THEIR PLACE IN NATURE. 29 



With the parasites, on the other hand, the conditions 

 are far from analogous. Through their activities 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 

 favorable to their development, and from which they 

 appropriate substances that are necessary to the health 

 and life of the organism to which they have found 

 access; at the same time they eliminate substances as 

 products of their nutrition that are directly poisonous 

 to the tissues in which they are growing. 



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 dia- 

 metrically opposite : — the saprophytic forms stand in the 

 relation of benefactors, in resolving dead animal and 

 vegetable bodies into their component parts, which serv^e 

 as food for living vegetation, and, at the same time, they 

 remove from the surface of the earth the remains of all 

 dead organic substances; while the parasitic group exists 

 only at the expense of the more highly organized mem- 

 bers of both kingdoms. It is to the parasitic group that 

 the pathogenic^ organisms belong. 



In addition to the saprophytes that are concerned in 

 the changes to which allusion has just been made, there 

 exist other saprophytic forms whose life-processes result 

 in specific changes of most interesting and important 

 natures. Some of these are characterized by their prop- 

 erty of producing pigments of different color; these are 

 known as the chromogenic? forms. Just what their 



> Pathogenic organisms are those which possess the property of producing 

 disease. 

 - Chromogenic :— possessing the property of generating color. 



