THE EVOLUTION AGAINST DISEASE 199 



against the diseases to which they are exposed is, I conceive, 

 clear proof that the organic world has not arisen by mutations 

 which are permanent and which therefore nothing but selection 

 can eliminate. We shall see in the next section of the present 

 volume how great that evolution has been, how closely it adjusts 

 each variety to its special environment, and how laden it is with 

 momentous consequences. Now, since all races evolve against all 

 the diseases to which they are exposed, it is evident that favour- 

 able variations against every disease occur in every race. Unless 

 disease miraculously evokes these variations, they are spontaneous, 

 and therefore, unlike the evolution which results from their selec- 

 tion, not dependent on the presence of the disease. Either they 

 are fluctuations which retrogress in the absence of selection, or 

 they are mutations which, it is said, only selection can eliminate. 

 If experimental workers are right and they are mutations, then, 

 when a variation which increases the power of resisting a disease 

 occurs in a country in which the disease is not present, it will not 

 be eliminated by selection, for it will be a harmless character. 

 Since such variations are spontaneous they will occur as abundantly 

 when the disease is absent as when it is present. If, then, evolu- 

 tion is by mutation, how does it happen that only races long exposed 

 to a disease are resistant to it, so that whole races tend to die out 

 when exposed to new diseases under stringent conditions for 

 example, the inhabitants of all the Western world when exposed 

 to tuberculosis ? The mutation theory is not complete as it stands. 

 The fact that races do not evolve against diseases to which they 

 are not exposed demands the corollary that every useless mutation 

 (e.g. capacity to resist measles) is also so harmful that nature 

 eliminates the individual possessing it. Otherwise it is impossible 

 to account for the fact which we shall be in a position to perceive 

 more clearly later that races evolve only in useful directions. 1 



327. " It was said even at the time of Drelincourt that no 

 less than two hundred and sixty-two groundless theories of sex 

 had been suggested ; and it may be added that since that time 

 there has been no falling off of interest in the sex question if the 

 number of new theories proposed is a criterion." The latest theory, 

 that which I venture to formulate, is sure, therefore, to meet 

 with scepticism. Nevertheless, the evidence supporting it when 

 gathered together appears strong. Whether I am right or wrong, 

 the correct interpretation in this case, as in other instances, 

 probably lies obvious on the surface, provided we do not allow 

 1 See chapter xiii. and especially 435. 



