CHAPTER IV
Once again we may resume the main thread of the argument. We have already noted that since, at the present time, men perish mainly by disease, man's present evolution 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 experience 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 undergone 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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