22 ORIGIN OF LOWEST ORGANISMS. 



belief in the truth of such a doctrine does, almost inevit- 

 ably, entail a belief in the de novo origination of living 

 things. No one who has looked into the evidence, 

 doubts the fact of the association between some of 

 these processes and the presence of organisms ; the only 

 question is, as to the relation in which they stand to 

 one another. If organisms are not the causes of those 

 fermentative changes with which they are invariably 

 associated, then they are, in all probability, the results 

 of such changes ; and they must certainly have been 

 produced de novo if it can be shown that fermentation 

 or putrefaction may take place under the influence of 

 conditions which make it certain that pre-existing 

 living organisms could have had nothing to do with 

 the process. 



Now, in order to lend some air of probability to 

 the former hypothesis, concerning the necessity for the 

 existence of living ferments, it was incumbent upon its 

 supporters to endeavour to show that the air did con- 

 tain such a multitude of " germs," or living things, as 

 were demanded by the requirements of their theory. 

 Spallanzani and Bonnet had, as far as the imagination 

 was concerned, done all that was necessary. They 

 had proclaimed the universal diffusion of " germs ' : of 

 all kinds of organisms throughout the atmosphere 

 -which were ready to develop, whenever suitable 

 conditions presented themselves. So far, however, 

 this was but another hypothesis. To establish the 

 doctrine that fermentation cannot take place without 



