34 BA CTERIOL OGY. 



world is not sufficient to meet the demands of the chlo- 

 rophyll plants, the importance of the part played by 

 bacteria in making up this deficit cannot be overesti- 

 mated. Were it not for the activity of these microscopic 

 living creatures all life upon the surface of the earth 

 would cease. Deprive higher vegetation of the carbon 

 and nitrogen supplied to it as a result of bacterial ac- 

 tivity, and its development comes rapidly to an end ; 

 rob the animal kingdom of the food-stuffs supplied to 

 it by the vegetable world, and life is no longer pos- 

 sible. It is plain, therefore, that the saprophytes, which 

 represent the large majority of all bacteria, must be 

 looked upon 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 metabolic activ- 

 ities 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 terrestrial life, the positions oc- 

 cupied by the two functionally different groups, the sap- 

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

 are diametrically opposed : the saprophytic forms stand- 

 ing as benefactors, in resolving dead animal and vege- 

 table bodies into their component parts, which serve as 

 food for living vegetation, and at the same time remov- 

 ing from the surface of the earth the remains of all dead 



