44 ORGANIC EVOLUTION 



have thus been able to prove that, on the average, 

 like tends to mate with like. This fact is, of course, 

 in a measure obvious. The individuals of any 

 species are, as a rule, unattracted by the individuals 

 of any other species. But these researches, so far 

 as they have gone, appear to show that this principle, 

 which Professor Pearson calls assort ative mating — as 

 distinguished from the preferential mating which we 

 have already discussed — holds good within the 

 species also. For instance, despite a popular im- 

 pression that extremes meet in marriage, it has been 

 shown by examination of thousands of the living, 

 that a blue-eyed man is more likely than a brown- 

 eyed to marry a blue-eyed wife. People with a 

 " strong constitution " (of which longevity is taken as 

 the criterion) tend to marry their like ; short men 

 tend to marry shorter women than do tall men, 

 and so forth over a whole host of characters. 

 Further, Professor Raymond Pearl, of the University 

 of Michigan, has found that the same is true 

 of certain animalcules — unicellular creatures about 

 one-hundredth of an inch in length — known as the 

 paramcecia. In all probability the same holds good 

 throughout the animal world generally, though no 

 inquiries have yet been made. This principle, the 

 mating of like with like, Professor Pearson calls 

 homogamy ; and he regards it as having been a most 

 important factor in the isolation and perpetuation 

 — indeed, in the " origin " — of animal species. This 

 is a mode of sexual selection which furnishes, it 

 would appear, a most important addendum to Dar- 

 win's " Descent of Man." It occurred to me that, 

 if homogamic unions could be shown to be more 



