368 HELEN DEAN KING 



Crampe's inbreeding experiments were begun, in 1873, with an 

 Albino female and a white and gray male. From the mating of 

 these rats he obtained the litter of five young which formed the 

 basis of his breeding stock. These animals were inbred, in various 

 degree of relationship, for seventeen successive generations. 

 Crampe states that many of the animals were sterile and that 

 others lost their reproductive instincts at the end of the first year. 

 Various kinds of malformations appeared; the animals were 

 seemingly too weak to resist disease of any kind, and they died 

 at a relatively early age. The weakness of these rats and their 

 susceptibility to disease, as well as the high degree of sterility 

 among them, all point to the probability, as Ritzema-Bos sug- 

 gests, that Crampe started his experiments with animals taken 

 from a defective stock. Since results similar to Crampe's were 

 obtained in the early part of my own experiments, I am inchned to 

 the opinion that inadequate nourishment was a factor that was 

 responsible, in great part, for his failure to maintain the stock in 

 good physical condition. 



Ritzema-Bos started his investigation in 1886 with a litter of 

 twelve rats that was obtained from the mating of an Albino female 

 and a wild Norway male. These rats were inbred, in various 

 ways, for six years, during which time, Ritzema-Bos states, 

 ''about thirty generations were obtained." There is evidently 

 some inaccuracy in this latter statement. The female albino 

 rat does not cast her first litter until she is about three months 

 old; wild rats do not breed, as a rule, before they are four or five 

 months old. Assuming that all of the females used in Ritzema- 

 Bos' experiments bred at the earhest possible age, i.e., three 

 months, only four generations could possibly be produced in a 

 year: this would give aj maximum of twenty-four generations at 

 the end of six years. In my own experiments an average of about 

 three and one-half generations a year were obtained. 



Ritzema-Bos gives data showing the average size of the litters 

 and the number of infertile matings during the various years in 

 which the work was in progress. These data have been repro- 

 duced in table 13. 



