226 THE CAUSATION OF DISEASE. 



this altering E will proceed. The ultimate result will depend 

 upon whether or not the E is a necessarily fatal one — i.e., alto- 

 gether incompatible with human life ; for if so, the process of 

 natural selection cannot be said to take place, the E leading 

 to the gradual extinction of the race. If,* however, the chang- 

 ing E be not necessarily fatal, the process of adaptation will 

 be set on foot. Some members of the community will probably 

 be very susceptible to the miasmatic fever, others less so, 

 while others again may be entirely proof '.against its evil in- 

 fluence. Such a community would afford an example of a 

 number of individuals varying in their power of withstanding 

 the evil effects of a particular E, independently of a previous 

 mal-E, and independently of general physical weakness, for, as 

 already observed, we have no right to suppose that the capa- 

 city to resist the evil effects of a miasmatic poison is in 

 direct proportion to the general bodily vigour of the indi- 

 vidual. 



Now, what will be the inevitable result of this process ? There 

 will be a weeding-out of those least capable of withstanding the 

 noxious E, and thus in course of time a race may evolve per- 

 fectly adapted to this particular E. It is, indeed, well known 

 that certain tribes thrive in ague-stricken districts which to 

 surrounding tribes are most deadly ; and this immunity has 

 •doubtless been acquired by a survival, during long ages past, of 

 such as, quite independently, it may be, of any extraordinary 

 physical vigour, happen to vary in the right direction. 



There are, however, other causes of unstable E than those 

 due to geographical change. As man has struggled from a 

 barbaric to a civilized state, there has been an alteration in 

 his E. But the chief cause of the instability of the E among 

 civilized peoples arises from the division of labour ; for, inas- 

 much as each calling has its special E,* it follows that, 

 unless generation after generation of the same family pursue 

 the same occupation, the E of successive generations will be 

 different. Hence the process of adaptation to these different 

 E's, by a survival of the fittest and by ' ' direct equilibration " 

 during successive generations, will be interfered with, adapta- 

 tion being for the most part personal and nothing more — that 

 * Vide Chapter V. Part I. 



