maintaining desirable character- 

 istics in the established varieties" (Shamel, '05); while in maize 

 inbreeding leads to a considerable loss in vegetative vigor but 

 not to degeneration (East and Hayes, '11; '12).' In swine, ac- 

 cording to Darwin ('76), close inbreeding invariably leads to 

 sterility and to a considerable loss in body size after only a few 

 generations, although this has recently been denied by Gentry 

 ('05). From available evidence inbreeding seems to be very inju- 

 rious to dogs (Darwin, '76; B, '06), to pigeons (Fabre-Domengue, 

 '98) and to goats (Ewart, '10). On the other hand, it is chiefly 

 through inbreeding, with selection, that the thoroughbred types 

 of cattle, of sheep and of horses have been developed and fixed, 

 and there is no evidence of degeneration in the descendants 

 of deer and of rabbits that have been inbred in isolated com- 

 munities for many years (Ewart, '10). If pure stock that ful- 

 fills standard requirements as to size, vigor and fertility is used" 

 for the investigation, and only vigorous, sound animals are 

 allowed to breed, there is, theoretically, no ground for beheving 

 that continued inbreeding will cause either loss of vigor or a 

 decrease in body size. In these experiments with the rat, the 

 bad effects of inbreediag per s,e, as far as they might manifest 

 themselves by a decrease in the body size of the individuals, have 

 apparently been entirely prevented through the use of a strain of 

 animals that seemingly had no inherent defects and by a careful 

 selection of breeding stock. Even after twenty-eight genera- 

 tions of continued brother and sister matings the inbred animals 

 have not deteriorated in any way, and they are still superior 



