EVOLUTION THE MASTER-KEY 



mating the extent to which people of strong con- 

 stitutions marry their like and conversely, since 

 longevity, as recorded on tombstones, may be 

 taken as a criterion of general bodily vigor. In 

 addition, thousands of married persons have been 

 examined with regard to height, eye - color, and 

 many other characters. 



The biometricians have thus been able to show, 

 by statistics analyzed and checked in a manner 

 quite impossible for any but the trained mathe- 

 matician and logician, that, for instance, a blue- 

 eyed man is more likely than a brown-eyed one to 

 marry a blue-eyed woman. People with a " strong 

 constitution" (estimated as we have seen) tend 

 to marry their like; short men tend to marry 

 shorter women than do tall men and so forth, 

 over as many characters as have hitherto been 

 examined. Various possible fallacies have had 

 to be excluded, such as the effect of resemblance 

 among local races, and the effect of exposing hus- 

 band and wife to the same environment; but the 

 essence of biometry is that it seeks all possible 

 explanations and then proceeds systematically to 

 test them. After so doing, the conclusion in this 

 instance is that " there is a real selection in marriage 

 between husband and wife on the basis of general 

 constitutional resemblance." 



Now if this be true of man, may we not reasonably 



expect homogamy to occur in lower forms of life ? 



This may surely be expected, unless we agree with 



Darwin that sexual selection depends upon the 



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