THE CAUSATION OF DISEASE. 225 



capacity for pain, like every other useful attribute, lias slowly 

 and laboriously evolved by natural selection. 



The notion that disease is solely due to ignorance or wilful 

 misconduct is, therefore, erroneous. No doubt, civilization has 

 multiplied the forms of disease, and vastly increased its pro- 

 portion, but a proper application of biological laws enables us 

 to assert, a priori, that disease must always have existed, apart 

 from the actual observation of facts pointing in the same 

 direction. # 



From the above observations, it follows that, even supposing 

 the E to be the most stable possible, varying but little (I say 

 "but little,'"' for we have seen that actual equality of E is not 

 possible for any two individuals) from generation to genera- 

 tion, the principle of natural variation must be ever at 'work, 

 weeding out a number of unlucky variations, and thus main- 

 taining the proper level of adaptation. 



It is quite impossible, even in savage communities, for the 

 E to be perfectly stable from generation to generation. It 

 could not be from the geological fact, if from no other cause, 

 that a ceaseless change is going on in the outer world. Not 

 only is it the detail of the E that is different for individuals 

 born at the same epoch ; but far greater and grander differ- 

 ences are caused by the long lapse of time. Thus, in addition 

 to marked astronomical differences, the geological state of the 

 earth is undergoing steady change, and these geological 

 changes entail meteorological changes also. Wherefore, as 

 Herbert Spencer observes, "throughout all time there has 

 been an exposure of organisms to endless successions of 

 modifying causes."' f 



As an example of changes in the E dependent upon the 

 natural physical processes taking place upon the earth, let us 

 suppose a savage tribe to inhabit a tract of country where 

 subsidence is going on — in the neighbourhood of a large lake or 

 a river-bed, it may be. A portion of the dry land thus becomes 

 gradually converted into a miasmatic swamp. A marked 

 change will thus have been gradually taking place in the E of 

 this savage tribe, and, pari passu, the process of adaptation to 



* See Lawson Tait, Dublin Quarterly Journal of Med. Science, Feb. 1874. 

 f Spencer's "Biology," ch. ix. 



