FACTORS OF ORGANIC EVOLUTION 



preferential mating. It probably exists in human 

 society at the present day, but great difficul- 

 ties are encountered in the attempt to measure 

 it. The second kind of sexual selection may be 

 called assortative mating. It depends on the fact 

 that like tends to mate with like. This principle 

 of homogamy may indeed turn out to be that " un- 

 known factor" in organic evolution which many 

 have declared to be the operation of design on the 

 part of a Creator. Homogamy may indeed have 

 been a necessary factor in the isolation of species 

 as we know them to-day. 



In its widest sense, homogamy is, of course, 

 an obvious fact. The bird does not mate with 

 the mammal, nor the reptile with the insect. 

 Furthermore, the dog does not mate save with 

 the dog, nor the sparrow with any bird not of its 

 own kind. These are obvious illustrations of that 

 kind of sexual selection which Professor Pearson 

 calls assortative mating. But what he has dis- 

 covered is the extension of this principle to mating 

 within the limits of the species ; or, at any rate, he 

 has shown it in the case of man. 



Professor Pearson and his coworker, Professor 

 Weldon, have made a most exhaustive research 

 upon human marriage from this point of view, 

 by studying, for instance, the tombstones of rural 

 Oxfordshire, the dales of Yorkshire, and the Lon- 

 don cemeteries; and by inquiries into pedigrees, 

 such as those furnished by the Society of Friends. 

 These studies have given them material for esti- 



