THE DOCTRIiNE OF CONTAGIUM VIVUM. 309 



to refer to it more than once in the course of this addiA iS. 

 I took a flask containing hay-infusion, which had been 

 sterilised by boiling, and inoculated it with a drop of fluid 

 swarming Avith Bacillus suhtilis. After the lapse of twenty- 

 four hours, the previously transparent infusion became turbid. 

 This turbidity increased, and, on the second day, a nlin or 

 crust formed on the surface of the infusion. On the third 

 and subsequent days, the crust broke up and fell in pieces 

 to the bottom of the vessel. In about a fortnight, the tur- 

 bidity passed away, and the original transparency of the 

 infusion was perfectly restored, so that it looked exactly as 

 it did before the process began, except that there was now a 

 sediment consisting of the spores of the little organism at the 

 bottom of the flask. In this case, again, there was the same 

 succession of events : a period of incubation, followed by a 

 period of disturbance, succeeded by a period of subsidence, 

 and, finally, restoration to the normal state. There was also 

 great increase of the infective material and immunity from 

 further attack by the same contagium. 



The yeast-plant and the Bacillus suhtilis may be taken as 

 representatives of a large class of organisms in regard to 

 which we are only beginning to realise their vast impor- 

 tance in the economy of Nature and in the life of man. 

 They are, as I shall presently show, the essential agents in 

 all fermentations, decompositions, and putrefactions. We 

 may group them together for the convenience of description 

 under the general designation of saprophytes, a term intended 

 to include under one heading all the organisms associated 

 Avith tlie decomposition and decay of organic matter. The 

 yeast plant and its allies, and all the numerous species and 

 varieties of bacteria, belong to this group. In size and form, 

 they are among the smallest and simplest of living things, 

 but their vital endowments are Avonderful. 



All the organisms hitherto found associated with infective 

 inflammations and contagious fevers belong to the tribe of 

 bacteria, and we cannot advantageously enter on a study of 

 that association without a knowledge of the origin and at- 

 tributes of these organisms. This brings us into a field of 

 active controversy. It has been alleged, as you know, on 

 high authority^ that these organisms, under certain con- 

 ditions, depart entirely from the universal law of generation, 

 which is expressed in the aphorism onne tivuin e vivo, and 

 that they may arise spontaneously by a process of abiogenesis. 

 It is also alleged that these organisms are not the actual 

 agents of decomposition, but are merely associated with that 

 process as secondary or accidental accompaniments. I pro- 



