CORRELATION OF BRAIN AND SPINAL CORD. 519 



of five different rat populations have obtained sex-differences in 

 body weight variability in the same direction, is indication that 

 we are dealing with a biological sex-difference of response to 

 factors contributive to variability, and that studies on man 

 should be so planned as to eliminate the possible interfering 

 factor of racial heterogeneity which might well mask a basic sex 

 difference. Further evidence supporting this view is the fact 

 that the body weight variability of the rat of homogeneous stock 

 is generally considerably less than that of man of heterogeneous 

 stock. The values recorded by Pearl (n) for man are 21.3 for 

 the male and 24.7 for the female. 



The female albino rat is also less variable than the male in body 

 length to a statistically valid degree, and shows a like tendency 

 in brain and spinal cord weight, though the degree of difference 

 here is too small to be valid. At this time I do not want to go 

 into a comparison of the entire array of organs in the rat, but 

 might point out that the sex difference is not uniform in direction 

 for all the organs, which fact has interesting implications as a 

 later study will show. 



However, the fact that in body weight and in body length the 

 female is less variable than the male albino rat indicates in this 

 species, at least, a greater stability of the female organism as a 

 whole to outside forces tending to disturb body size equilibrium. 

 Teleologically this might be considered an expression of a pro- 

 tective mechanism, tending to enhance resistance and thus favor 

 the essential purpose of the female, namely reproduction. 



From Table III. it is seen that body length is much more stable 

 than body weight. The same holds true for man. The C.V. for 

 stature in the male was found to be from 3.8 to 4.3 (Pearl) and 

 3.6 to 4.5 (Blakeman) : and in the female from 4.0 to 4.7 (Pearl) 

 and 3.8 to 4.2 (Blakeman). Attention is directed to the fact that 

 the values for man are of the same order of magnitude as those for 

 the rat. 



It is hardly necessary to point out that the lesser variability of 

 body length or stature is a consequence of the greater inherent 

 metabolic stability of the skeleton as the chief component de- 

 terminative of this measurement, as compared with that of the 

 body weight with its predominant element of metabolically 



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