BIOMETRICS 165 



But Pearson has extended still further the method of 

 measuring the correlation between given characteristics 

 of two individuals. He compared husband and wife with 

 regard to stature and eye-colour, and found a distinct cor- 

 relation for both characteristics in married couples. By 

 constructing, according to the usual method, a correlation 

 table of the statures of 1,000 husbands and wives, he 

 found a decided tendency for " like to mate with hke " — 

 i.e., tall men married on an average tall women, medium 

 men women of medium height, and small men small women. 

 A similar result was obtained for eye-colour. This shows 

 definitely that, contrary to the popular belief that " oppo- 

 sites attract each other," there is, as Pearson has called it, 

 " assortative mating " with regard to stature and eye- 

 colour. " In fact, husband and wife are," according to 

 him " for one of these characters " (stature) " more alike 

 than uncle and niece, and for the other " (eye-colour) 

 " more alike than first cousins." 



(h) Fertility. 



So far we have dealt in detail with the correlation of 

 such characters as had a definitely visible outward ex- 

 pression, and could therefore easily be measured. But, 

 as we said at the beginning of our chapter on correlation, 

 there are other characters which, being physiological or 

 functional variations of the organism, are more elusive, 

 though they, too, can be shown to come under the law of 

 correlation. One of these characters is fertility. 



That relative fertility plays an essential role in the origin 

 of species has been an acknowledged fact ever since Darwin, 

 and various theories have been advanced in order to explain 

 how the divergence of new varieties from the mother-type 

 and their final separation into distinct species take place. 

 Relative sterility between the incij)ient species seems to be 

 an essential factor. Now, Pearson has been able to show 

 that there is a definite correlation between certain characters 



