178 THE PRINCIPLES OF HEREDITY 



298. Zymotic disease at any rate disease against which 

 immunity can be acquired when endemic is far less terrible 

 than when epidemic. Modern examples of ancient epidemics 

 may be seen in isolated regions. In Pacific islands, for 

 example, air-borne disease spreads like a flame. The whole 

 community is stricken down. The sick are left untended 

 and perish in multitudes. The fields, the entire business of 

 the community, is neglected, and famine frequently follows. 

 Under such conditions measles or whooping-cough, diseases 

 which we in England are accustomed to regard as scarcely 

 more than nuisances, may rise to the level of a great national 

 disaster. Thus in 1749 thirty thousand natives perished of 

 measles on the banks of the Amazon. 1 In 1829 half the 

 population died in Astoria. 2 In 1846 it committed frightful 

 ravages in the Hudson Bay Territory. 3 More recently a 

 quarter of the total inhabitants was swept away in the Fiji 

 group. 4 



299. At the dawn of history, long after the evolution of 

 zymotic disease, the population of the Eastern Hemisphere 

 was still sparse and scattered. Even as late as the Norman 

 Conquest that of England was barely two millions about 

 one-sixth of the number now present in London. Means of 



to disappear for a considerable time until the number of new births 

 provides a fresh supply of food. Consequently severe, and therefore 

 rare, epidemics occur more especially in small and isolated communities. 

 Large and less isolated communities provide a perennial supply of food, 

 and maintain or receive a perennial supply of microbes. Amongst them, 

 therefore, the disease tends to be endemic. Of course if the microbe is 

 of a kind which is much exposed and very susceptible to conditions 

 external to the human body (e. g. the bacillus of cholera), epidemics 

 tend to recur only when these conditions also are favourable. If they 

 are seldom satisfied, the disease will seldom occur except in an epidemic 

 form. It will be endemic only in favourable localities in which the 

 population is large. When a disease is endemic the elimination of the 

 unfit is very thorough. Almost every individual who is weak against 

 the disease perishes, but hardly any individual dies through mere lack 

 of attendance or from famine caused by the disease. But when the 

 disease is epidemic many of the unfit may survive and have offspring 

 during the long intervals which, perhaps, intervene between one visita- 

 tion and the next. Moreover, during the visitation many individuals 

 who are fairly resistant may perish from mere lack of attendance or 

 from famine. An endemic disease therefore does its work of elimination 

 more " cleanly " and completely than an epidemic disease. It follows 

 that races that have suffered from endemic disease should be as they 

 are more resistant than those who have suffered merely from epidemic 

 disease. 



i Hirsch, vol. i., p. 167. 2 Op. cit., p. 167. 



3 Op. cit., p. 158. * Op. cit., 167. 



