208 THE CAUSATION OF DISEASE. 



events which does not cause death in all. The one is incom- 

 patible, the other compatible, with human existence. 



Now, it is obvious that natural selection only operates as 

 regards the not-necessarily-fatal-E, for if the E be inconsistent 

 with the permanent continuance of the race, there is no 

 natural selection in the proper sense of the term, natural selec- 

 tion implying adaptation ; but by natural selection a race may 

 become adapted to the not-necessarily-fatal mal-E's. I was 

 careful to point out that a strictly accidental death has two 

 characteristics — first, that it is independent of carelessness or 

 other mental deficiency on the part of the individual ; secondly, 

 that it is due to an E which must necessarily lead to the death 

 of any one exposed to it. Such an E corresponds to what we 

 have called the necessarily-fatal-E, with this difference that it 

 signifies an E which necessarily causes death in the first 

 generation, while the necessarily-fatal-E of which I am now 

 speaking is one which need not cause death in the first genera- 

 tion, but which sooner or later leads to extinction. 



While speaking of mal-E's it is necessary to observe that no 

 one can pass through life without meeting with a certain quan- 

 tity of mal-E, such as sudden changes in the weather — damp, fog, 

 and so on. We may speak of this as the necessary-mcd-E. The 

 quantity of this varies, of course, for all, but there is a certain 

 mean or average quantity from which none can escape. It is 

 true that it can be reduced to a minimum, but it can never be 

 quite obliterated, and it is only a certain number who success- 

 fully run the gauntlet of this necessary -mal-E ; a large number 

 succumb, whether it be from a cold or a bacillus : and these 

 individuals are weeded out, being imperfectly adapted to an 

 average E. Take the case of children, for instance. Athough, 

 doubtless, their survival or non-survival depends rather upon 

 differences of E than of S,* yet it may, and often does, 

 depend upon differences in the capacity to resist certain 

 noxious agents. Thus, one child may succumb to diphtheria, 

 a second to tubercle, while two others, to all appearances 

 equally exposed to the poisons of these disorders, survive. 

 We may suppose that the two former children have been 



* The large mortality of children among the poorer classes of large cities is 

 dne, not to differences in the children, but to differences in the E. 



