8 SOCIAL EVOLUTION 



by the statement that there is a relation between the 

 frequency of a particular variation and the amount of its 

 deviation from the mean stature of the group. 



The task of registering the variations that occur in 

 any group of creatures may at first sight seem tedious 

 and far removed from the warm pulsations of life, but 



CM. 109 113 117 m 125 129 133 137 141 145 149 ISA 



From Thorndike, "Individuality." 



FIGURE 1. Distribution of Stature of American Boys 10J years old. 



a little experience in the measurement of such things as 

 length of rose petals, the length of bird wings, or of star- 

 fish arms, will convince the student that biometrics may 

 lead him into the very heart of the matter. If the regis- 

 tration of the dimensions of a particular character be 

 carried on year after year in similar material, and show 

 a consistent increase in asymmetry or skewness of the 

 curve (asymmetry or skewness means a curve in which 

 the hump as in the figure, is not over the middle, but 

 nearer one end, making the slope at that end more abrupt 

 and at the other end more gradual) this must mean that 

 the species is moving in a definite direction as regards 

 the particular character measured. Similarly, the per- 

 sistent occurrence of a well-substantiated double-humped 



