SOCIAL AND WORLD ASPECTS 207 



relations. Obviously, if they all resemble each other 

 in a particular character, the family will be homozygous 

 for that character. But it appears that in inodern 

 civilised races, at any rate, this is seldom the case, 

 even for a single character. Nevertheless, many 

 Scandinavian families are probably homozygous for 

 blue e^^es, and other similar cases might be mentioned, 

 but they are obviously exceptional. This brings us 

 to consider briefly some of the mathematical laws 

 connected with in-breeding and cross-breeding. 



A considerable mathematics has grown up about 

 the stud}' of Mendelian characters. It is not my 

 purpose to deal with that literature here, but a few 

 of the consequences of Mendelian inheritance as 

 regards the whole population may be pointed out. 

 There is considerable evidence that man}' trivial 

 characters in plants and animals, as well as in man, 

 are quite innocuous as regards the welfare of the 

 organism, at least under usual conditions. It is 

 difficult to believe that a long nose or blue eyes is of 

 an}^ direct biological advantage to their possessor 

 in a civilised communit}^ in temperate countries. 

 But under a burning sun in a condition of nature 

 it is at least highly probable that a pigmented iris 

 as well as a pigmented skin are a distinct protection. 

 This is, perhaps, why such mutations in loss of pig- 

 mentation as have occurred to native races in the 

 tropics, have never been able to establish themselves 

 or give rise to a distinct type. 



While natural selection as regards blue or brown 

 eye colour is therefore probabl}' inoperative in 

 temperate countries,* it might easity become opera- 

 tive under wild conditions in the tropics. W^arren 

 (191 7), in a paper on the numerical effects of natural 

 selection acting upon Mendelian characters, shows 

 the number of generations which would be required 



* What sexual selection may do is another matter. 



