246 INBREEDING AND OUTBREEDINO 



give information based upon the law of averages which 

 has some importance. 



Man is a single species if one may judge from the 

 interf ertility and the blood chemistry of existing peoples, 

 but mankind are not all brothers in spite of this oft- 

 repeated laoonicism of idealists and radicals; through 

 some 300,000 years of evolution the relationship between 

 the extremes is rather vague. During this period the 

 black race, the yellow race, the white race — three well- 

 marked varieties of the species — have come into existence ; 

 and the total number of heritable variations differentiat- 

 ing sub-races and individuals is almost incalculable. 

 Naturally, the selective agents concerned in this process 

 of segregation were numerous ; but isolation in a broad 

 sense, necessitating as it did a variety of group struggles 

 for existence amid different enviroimients, probably may 

 be regarded as the factor chiefly responsible. In the im- 

 mediate past, however, a short period as evolutionary 

 time is marked, there has been an increasingly swift 

 reversal of this process of racial separation. History, in 

 fact, has been hardly more than a record of successive 

 race migrations with the inevitable mingling of the con- 

 queror vnth the conquered. 



With the twentieth century the world enters a new 

 phase of development. Witliin a single generation man 

 has reached out and grasped the mastery of his environ- 

 ment. Space has been annihilated with the telegraph and 

 telephone, the railway, the steamboat, the submarine, the 

 aeroplane. As a result of this freedom of communication 

 there will be even more colonization until the limit of the 

 food supply is reached ; and then, a stationary population, 

 through an increased death rate or a decreased birth rate. 



