CHAPTER VIII. 



Natural Selection (continued) — Adaptation to Micro-organisms — The Insta- 

 bility of these latter interferes with the Process of Adaptation. 



Instability of E has been a great check to perfect adaptation. 

 Granting stability of E, adaptation to it must sooner or later 

 occur, provided only that it be not necessarily fatal. 



Such being the case, the question may be asked, How is it 

 that mankind has not become perfectly adapted to the poisons 

 of scarlatina, measles, typhoid, and so forth ? To this it might 

 be replied that these are ' ; necessarily fatal " forms of E, 

 and that man has only survived them because, fortunately, 

 they are limited in quantity ; but this is not the correct 

 explanation. The failure of adaptation to these poisons lies in 

 their instability. We have seen that adaptation to the specific 

 mal-E constituted by the ague poison is possible. In this 

 case the poison is probably fairly constant from one generation 

 of man to another. In the case of the specific fevers the poisons 

 are essentially fickle, as is very simply proved by the fact that 

 each epidemic has its own special characters. Nor should this 

 fickleness, or " variability," surprise us, seeing that the disease- 

 causing agent consists of a living organism, and remembering 

 that all organisms must necessarily vary from generation to 

 generation. We cannot exempt the countless varieties of 

 unicellular organisms from this universal biological law. In 

 a very short space of time they pass through many thousands 

 of generations ; hence, during each successive generation of 

 man, they have ample opportunity of becoming modified. 



But despite their instability, a very active adaptation to 

 these specific E's does take place among men. This adaptation 

 is in no way personal, but results from the survival of the 

 fittest, .viz., the survival of those best able to resist their 

 harmful effects. It is well known that the introduction of 



