264 THE FLOATING-MATTEE OF THE AIR. 



an atmosphere charged with the germs of different 

 organisms ; the mere accident of first possession ren- 

 dering now one organism, now another, triumphant. 

 In different stages, moreover, of its fermentative or 

 putiefactive changes, the same infusion may so alter 

 as to be successively taken possession of by different 

 organisms. Such cases have been adduced to show that 

 the earlier organisms must have been transformed into 

 the later ones, whereas they are simply cases in which 

 different germs, because of changes la the infusion, 

 render themselves valid at different times. 



By teaching us liow to cultivate each ferment in its 

 purity — in other words, by teaching us how to rear the 

 individual organism apart from all others, — Pasteur has 

 enabled us to avoid all these errors. And where this 

 isolation of a particular organism has been duly effected 

 it grows and multiplies indefinitely, but no change of 

 it into another organism is ever observed. In Pasteur's 

 researches the Bacterium remained a Bacterium, the 

 Vibrio a Vibrio, the Penicillium a Penicillium, and the 

 Torula a Torula. Sow any of these in a state of purity 

 in an appropriate li [uid ; you get it, and it alone, in 

 the subsequent crop. In like manner, sow smallpox 

 in the human body, your crop is smallpox. Sow there 

 scaiiatina, and your crop is scarlatina. Sow typhoid 

 virus, your crop is typhoid — cliolera, your crop is 

 cholera. The disease bears as constant a relation to its 

 contagium as the microscopic organisms just enumerated 

 do to their germs, or indeed as a thistle does to its seed. 

 No wonder then, with analogies so obvious and so 

 striking, that the conviction is spreading and growing 

 daily in strength, that reproductive parasitic life is at 

 the root of epidemic disease — that living ferments find- 

 ing lodgment in the body increase there and multiply, 

 directly ruining the tissue on which they subsist, or 



