34 HELEN DEAN KING AND HENEY H. DONALDSON 



in wild Norways. This lessening of nervous tension, and its 

 far-reaching effects through the nervous system, may be a 

 factor that influenced the course of body growth in gray rats 

 of the later generations, since these animals were relatively 

 very tame as compared with wild Norways. 



VAEIABILITY IN BODY WEIGHT 



Since changes in the rate and in the extent of body growth 

 in captive gray rats took place slowly, as shown in the previ- 

 ous section, it seemed probable that any change that occurred 

 in body- weight variability was also a gradual one. Coef- 

 ficients of variation for body weights at different age periods 

 were computed, therefore, only for individuals in the first 

 and for those in the tenth generations. A comparison of these 

 two series of coefficients will indicate to what extent vari- 

 ability in body weight had changed at the end of ten genera- 

 tions. 



Coefficients of variation, with their probable errors, for 

 males in these two generations are given in table 4 ; those for 

 females are shown in table 5. Each of these tables also con- 

 tains a series of coefficients for body weights in the stock 

 Albinos used for comparison with the Grays in the analysis 

 of body growth. The coefficients for Grays of the first gener- 

 ation, as well as those for Albinos, are reproduced from previ- 

 ous publications (King, '15 a, '23). Grouped data had to 

 be used in calculating the coefficients for the thirteen- and 

 for the thirty-day periods, since only the average body 

 weights for rats of each sex were recorded in the weighings 

 of the litters at these ages. Coefficients for all other age 

 periods are based on individual data. 



Variability in body weight, as indicated by the size of the 

 coefficients in table 4, had the same general trend in all three 

 groups of males : it was relatively low in young animals, rose 

 quickly to a maximum, and then fell gradually to a level that 

 was maintained, with unimportant fluctuations, throughout 

 adult life. At the various age periods, however, variability 

 in body weight differed considerably in the three groups. 



