THEIR PLACE IN NATURE. 29 



looked upon by us in the light of benefactors, without 

 which existence would be impossible. 



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 favor- 

 able to their development, and from which they appro- 

 priate substances that are necessary to the health and 

 life of the organism to which they may 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 

 diametrically opposite : the saprophytic forms stand 

 in the relation of benefactors, in resolving dead animal 

 and vegetable bodies into their component parts, which 

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

 time, they remove from the surface of the earth the re- 

 mains of all dead organic substances ; while the parasitic 

 group exists only at the expense of the more highly 

 organized members of both kingdoms. It is to the 

 parasitic group that the pathogenic 1 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 im- 

 portant natures. Some of these are characterized by 

 their property of producing pigments of different color; 



1 Pathogenic organisms are those which possess the property of producing 

 disease. 



