42 HELEN DEAN KING 



for stock males, taking all ages together, was 13.5, while that for 

 the inbred males was 12.4. Since the difference between these 

 coefficients is only slightly greater than the probable error, it is 

 evident that close inbreeding did not decrease the variability 

 of the entire male population more than about 8 per cent. 



When the coefficients for the inbred females of the combined 

 series (table 15) are compared with those for stock females 

 (table 17) it is found that in this sex, also, inbred animals were 

 more variable in body weight than stock animals during the early 

 growth stages. After the age of ninety days, however, stock 

 females tended to be slightly more variable in body weight than 

 inbred females. The average coefficients for the two groups 

 differ by less than one point, so it appears that the inbred females 

 had practically the same range of variability in body weight as 

 the stock females. 



In a btudy of the variability in body weight of the albino 

 rat, Jackson ('13) found that: ''variabihty in body weight is 

 lowest at birth (the coefficient being about 12) and is not much 

 higher at seven days (16). It appears highest at three weeks 

 (28), and at later periods varies from 19 to 21. The average co- 

 efficient, taking all ages together, is 19." In Jackson's series the 

 maximum variability in body weight comes at an earlier period 

 than it does in either the inbred series or in the stock controls, 

 and his coefficients are much higher for all ages. The fact that 

 Jackson used data obtained from rats that represented "for the 

 most part a random sample of the general population at each 

 age" undoubtedly accounts for the rather wide range in our re- 

 sults, although a difference in the strain of albinos used and in 

 the environmental conditions to which the rats were subjected 

 may have contributed to the result. 



In order to determine whether the animals in the later inbred 

 generations were any less variable in body weight than those in 

 the general inbred population, the coefficients of variation for the 

 body weights at different ages of individuals in eight fitters of 

 the fifteenth inbred generation were determined and are given 

 in table 17. All of these coefficients were decidedly lower than 

 the corresponding ones for the individuals of the combined (A, B) 



