THE APPLICATION TO MAN 241 



cross-pollinated plants were to the self-pollinated 

 ones as 100 to 53, and his conclusion was conse- 

 quently, that cross-pollination is beneficial and self- 

 pollination is detrimental. 



Ritzema-Bos inbred rats for twenty generations. 

 For the first ten generations the average number of 

 young per litter was 7.5, while for the last ten genera- 

 tions it fell to 3.2. 



Weismann inbred mice for twenty-nine genera- 

 tions and obtained a parallel result. For the first 

 ten generations the average number per litter was 

 6.1, for the second ten generations 5.6, and for the 

 last nine generations 4.2. 



Shull found in growing Indian corn that loss of 

 vigor results from continual self-fertilization, and 

 many breeders have had similar experiences with 

 other plants and animals. 



On the other hand, in the case of the pomace fly, 

 Drosophila, Castle inbred brother and sister for 

 fifty-nine generations without diminishing the fer- 

 tility of the line. No arbitrary law with respect to 

 the effects of inbreeding upon vigor and fertility 

 can be laid down, therefore, which will apply equally 

 to all cases. 



10. THE INFLUENCE OF PROXIMITY 



Inbreeding is often the result of proximity. In- 

 sular or isolated communities, slums in cities, where 

 those of one language herd together, or hovels in 

 the backwoods, where degenerates of a kind are kept 

 in intimate association, as well as asylums of various 



