SPONTANEOUS GENERATION 51 



in all those cases where bacteria appeared in liquids after protracted boiling 

 they arose from highly resistant spores, whose vitality the heat had been 

 unable to diminish, and we know further that those experimenters were 

 right who sought to render air sterile by passing it over sulphuric acid or 

 through heated tubes, and that it was for this reason that the infusions 

 sometimes remained clear. 



Infusions of organic substances, if they be only boiled long enough, will 

 remain sterile for years, even in vessels closed only by a plug of cotton 

 wadding. A spontaneous generation of new life never takes place ; for 

 bacteria, like all other organisms, can only originate from pre-existing 

 germs. The fantastic theories that spring up from time to time regarding 

 the derivation of bacteria from the disintegrated protoplasm of the higher 

 creatures are apocryphal and, like the stories of organisms that consist 

 only of carbon, oxygen, and hydrogen, must be received with the scepticism 

 they deserve (26). 



Bacteriological research has thrown light from another side also upon 

 the irrepressible theory of spontaneous generation. Before the discovery 

 of prototrophic micro-organisms it was inconceivable how the first forms 

 of life to appear upon the earth could have obtained their nourishment, 

 for all organisms, including the infusoria and other protozoa, seemed to 

 be metatrophic. Now, however, that we are acquainted with the meta- 

 bolism of the prototrophic bacteria, particularly the nitrifying bacteria, we 

 can form some conception of the mode of nutrition of the earliest terrestrial 



organisms 



E 2 



