EFFECTS OF INBREEDING ON BODY WEIGHT 95 



Table 12 shows that in each inbred series the coefficients of 

 variabiUty for both sexes decrease in size with advancing age 

 until the animals attained an age of about 300 days, and then 

 tend to become somewhat larger; a similar change in the size 

 of the coefficients at various age periods was also noted for the 

 animals in the earlier generations of the inbred strain as well as 

 for those in the two stock series reared as controls. After reach- 

 ing the height of their reproductive activity at the age of from 

 seven to ten months, certain individuals, especially males, tend 

 to accumulate an excess of adipose tissue; while other individuals, 

 even members of the same litter, will show little change in body 

 weight for a period of several months, or they may even decline 

 steadily in body weight although they are apparently in good 

 physical condition. The increased variability in the body 

 weights of older rats is, therefore, due in great part to the ac- 

 cumulation of a greater or less amount of adipose tissue; it is 

 not a growth phenomenon comparable to that shown during 

 early postnatal life. 



In order to make a closer analysis of the relative variabihty 

 in the body weights of animals in successive generations of the 

 inbred strain, coefficients of variability were calculated from the 

 body-weight data for the animals in three generations combined 

 as summarized in table 7. This series of coefficients is shown in 

 table 13. 



In table 13 the average coefficients for the male groups com- 

 prising the individuals of the sixteenth to the twenty-fourth 

 generations vary by less than one point, so it is evident that in 

 the later generations of the inbred strain the variability in the 

 body weights of the males did not decrease with the advance of 

 inbreeding, as was the case in the earher generations (King, '18; 

 table 16). The series of coefficients for the males of the twenty- 

 fifth generation are, as a rule, smaller than the corresponding 

 coefficients for the males of the preceding generation group. 

 But the difference between the average coefficients for the^two 

 groups is less than three times the probable error, so it cannot 

 be considered as significant, especially as the number of body- 

 weight records used in calculating the coefficients for the animals 



