376 HELEN DEAN KING 



light, of temperature, and of nutrition. Although the two series 

 were descended from the same ancestral stock, apparently there 

 was an inherent difference in the gametic constitution of the two 

 pairs of rats with which the experiment was started, which 

 persisted from generation to generation and produced the effects 

 noted. 



While the inbred strain of rats that has been developed in the 

 course of these experiments is seemingly superior to the average 

 run of stock Albinos in body size, in fertility, and in longevity, 

 I do not claim that this superiority is due solely to the fact that 

 the animals were inbred, neither do I wish to assert that, in gen- 

 eral, inbreeding is better than outbreeding for building up and for 

 maintaining the general vigor of a race. The two forms of breed- 

 ing are not mutually exclusive: each has its merits, and the one 

 should supplement the other to bring out the best in any stock. 

 The favorable results that have been obtained in these experi- 

 ments have been achieved through the constant selection of 

 only the best animals from a larger number available for breeding 

 purposes and by keeping the environmental conditions as uniform 

 and as favorable as it was possible to make them. These experi- 

 ments have fully demonstrated, I think, that even in mammals 

 the closest form of inbreeding possible, i.e., the mating of brother 

 and sister from the same litter, is not necessarily injurious either 

 to the fertility or to the constitutional vigor of a race even when 

 continued for many generations. Success or failure in inbreeding 

 experiments depends chiefly, it would seem, on the character of 

 the stock that is inbred, on the manner in which the breeding 

 animals are selected, and on the environmental conditions under 

 which the animals are reared. There is no warrant, therefore, 

 either in theory or in fact, for the dogmatic assertion of Kraemer 

 ('13) that : ' ' continued inbreeding must always result in weakened 

 constitution, through its own influence." 



4. SUMMARY 



1. The present paper gives data showing the fertilitj^ the time 

 of puberty, and the longevity in two series of albino rats (A,B) 

 that were inbred, litter brother and sister, for twenty-five gener- 

 ations. 



