24 DEVELOPMENT OF THE GERM THEORY. 



US was also filled with germs of every kind. Without laying claim 

 to being the first discoverer of germs in connection with disease, 

 he was the first to recognise the vast importance of these minute 

 organisms in the economy of nature. It was while working at 

 molecular physics that the germ theory took root in his mind, and 

 caused him to pursue the studies on fermentation, which even- 

 tually led to the investigation of " ferment " diseases affecting 

 human beings and cattle. He divided micro-organisms into two 

 great classes, : the aerobic and the anaerobic — those which require 

 free oxygen for their existence and those which are killed by it. 

 The former begin their work on the surface of things, their 

 mission being to clear the earth, by a process of slow combustion, 

 of all that is dead ; the latter, working simultaneously, spring into 

 activity underneath the surface of putrescide matter and die on 

 exposure to the free oxygen of the air, and are in their turn swept 

 away by those on the surface.''" Thus the two great classes of 

 minute living organisms co-operate towards the fulfilment of a 

 common end, the one beginning work which the other takes up 

 and completes. They also attack our plants as parasites, bring- 

 ing degeneration and death to their hosts, and at times cause the 

 severest diseases in the lower and higher animals, and threaten 

 even man himself with fatal epidemics. On the other hand, but 

 for their united efforts, we should cease to live, for the earth would 

 be littered with fallen debris and organic matter of every kind, all 

 of which It is their duty to transmute into the very elements which 

 are essential to life. 



Pasteur had discovered that diseases affecting beer or wine 

 were due to microbes which reached the vats through the air, and 

 that no disease appeared if the air were filtered or sterilised. He 

 had established the vitality of all ferments, harmless or noxious, 

 and had proved that the spores were carried to their respective 

 breeding-grounds through the pollen of flowers or dust of the air. 

 This was the first true indication of how disease travelled, and 

 that it was intangible, invisible, and belonged to the lowest forms 

 of vegetable life. He then turned his attention to the disease 

 affecting silkworms, which was then threatening to extinguish a 

 vast industry, and by a series of the most careful experiments 



* Realm of the Microbe (Priestley). 



