290 Heredity and Eugenics 



the blonde; the straight hair and the curly; the flaxen 

 hair and the brown and the red; the blue eyes and the dark; 

 the straight nose, the aquiline, and the pug; the broad 

 head and the high and the narrow; the thin Hps and the 

 thick, and so through the categories. The only reason why 

 we do not have distinct species of men distinguished by 

 such traits is because of the extensive hybridization that 

 man is undergoing. Everywhere, brown eyes mate with 

 blue, black hair with flaxen, curly with straight, and so on. 

 Man's potential races are not realized just because of the 

 universal interfertility of the different races and because 

 of the mobility of man's habitat. 



Now is there any evidence aside from a-priori consid- 

 erations for testing this view ? Are the potentialities that 

 we assume anywhere realized? Is the theory of man's 

 universal hybridization more than a figment of the imagi- 

 nation ? 



First, let us admit that evidence for the unity of, say, 

 the Caucasian race has been offered by the biometricians. 

 They have said if the race is homogeneous it will show itself 

 by the biometric test. Measure a trait in 10,000 individ- 

 uals; and plot the relative frequency of the diff'erent values 

 found. If that frequency rises in a gentle curve from its 

 lowest value to a maximum at some middle value and then 

 falls again smoothly to the highest value the curve is a 

 simple curve and this has been regarded as proving a unit- 

 population. But it does not prove it; for we now realize 

 that even the apparently simple curve may be the result- 

 ant of many more elementary curves whose number dimin- 

 ishes uniformly on both sides of the center. The elementary 

 curves are the ones that include the fluctuations of the real 

 units. 



