518 FREDERICK S. HAMMETT. 



forces concerned in differential development is the coefficient of 

 variability when used comparatively. This figure is the quotient 

 times 100 of the mean of the variates into their standard devi- 

 ation, or, C.V. == cr/J/ X 100 per cent. It is an abstract value 

 which makes possible inter-group, inter-sex, inter-structural and 

 inter-organ comparisons of sensitivity to the totale of forces 

 contributive to variation which play upon the organism. In a 

 study such as this, \vhere a comparison is being made of the 

 organs as parts of a whole, the differences in the coefficient of 

 variability exhibited are indices of differences allied to differential 

 development. They are worthy of investigation because they 

 represent deep-seated biological relationships. 



In Table III. are given the coefficients of variability and their 

 probable errors of the body weight, body length, brain weight and 

 spinal cord weight of the male and female albino rats at 150 days 



of age. 



TABLE III. 



COEFFICIENTS OF VARIABILITY OF THE BODY WEIGHT, BODY LENGTH, BRAIN 



WEIGHT AND SPINAL CORD WEIGHT OF THE MALE AND .FEMALE 



ALBINO RATS 150 DAYS OF AGE. 



Structure. Male. Female. 



Body Weight. . . . 14.19 0.63 n-39 0.49 



Body Length 4.42 0.19 3.40 0.15 



Brain Weight 5.19 0.22 4.92 0.21 



Spinal Cord Weight 6.69 0.29 6.25 0.27 



The values show definitely that body weight in the female is less 

 variable than in the male. This difference has already been 

 noted by King for albino stock (12), for inbred albino stock (13) 

 and for Norway rats (14). Jackson's (8) values show a like 

 direction of difference in the albino rat. 



This sex difference is not exhibited in adult man (n). Never- 

 theless it must be remembered that our data are derived from a 

 racially homogeneous stock, while that of Pearl were not so 

 constituted. It would be rash to state that the results of 

 biometrical analysis of groups of humans is inadequate to divulge 

 such relations. It is better to say that the lack of sex difference 

 in value in a heterogeneous stock is no indication of its non- 

 existence in an homogeneous population. While it would also be 

 rash to generalize from rat to man, the fact that three observers 



