242 GENETICS 



sorts in which similar defectives are promiscuously 

 housed under the same roof, are all potent agencies 

 to insure human inbreeding. 



Similarly, localities which have been devastated 

 by migrations of the most effective blood, as, for 

 example, parts of Ireland or many rural villages 

 in New England, are frequently characterized by a 

 population showing a large percentage of defective- 

 ness. The able-bodied and ambitious go forth into 

 the world to seek their fortunes, while the deficient 

 in body or spirit are left behind where, under the 

 spell of proximity, they perpetuate their deficiencies. 



The part that improved transportation has played 

 in mixing up populations and in counteracting the 

 effects of stagnation on human heredity, through in- 

 breeding under the inertia of proximity is very great. 

 Before the days of railroads, cousin-marriages were 

 much more frequent than they are now. 



From a biological point of view there is something 

 to be said in favor of the rape of the Sabines in the 

 past and for the pursuit of American heiresses by 

 European nobility to-day. 



11. Inbreeding in the Light of Mendelism 



Inbreeding in itself may not necessarily be injuri- 

 ous. The consequence of inbreeding as shown by 

 the working of Mendelian laws is that latent or 

 recessive characters tend to become homozygous and 

 so brought to the surface, while outcrossing brings 

 about the formation of heterozygous traits which 



