THE ORIGINS OF ZYMOTIC DISEASES 181 



has previously undergone evolution against tuberculosis. 

 But of this more anon. 



302. So during the long sweep of the ages microbic 

 diseases strengthened their hold on the inhabitants of the 

 Eastern Hemisphere, who, in turn, slowly evolved powers of 

 resistance. In like manner antelopes grow swift and wild 

 sheep active when persecuted by beasts of prey. Then, when 

 the germs of disease were rife in every home and thick on 

 the garments of every man, there occurred the greatest event 

 in human history, the vastest tragedy. Columbus, sailing 

 across an untracked ocean, discovered the Western Hemi- 

 sphere. The long separation between the inhabitants of the 

 East and the West ended. The diseases of the Old World 

 burst with cataclysmal results on the New. 



