24 HELEN DEAN KING 



graphs in figure 6 show, however, that males tended to be 

 more variable than females during early postnatal life, and 

 less variable during later life. It seems probable that the 

 greater body-weight variability of gray females during adult 

 life can be ascribed to reproductive activity, as Kopec ('32) 

 has suggested is the case in mice. Pregnancy and lactation 

 have marked effects on the body weights of breeding females. 

 When weight determinations are made at stated intervals 

 over a considerable period of time, the data obtained often 

 include weights for lactating females and for those in early 

 stages of pregnancy, even though, as in this investigation, 

 weighings are omitted when females are known to be preg- 

 nant or when their weights are greatly reduced because they 

 are suckling large litters. Coefficients of variation for body 

 weights of adult females, calculated from data thus obtained, 

 are undoubtedly larger than would be a series of coefficients 

 based on data for unmated females whose weights had not 

 been affected by reproductive processes. Taking this fact 

 into consideration, the greater body-weight variability of 

 females during adult life has no significance. For age periods 

 during early life, where the two series of data are strictly 

 comparable, body-weight variability in males was somewhat 

 greater than that in females. A similar sex relation in body- 

 weight variability has been found in albino rats (Jackson, '13; 

 King, '18, '19) and in mice (Robertson, '16; Sailer, '27; 

 Kopec, '32). These findings accord with the view that 

 throughout the organic world males, in general, tend to be 

 more variable than females. 



Darwin (1875) was of the opinion that variability of every 

 kind is caused directly or indirectly by changed conditions of 

 life, and that, "if it were possible to expose all individuals 

 of a species during many generations to absolutely uniform 

 conditions of life there would be no variability." In light 

 of our present knowledge of genetics, it is probable that 

 changed conditions of life in captivity had little effect on the 

 body-weight variability of gray rats. The gradual decrease 

 in the variability of these rats as the generations advance can 



