382 FRANCIS B. SUMNER 



than either parent race, taken by itself, and the chances of the 

 segregation of extreme types would be much better in the F2 

 than in the Fi series. All this seems so obvious that it is 

 difficult to understand how such an objection could be raised 

 seriously. 



Passing to a very interesting incidental result of these studies, 

 certain striking differences have been observed between the 

 sexes. The most certain of these relate to the feet and the 

 pelvic bones. When mice of the same size are compared, it is 

 found in all but one of the eight local collections that the aver- 

 age foot length of the males is greater than that of the females. 

 In most cases this difference is statistically significant, whereas 

 in the single exceptional case there is a practical equality between 

 the figures. Also, in all of the four races whose skeletons have 

 thus far been measured, the average length of the innominate 

 bone (pelvis) is greater in the females, this difference, in three 

 of the cases, being large in proportion to its probable error. 



It is of interest to note that the two differences just men- 

 tioned are of opposite sign. We may profitably consider the 

 bearing of these relations upon certain prevalent ideas regard- 

 ing the origin of secondary sexual characters. Although many 

 theories have been put forth in this field, it is my understand- 

 ing that secondary sexual differences, in the higher vertebrates 

 at least, are now commonly supposed to be due largely to the 

 action of internal secretions or 'hormones,' produced by che 

 gonads or by certain cells of these. To simplify the situation, 

 we might assume, in each sex, the existence of a single charac- 

 teristic hormone which determined all of the secondary sexual 

 characters. In the present instance, the male hormone would, 

 among other things, stimulate the growth of the feet and inhibit 

 that of the pelvic bones. The female hormone, on the con- 

 trary, might be supposed to stimulate the growth of the pelvic 

 bones and inhibit the growth of the feet. 



Now, according to the foregoing hypothesis, such a state of 

 affairs would inevitably bring about in each sex a negative cor- 

 relation between feet and pelvic bones. For in each sex there 

 would surely be wide individual differences in the amount of 



