EFFECTS OF INBREEDING ON BODY WEIGHT 43 



series of inbreds (table 15). The difference between the average 

 coefficients was quite large, amounting to 4.3 points for the males 

 and to 3.8 points for the female groups. This result indicates 

 that the animals of the fifteenth inbred generation were about 35 

 per cent less variable in body weight than the animals in the gen- 

 eral inbred population. 



When the coefficients of variation for the individuals in the 

 fifteenth inbred generation are compared with those for the stock 

 controls (table 17) the results are equally striking and significant, 

 At only one age period, i.e., thirty days, was the coefficient of vari- 

 ation for the inbred males slightly in excess of that for the stock 

 males; at all other ages the coefficients for the stock males were 

 much the larger. The average coefficient for the stock males 

 was 13.5, while that for the inbred males was only 8.1. It- 

 appears, therefore, that the variability in the body weights of 

 the males of the fifteenth inbred generation was, on the average, 

 about 40 per cent less than that in stock males. 



Judging from the size of the corresponding coefficients (table 

 17) females belonging to the fifteenth inbred generation were 

 more variable in body weight at ninety days of age than the fe- 

 males of the stock controls, at all other ages the stock females 

 were the more variable. The difference of 4.5 between the 

 average coefficients for the two groups indicates that the females 

 of the fifteenth inbred generations were about 40 per cent less 

 variable in body weight than stock females. 



From a study of fraternal variability in the albino rat, Jackson 

 concludes that ''in general the variation in body weight within 

 a given litter of albino rats is probable less than half that of the 

 general population of the same age under similar environment." 

 In the series of stock animals reared as controls for the present 

 inbred series it was shown (King, '15 a) that "the range of varia- 

 bility within the litter is about 70 per cent that of the general 

 population in the case of the males, while for the females it is 

 about 55 per cent." 



By comparing corresponding coefficients, as given in table 17, 

 it is possible to determine whether variability in the body weight 

 of the animals in the fifteenth inbred generation was greater or 



