24 BACTERIOLOGY. 



looked upon by us in the light of benefactors, without 

 which existence would be inapossible. 



With the parasites, on the other hand, the conditions 

 are far from analogous. Through their existence there 

 is constantly a loss, rather than a gain, to both the 

 animal and vegetable kingdoms. Their host must 

 always be a living body in which exist conditions favor- 

 able to their development and from which they appro- 

 priate substances which are necessary to the health and 

 life of the tissues of the organism to which they may 

 have found access. At the same time the substances 

 which they form as products of their nutrition are direct 

 poisons for the surrounding tissues. 



In their relations to humanity the positions occupied 

 by the two biologically different groups, the saprophytes 

 on the one hand and the parasites on the other, are 

 diametrically opposite. The saprophytic forms standing 

 in the relation of benefactors, in resolving dead animal 

 and vegetable bodies into their component parts, which 

 serve for food for living vegetation, and, at the same 

 time, removing from the surface of the earth the re- 

 mains of all dead organic substances ; while the parasitic 

 group exist only at the expense of the more highly 

 organized members of both kingdoms. It is to the 

 parasitic group that the pathogenic' organisms belong. 



In addition to the saprophytes which are concerned 

 in the changes to which allusion has just been made, 

 there exist other saprophytic forms which are recognized 

 by their property of producing pigments of different 

 color. These are known as the chroraogenic" forms. 



1 Pathogenic organisms are those which possess the property of 

 producing disease. 



'■" Chromogenic, possessing the property of producing color. 



