308 GENETICS 



but collaterals showed the disease latent for six gen- 

 erations. 



Inbreeding is often the result of proximity. Insular 

 or isolated communities, slums in cities, where those of 

 one language herd together, or hovels in the back- 

 woods, where degenerates of a kind are kept in intimate 

 association, as well as asylums of various 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 Eng- 

 land, are frequently characterized by a population 

 showing a large percentage of defectiveness. 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. 

 There were, obviously, geographic reasons for the 

 well-known love story of Adam and Eve. Before the 

 days of railroads, cousin-marriages were much more 

 frequent than they are now. 



Many cases of human defects, such as imbecility or 

 insanity, are extremely difficult of analysis from the 

 standpoint of heredity because, in the first place, the 

 defective conditions descriptively included under these 



