INBREEDING EXPERIMENTS 105 



maintain the impossibility of injury to any strain of any 

 species through inbreeding per se, but it is proper to say 

 that the evidence in favor of it is practically nil. 



Doubtless we could make our case more convincing to 

 the stockman could the enormous number of really well- 

 kept herd records be cited and analyzed. But it is not 

 possible at present to say whether many of these records 

 satisfy the requirements of modern genetic research. 

 This is a task which must be left to the breeding organiza- 

 tions of the future. We can appeal at present to only two 

 investigations on mammals where the effect of Mendelian 

 recombination has been largely eliminated; and these 

 again are on small mammals, the rat and the guinea-pig. 



The first of these investigations to be reported was 

 that of King. 119 ' 120 ' 121 It was started with a litter 

 of four slightly undersized but otherwise normal albino 

 Norway rats, two males and two females. From these 

 females two lines, A and B, were carried on for twenty- 

 five generations by mating brother and sister. In 

 the earlier generations practically all of the females were 

 used for breeding, but in every generation after the sixth 

 about twenty females were selected from approximately a 

 thousand available young. 



At first the inbred rats exhibited many of the defects 

 reported by Crampe. Numerous females were either ster- 

 ile or produced but one or two small litters. Other ani- 

 mals were characterized by low vitality, dwarfing, and 

 malformations. Stock rats exhibiting the same charac- 

 teristics at this time, however, led to a change in the food, 

 following which the "dire effects of inbreeding" prac- 

 tically disappeared. Whether this improvement in the 

 colony was due entirely to the change of diet or may be 



