D. B. WARD, M. D. 189 



plied to the whole class. They are undoubtedly plants, 

 and are closely allied to the fungi and algse. 



I think, in view of their prodigious importance in the 

 economy of nature, that they should be put into a sep- 

 arate class by themselves and not incorporated with any 

 other. It may be said of all of them that their mission is 

 destruction. They are the active cause of all decomposition 

 of dead animal and vegetable tissues. They attack these 

 substances and resolve them into their simpler elements, 

 or rather proximate principles, so that these principles 

 are free to enter into new combinations. You see, then, 

 that what at first might appear to be a mischievous and ma- 

 levolent action is really absolutely necessary to the main- 

 tainance of life upon this earth. Let us suppose that 

 every plant and every animal remained unconsumed 

 when dead. It would not be very long before all the 

 available material of which plants and animals are made 

 would be used up entirely and then all life would cease. 

 I think we are too apt to overlook this beneficent func- 

 tion of the bacteria and to magnify the mischief done by 

 a few disreputable microbes which have departed from 

 their original functions and now prey upon our unhappy 

 race. Then there is a group of bacteria which is quietly 

 at work in the soil. They are called the nitrifying bac- 

 teria because, as a result of their work, the nitrogen is 

 presented to the roots of plants in a form which they can 

 assimilate. Without them no plant life, hence no life. 

 So you see we owe our very existence to this humble 

 class of almost invisible plants. 



We can divide all the bacteria into two great groups : 

 First, those which live on dead matter only, and which 

 we call saprophytes, and, second, those which attack liv- 

 ing organisms and which are called parasites. These 

 latter are the cause of disease. 



If we take a little of any infusion of animal or vegeta- 

 ble substances — beef tea, for instance — and expose this 



127 



