I. LIFE PROCESSES 37 



influence the size of coefficients of variation for body weight 

 that are calculated from individual, not from average, body 

 weights. 



It was not possible to determine the variability in body 

 weight at birth in the gray rats used in this study. C<ct 

 ficients of variation were calculated from the birth weights 

 of 226 males and 212 females in seventy-one litters cast by 

 gray females of the eighth to the tenth generation, inclusive, 

 in order to obtain some idea of body-weight variability in 

 newborn gray rats. The coefficient for the males was 12.83 

 0.40 ; that for the females was 12.62 0.42. Variability 

 in body weight at birth was, therefore, about the same in the 

 two sexes. Birth weights in the males were slightly more 

 variable than those in the females, ranging from 3.5 to 7.7 

 grams in the former and from 3.6 to 7.2 grams in the latter. 



Data given above indicate that both in the gray and in the 

 albino race of Norways males tend to be more variable in 

 body weight than females during adolescence and early ma- 

 turity. A slight, but constant, difference in body-weight 

 variability of the sexes has been noted in other series of 

 Albinos (Jackson, '13; King, '18, '19), and also in mice 

 (Robertson, '16; Sailer, '27). These observations are in 

 accord with the theory, held by Darwin (1871 ) and by Brooks 

 (1883), that throughout the organic world males, in general, 

 are more variable than females. 



In man, according to Boas (1897) and Porter ('05), vari- 

 ability in body weight is correlated with the growth rate. 

 A like correlation was found in the stock Albinos used in \\\\^ 

 study, since variability was greatest at an age period at 

 which growth was very rapid, and then gradually declined 

 as the growth rate decreased during adult life. A rapid 

 growth rate does not necessarily indicate a very high v,-iri 

 ability in body weight, however, as in both sexes of albino 

 rats the body-weight variability was less than that in gray 

 rats in which the growth rate was relatively slow during 

 earlv life. 



