CHAPTER XV. 



Action of E upon Structure independently of Selection. 



A particular E acting during several successive generations is 

 able to modify the structure of the organism in a particular 

 way independently of selection, natural or sexual. This factor, 

 however, has played a very subordinate part in evolution. It 

 is chiefly natural selection, and in a lesser degree sexual selec- 

 tion, which have brought about the complex specializations of 

 organisms. 



Now, selection can have played no part in the evolution of 

 disease, since all pathological variations, far from being selected, 

 tend to be vjeedcd out. " Natural selection acts exclusively by 

 the preservation and accumulation of variations which are 

 beneficial under the organic and inorganic conditions to which 

 each creature is exposed at all periods of life. The ultimate 

 result is that each creature tends to become more and more 

 improved in relation to its conditions/'* Thus selection is ever 

 busy tending to prevent disease, and I wish to draw particular 

 attention to this fact because we hear a great deal about the 

 " evolution of disease." But it is obvious that if a disease 

 evolve during several successive generations, it must essentially 

 be by this third method — viz, by the action of E upon 

 structure, independently of selection. 



I use the term " evolution of disease," but it must be 

 confessed that it is somewhat paradoxical, for disease is 

 essentially a dissolutionary process, and it would, therefore, 

 be more accurate to speak of the dissolution of disease. Indeed, 

 if we hold by the meaning attached to the word by Herbert 

 Spencer, the illustrious propounder of the doctrine of evolution, 

 we cannot, without contradiction, speak of the evolution of 



* " Origin of Species," p. 97. 



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