THE CAUSATION OF DISEASE. 213 



of sunlight ; pestilences, moreover, are apt to arise where many 

 human beings congregate, unless special precautions be taken. 

 Now, primitive tribes are less exposed to these several forms 

 of mal-E on account of their wandering habits, but they and 

 even the lower animals may be carried off by epidemics when 

 their number gets too abundant. This, for instance, not un- 

 frequently occurs in the case of grouse and salmon. Where- 

 fore it is obvious that man has to struggle against many 

 unfavourable circumstances besides the mere absence of food, 

 and it is these which cause the active weeding-out of civilized 

 man, not the scarcity of food. Although poverty does not 

 often entail death from sheer starvation, it may lead to ex- 

 posure to a variety of mal-conditions, and so a very active 

 elimination occurs among the poorer classes, especially in large 

 cities. 



