THE CAUSATION OF DISEASE. 255 



the various civilized communities one with another, the food 

 supply has by this process hitherto been made sufficient for the 

 most rapidly growing populations ; for, if the total food supply 

 of a country be insufficient at any time, one of two things 

 happens : either there is an importation of the necessary food 

 to it — this, of course, only occurring when the country contains 

 an equivalent amount of wealth in some other shape, which it 

 can offer in exchange — or there is a migration of the surplus 

 population to some other part of the world where the adequate 

 supply can be obtained. Hence, among the civilized, that 

 great natural check to increase of population, the limitation of 

 the food supply, has been removed. 



We have further to observe that, by the mutual consent of 

 the better-off, a sufficiency of food is placed at the disposal of 

 the needy, so that, excepting those rare and isolated cases of 

 starvation, and the now still rarer cases of widespread famine, 

 there is no weeding-out among the civilized through lack of 

 food, and hence the survival of many who, except for such 

 intervention, would be destroyed. 



Among this number we must distinguish two classes. There 

 are, first, those who cannot obtain food through lack of the 

 necessary mental qualifications — the lazy, the dissolute, or the 

 actually incapable, it may be. Such are more interesting to 

 the political economist than to the pathologist. Secondly, 

 there is the class of the physically unfit. Very many indi- 

 viduals, simply because physically incapable of providing for 

 themselves, would, in the ordinary course of nature, assuredly 

 succumb. Such are either helped by their friends or provided 

 for by the community at large, and it may be said of them 

 that, in order to secure their survival, their E is rendered less 

 rigorous. 



Let me define this term. By it I intend to signify the 

 amount of struggle which the E entails, or, what comes to the 

 same thing, the difficulty of adaptation. I do not include 

 under the rigorous E's those which are necessarily fatal : the 

 term only applies to those to which the community is capable 

 of becoming adapted. In contradistinction to a rigorous E 

 we may speak of an " easy " one. 



Now we shall find that the efficiency of S is in direct pro- 



