THE ORIGINS OF MICROBIC DISEASES 267 



or other causes had lowered the resisting power of a community of 

 its present hosts. 



441. The microbes of the different diseases differ so sharply in 

 the effects they produce, in their methods of protecting themselves 

 when in the body, and in their modes of passage from one in- 

 dividual to another, that it seems more probable that each patho- 

 genic species was derived direct from a separate saprophytic type 

 than that any two species were derived from a common parasitic 

 stock an ancient disease ancestral to two modern diseases. Very 

 possibly the air-borne species arose from saprophytic types that 

 were originally air-borne, the water-borne species from water-borne 

 saprophytes, and the contagious types from saprophytes that 

 normally and harmlessly inhabited cavities of the body, and so on. 

 Again, since the constant tendency in nature is towards differentia- 

 tion, not approximation, in type, since the microbes of disease are 

 highly specialized, and since the passage from saprophytic to 

 parasitic ways of life is very difficult, whereas, when once the 

 parasitic habit is established, the passage from one host to another, 

 and so from one country to another, is comparatively easy, it seems 

 probable that no disease had multiple origins in different parts of 

 the world, but that every disease spread from a single centre. 1 

 However, as to all this we have no direct evidence on which to base 

 speculation. 



1 At least we must consider it improbable that diseases had multiple origins 

 if we accept the selectionist theory of evolution. Doubtless, however, supporters 

 of the mutation theory, who insist that the same mutation tends constantly to 

 occur in the same species, would hold a contrary opinion. But probably recur- 

 rent mutations are in most, if not all cases, reversions (see footnote, 297). 

 None of them have been known to be adaptive, and a mutation which adapted 

 a saprophytic species in all its structures to a parasitic mode of life would be a 

 very wonderful thing. It would involve, not only the sudden development of 

 great virulence and all that that implies, including a capacity to resist the enzymes 

 of the host, but, at the same time, a power and a tendency to pass from the 

 infected person to others in the neighbourhood. Moreover, this amazing con- 

 glomeration of correlated mutations would have to occur, not in one or a few 

 microbes but simultaneously in thousands ; for if only a few microbes invade 

 the body they perish. Disease results from the invasion of very many (see 

 402). Thus the microbes of vaccinia are unable to persist naturally because 

 as a variety they lack means of passing from one host to another. It follows, if 

 evolution is by mutation, then only the simultaneous mutation of thousands of 

 vaccinia or other microbes could render them independent of artificial propagation. 

 On the other hand, if evolution proceeds by the selection of fluctuations, a passage 

 through a few weakly (e.g. non-resistant or starving) persons might create the 

 necessary virulence (see 400). It must be borne in mind that it has been ex- 

 perimentally demonstrated that microbes gain in virulence only slowly after the 

 passage of many generations, and unless greatly injured, as by heat, lose their 

 virulence as slowly. 



