NATURAL SELECTION AMONG CIVILIZED MEN 111 



monstrated by the familiar causes of death among the living 

 beings we know best. Moreover, apart from its practical 

 and scientific importance, the study of heredity in connection 

 with diseases is of incalculable interest from political, social, 

 and historical standpoints. If at this stage I declared that 

 disease has so influenced the evolution of certain races of 

 mankind that they alone are capable of undergoing civiliza- 

 tion, and therefore that nearly half the races of the world are 

 doomed to early extinction, no doubt I shall be thought guilty 

 of extravagance. The impression will be deepened if I add 

 that disease has so dealt with one race, recently of insignifi- 

 cant numbers, that its future world-predominance is assured. 

 Nevertheless all this is true and capable of clear demon- 

 stration. 



179. When a zymotic disease, such as measles, is very 

 prevalent in any area, experience teaches that the inhabitants 

 may be divided, roughly, into three categories. There are 

 those who, under the circumstances, are totally immune to 

 the disease ; l there are those who take the disease, but are 

 sufficiently resistant to recover ; and lastly, there are those 

 who take it and die. The three categories, however, shade 

 into one another. In the first are individuals who, though 

 immune under ordinary conditions of health and environment, 

 may take the disease, and even perish of it, under worse 

 conditions ; in the second are people who may be immune 

 under exceptionally good conditions, but who may perish 

 under exceptionally bad conditions ; while in the third are 

 individuals who under good conditions may survive, and even 

 not take the disease at all. 



180. It is universally admitted that men differ greatly in 

 their susceptibility to infection and in their powers of subse- 

 quent resistance. If, then, a lethal disease be very prevalent 

 it is evident that it presents a very stringent form of Natural 

 Selection. In England, for example, hardly any one escapes 

 measles, whooping-cough, or tuberculosis unless he be 

 immune, or death unless he be resistant. In other parts of 

 the world no one weak against malaria, typhoid fever, or 

 dysentery is able to survive. Whenever any form of selection 

 is stringent, it is accompanied by an evolution of those 



1 Inborn immunity is probably never absolute, but it is an error to 

 state, as is sometimes done, that there is no such thing as absolute 

 immunity. Acquired immunity is absolute, at any rate for some time ; 

 otherwise there could be no recovery from a general infection. Worse 

 conditions could not well be produced than occur at the height of such 

 an infection. Yet the sufferer recovers. Such recovery, therefore, 

 implies absolute immunity. 



