96 LECTUEES ON BIOLOGY 



long before a rich variety of life began to spring up in the 

 tubes in spite of the preceding boiling. When, however, air 

 was admitted which had previously been passed through a red- 

 hot tube or sulphuric acid or caustic potash and thus rendered 

 germ-free, no organism was ever observed to originate. 



These experiments were almost universally regarded in 

 scientific circles as an absolute proof against the spontaneous 

 generation of these organisms. But this is in my opinion a 

 hasty conclusion, for it is not only possible but even certain 

 that the air undergoes in its composition, as a result of the 

 various processes, more or less radical changes. The carbon 

 dioxide of the air is, for instance, more or less decomposed by 

 caustic potash, and the ammonium by the sulphuric acid, and 

 there is, therefore, at least the possibility that these changes, 

 no matter how slight, are the cause of rendering an abiogenesis 

 of organisms impossible. At any rate, the possibility is not 

 excluded, even if the probability has become extremely slight. 



However, as shown later by Schroder and by Dusch, it is 

 not even necessary to adopt such circumstantial precautionary 

 measures in order to prevent an appearance of living organisms 

 in sterilized test-tubes, for it is sufficient merely to close the 

 tubes with pads of cotton-wool. Like extremely fine filters these 

 plugs of wadding keep back all germs and permit only the 

 purified air to enter. Further, according to Pasteur's exhaustive 

 experiments, it is only necessary to conduct the air to the germ- 

 free nutrient medium through long sinuous tubes in order to 

 prevent fermentation and putrefaction. The air itself passes 

 freely through the windings of the tubes, but all particles of 

 dust or germs of organisms are deposited at the lowest points of 

 the bends. 



Confronted with these results, even a sceptic must now 

 admit that abiogenesis of bacteria and protozoa does apparently 

 not take place. Indeed, if at the time when these investiga- 

 tions excited scientific circles scientists had possessed that 

 knowledge of the structure of the protozoa and of the still 

 more delicate structure of bacteria which we possess to-day 

 no one would probably ever have conceived the idea that these 

 differentiated forms could owe their existence to a transforma- 

 tion from inorganic matter. We shall have to deal with these 



