SELECTED PAPERS 



width amounts to only 300-400 nifj. appear before his eyes. But then 

 he will recognize that optical limitations prevent the direct observa- 

 tion of still smaller forms. Should one now conclude that here a most 

 unexpected coincidence has come into play, and that the lowest limits 

 of life concur with those that are specific to the optics of the micro- 

 scope? This seems improbable, and inductively the microbiologist will 

 reach the conclusion that there may well exist a realm in which the 

 individuals are forever hidden from direct observation. It is generally 

 known that numerous observations of various sorts have gradually 

 accumulated which at first sight clearly force us to the same con- 

 clusion. 



A single observation of this kind may here be briefly sketched. It 

 might happen, and it actually has happened, that the same micro- 

 biologist who a while ago delighted in watching the antics of his tiny 

 bacteria, would be induced to repeat his observations half an hour 

 later, and in so doing, and to his utter astonishment, would find that 

 the merry 'little animals' of a moment ago had completely disappear- 

 ed from his slide ; not even bacterial corpses interrupt the vacuity of the 

 field. While our microbiologist tries to recover from the shock he has 

 experienced, it might happen that the door to his laboratory is opened, 

 and that his famous colleague, Felix d'Herelle, enters. If then he com- 

 municates his experience, he would undoubtedly notice a pitying 

 smile spreading over his visitor's face, and in all probability he would 

 hear the answer: 'My dear colleague, don 't you realize that your 

 bacterium has fallen prey to the obligatorily parasitic ultramicrobe 

 that I have discovered and named Protobios bacteriophagusV And, with 

 a view to reinforcing this verdict, the visitor might show that a minute 

 fraction of the drop on the slide, added to a young, growing culture 

 of the same bacterium, causes the latter with equal suddenness to 

 clarify; that a tiny quantity of this culture effects the same in a sub- 

 sequent culture; and so on and so forth, ad infinitum. He might also 

 demonstrate that such a 'dissolved' culture can be passed through a 

 filter that retains all particles of bacterial dimensions without thereby 

 reducing its lytic power. He might, furthermore, show that it is fre- 

 quently possible to dilute a lysed culture as much as io 9 fold with a 

 previously sterilised solution, whilst preserving its lytic capacity. And he 

 might provide the evidence that this power is destroyed by a brief heat 

 treatment of the liquid, say at 7o°C. In view of all this the conclusion 



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