CHAPTER IV 



Once again we may resume the main thread of the 

 argument. We have ah-eady noted that since, at the 

 present time, men perish mainly by disease, man's 

 present evohition must be principally against disease. 

 The most cursory examination reveals how considerable 

 this evolution is and of what importance. It is time 

 to attempt some estimate of it, and this, perhaps, will 

 best be done by contrasting the effects produced by 

 some of the more death-dealing diseases when afflicting, 

 on the one hand, races that have had extended experi- 

 ence of them, and have therefore been rendered resistant 

 by the survival of the fittest, with the effects produced 

 by them when afflicting, on the other hand, races which 

 have had no extended experience of them, and therefore 

 have underQ[one no evolution in relation to them. But 

 before entering into this detailed examination, it will be 

 well if we pause for a space and endeavour by means of 

 a general survey to trace the course of this evolution ; a 

 proceeding which will enable us to form some idea of 

 the vast, the vital importance of the subject with which 

 we are dealing. We have seen that the microbes of 

 some diseases, of which malaria may be taken as the 

 type, are perfectly capable of maintaining, and normally 

 do maintain, an entirely saprophytic existence. A 

 human prey is not essential for their persistence ; they 



are able to find their nutriment and to multiply in 



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