36 HELEN DEAN KING 



the stock rats had the advantage of birth in winter. Even 

 under these conditions the males of the fifteenth inbred genera- 

 tion were, on the whole, heavier than the controls after they 

 had attained maturity. 



Figure 14 gives graphs showing the weight increase with age 

 for females of the fifteenth inbred generation and for females of 

 the stock control series. 



A comparison of the growth graphs for the two groups of fe- 

 males (fig. 14) leads to the conclusion that there was no essential 

 difference in the rate of growth of stock and of inbred females 

 during the early growth period, but that in the adult state the 

 inbred females were slightly heavier at any given age than the 

 females in the control series. 



3. VARIABILITY IN THE BODY WEIGHTS OF INBRED RATS 



For the purpose of ascertaining the extent of variability in 

 the body weights of the rats in the two inbred series, their co- 

 efficients of variation for the body weights at different ages 

 were computed, together with the probable errors. Only rec- 

 ords for animals belonging in the seventh to the fifteenth inbred 

 generations were used for this purpose. No attempt was made 

 to find the extent of variability in the body weights of the ani- 

 mals in the first six inbred generations, since only one weight 

 for each animal was recorded and the age period covered by the 

 data at hand was comparatively short. 



Table 15 gives the coefficients of variation for the body 

 weights at different ages, with their probable error, for the in- 

 dividuals in the seventh to the fifteenth generations of each of 

 the two inbred series, and for the animals in the two series com- 

 bined. Grouped data were used in making the calculations for 

 the thirteen and for the thirty day periods, as only the average 

 body weight of the individuals of each sex was recorded in the 

 weighings of the various litters at these ages; foi* all other ages 

 the individual data were employed. 



Comparing the corresponding coefficients for the males and 

 for the females it is found that in each series the female rats 



