236 THE CAUSATION OF DISEASE. 



Enough has now been said to show that a continual adapta- 

 tion to those morbific agencies which are not necessarily 

 fatal is taking place, and that as regards contagia, the success 

 varies in different instances. Perfect success has apparently 

 been achieved in the case of the ague poison (or, perhaps, it 

 would be more correct to say, certain varieties of the ague 

 poison, for there are many different kinds), but only very 

 imperfect adaptation to the ordinary zymotic poisons has been 

 effected. Yet I doubt not that man has by natural selection 

 become adapted to many microbes which in the past were 

 fraught with the greatest evil for him. 



Many zymotic diseases which, as we know from history, once 

 wrought great havoc, have completely vanished, but how many 

 more must have existed of which history is silent ? We may 

 be sure that man suffered from zymotic disease long before he 

 took on his present shape, far down in the genealogical tree. 



How, then, are we to explain the disappearance of these 

 diseases ? Many of them have doubtless vanished with the E 

 which brought them into being. The great epidemics of the 

 Middle Ages arose out of grossly impure hygienic conditions, 

 and the pathogenic germs causing them disappeared with the 

 hygienic improvement. It may be that the germs died 

 right out ; or we may suppose them to have lost their patho- 

 genecy through natural variation, for it is well known that 

 unicellular organisms, in common with the more complex forms 

 of life, are capable of being modified by a modification of 

 their E. It is highly probable, however, that perfect adapta- 

 tion to them has taken place in the past by natural selec- 

 tion. Let it ever be borne in mind that man, like all other 

 living things, has gradually evolved by a continued adaptation 

 to his E. He has been gradually moulded so as to fit more or 



fute it, but this is no argument against isolation ; the number of individuals 

 attacked " during any given epidemic " might be greater, but the proportion 

 of individuals attacked during a given long period of time, would not be nearly 

 so great, although the process of natural selection would be interfered with, and 

 the number of the susceptible thus increased, but no mention was made of the 

 influence of natural selection during the discussion of this subject in theBritish 

 Medical Journal. A further point was left out of consideration in the discus- 

 sion. If the disease is to kill or cripple its victim, is it not better that it should 

 do so as late in life as possible ? If a man dies of scarlatina at forty instead of 

 at three, he has gained thirty-seven years of a possibly healthy life. 



