DISTRIBUTION OF INHERITABLE TRAITS 189 



whose line of descent has not received contributions from 

 Ethiopia within the last millenimn — when we stop to con- 

 sider the slaves, not only white and yellow but also brown 

 and black, that were brought to Rome, became free there 

 and contributed elements to the population of Italy and to 

 all Europe. 



Returning from this digression, we may recognize that, 

 however vague scientifically the term consanguineous may 

 be, popularly, it means related as first or possibly as second 

 cousin. This is, of course, from the standpoint of modern 

 heredity, an absurd limitation of the term since fifth or 

 tenth cousins may carry the same ancestral traits. Our 

 question may then be transformed in this fashion: What 

 proportion of the population marries within the grade of 

 fifth (or tenth) cousin? The answer to this question for 

 the United States as a whole would require a special census, 

 and the proportion, expressed in a single figure would have 

 little significance. Much more important is it to know for 

 each of several small communities the grades of relationship 

 of consorts; and the association of degree of consanguinity 

 with physiographic and other barriers. 



3. Barriers to Marriage Selection 

 Barriers, indeed, to free and wide marriage selection 

 favor consanguineous marriages, and for the same reason 

 they favor the formation of races of men with peculiar 

 traits, even as it has long been recognized that they facili- 

 tate the formation of races of plants and animals, by per- 

 mitting newly-arisen traits to infect, as it were, the entire 

 population and thus to form a new species. The barriers 

 may be classified as physiographic and social. 



A. Physiographic Barriers 

 Physiographic barriers are for man, a land animal, stretches 

 of water, such as parts of the ocean, sounds and bays that 



