ESSAYS ON BACTERIOLOGY. 9 



really wider than is commonly supposed. As we look 

 about us and see the havoc wrought, directly and in- 

 directly, we may fairly say that it is hardly possible to 

 • exaggerate the importance of infection and of the 

 minute agents which produce it. The invisible ene- 

 npiies are here, as elsewhere,, the most numerous and 

 dangerous. 



The germ theory of disease is no new thing. 

 Though it has but recently come into prominence, 

 and its truth but recently been demonstrated, it is, as 

 a theory, as old, almost, as history. Crude, ill-de- 

 fi,ned, and as crudely expressed, it has nevertheless 

 been for ages essentially the same. Hippocrates saw 

 and believed in it; and from his day to ours it has 

 served as a rallying cry for warring factions and for 

 the disputes of the learned. At times it has assumed 

 prominence, throtigh the influence of some master 

 mind great in its capacity to grasp a simple and far- 

 reaching truth. Again it has sunk almost into obliv- 

 ion, carried down by the weight of its own artificially 

 and ignorantly imposed absurdities. Thus it has 

 risen and fallen, come and gone, with the varying tide 

 of speculative fashion. In this unstable but endur- 

 ing history there must be some reason, and for it we 

 have not far to look. 



We can readily see why the germ theory of disease 

 so persistently suggested and asserted itself to men's 

 minds. They saw many of the facts which we see. 

 They saw a small amount of poison enter the body of 



