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tries to distinguish life in these regions does not observe sharp con- 

 trasts and contours ; there appears to be nothing but misty fringes . . . 

 This situation may justify the title I have chosen for my lecture. 



It will be evident that the above account was meant to sketch out the 

 problem and to indicate its portent at least in rough outline. If I 

 should have had to limit myself to this I should not have ventured to 

 discuss this subject at the end of a congress during which so many im- 

 pressive advances in the field of natural science have once again been 

 reported. Even though I shall not succeed in dissipating the mist in 

 which life appears to lose itself, there have been numerous investiga- 

 tors who have managed to penetrate further, and in the yet dim glow 

 of the lights they have kindled a few vague outlines are nonetheless 

 beginning to take shape. Some of these developments I should now 

 like to communicate. 



In the first place the question arises whether Stanley's crystalline 

 tobacco mosaic virus implies the death-blow of submicroscopic life in 

 general. It is understandable that nowadays the immediate reaction 

 is prone to be a denial of the necessity to consider all filtrable infect- 

 ious agents right away from a single viewpoint. This implies therefore 

 that one certainly wishes to retain the belief in the existence of living 

 entities, essentially similar to bacteria, though distinguishable from the 

 latter because by their size they evade direct microscopic examination. 

 This view is supported by the fact that recently Laidlaw and Elford 

 isolated from sewage a microbe that exists on the one hand in the 

 form of structures with a diameter not exceeding 1 25-1 75 mfx, as shown 

 by strictly dependable filtration procedures, and on the other hand 

 gives rise to larger bodies, directly observable by microscopy. Its most 

 interesting aspect is, however, that it is a typical saprophyte, thus 

 emphasizing the similarity with the majority of the smallest visible 

 organisms. We may therefore conclude that submicroscopic life actual- 

 ly exists; and yet, at the other extreme, we find Stanley's crystals. 



How may we expect to answer for each of the hundreds of viruses 

 described thus far the question, inframicrobe or not? Obviously we 

 may assume that an accurate determination of particle size will help 

 out in this connexion. As early as 1906 Errera has pointed out that a 

 spherical particle of 100 mfx in diameter can contain only 10,000 protein 

 molecules, a number that Errera considered as approximating the 



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