36 BACTERIOLOGY 



supplied to it as a result of bacterial activity, and its develop- 

 ment 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 possible. It is plain, therefore, that in this 

 cycle of life phenomenon 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 analagous. Through their metabolic 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 on 

 which they are preying; at the same time they elaborate 

 substances as products of their nutrition that are directly 

 poisonous to the tissues in which they are growing. 



In their relations to terrestrial life, therefore, the posi- 

 tions occupied by the two functionally different groups, the 

 saprophytes on the one hand, and the parasites on the 

 other, are diametrically opposed. 



SPECIFIC FUNCTIONS OF SAPROPHYTIC BACTERIA. 



Appropriate investigation of the saprophytic group of 

 bacteria has shed important light upon certain specific 

 characteristics with which many of the species are endowed. 

 We know that numerous common phenomena are the results 

 of their activities. The souring of milk, the ripening of 

 cheese; certain of the fermentations resulting in the forma- 

 tion of various acids of the fatty series; the elaboration of 



