222 GENETICS AND EUGENICS 



get an idea of how frequently genetic changes are happening. 

 Such purification of stock has rarely been undertaken. Miss 

 King has inbred rats, brother with sister, for 25 generations 

 and the resulting stock has been studied as to genetic char- 

 acter chiefly in respect to sex ratio. In the course of the 

 inbreeding, selection was made in two different lines for 

 opposite changes in the sex ratio; in Series A for a high ratio 

 of males to females, and in Series B for a low ratio of males 

 to females. The result in 6,274 young of the 25 generations 

 of the A series was a ratio of 122.3 males to 100 females. In 

 5,893 young of the B series, the ratio was 81.8 males to 100 

 females. These very different results were secured within 

 the first 10 or 12 generations of selection, after which progress 

 in the direction of the selection was negligible. This indicates 

 that the genetic factors responsible for the changes were 

 already in existence in the stock at the beginning of the 

 selection and were gradually sorted out and rendered ho- 

 mozygous in the early part of the selection period, and that 

 new genetic changes appreciable in amount did not appear 

 subsequently. 



Selection was made simultaneously for large size in the 

 course of the inbreeding experiments of Miss King and this 

 resulted in producing inbred races which were larger than 

 the unselected stocks from which they were derived. The 

 maximum size was attained as early as the seventh inbred 

 generation, possibly earlier, as the seventh generation is the 

 earliest one for which comparable data are available. The 

 inbred races maintained throughout the entire period, up to 

 the twenty-fifth inbred generation, their superiority in size 

 over the control stocks from which the inbred strains had 

 originated, but no evidence was found that genetic size factors 

 had changed in the period between the seventh and twenty- 

 fifth inbred generations. Observations were made also on 

 fecundity as indicated by size of litter in the inbred rats. 

 No change was observed in this character as a result of the 

 inbreeding. The average size of litter was for the inbred 

 series 7.5 young, for the stock albinos used as a control, 6.7 



