PROBLEMS OF MODERN SCIENCE 



diseases in man, have been shown to be due to 

 such organisms, and the same is apparently the 

 case with certain plant diseases, such as the mosaic 

 diseases and the infectious chlorosis of Abutilon, 

 though the latter is transmitted only by grafting. 

 Until very recently, yellow fever was believed to 

 be due to another ultramicroscopic organism, but 

 Noguchi has found that this disease is produced 

 by a spirochaete. In one stage, however, its virus 

 passes through the finest porcelain filter, so it would 

 appear that this must represent an ultramicroscopic 

 reproductive stage in the life-cycle of the spirochaete 

 organism. There are also indications that some 

 of the bacteria have, in addition to their ordinary 

 methods of reproduction, a more minute or gonidial 

 stage which is also in some cases a filter-passer. 



The existence of such organisms, far more 

 minute than bacteria, exhibits the problem of the 

 origin of life in an entirely new light. Some of 

 these organisms can pass through filters which will 

 stop others. They therefore differ in size among 

 themselves, and it becomes a question what is the 

 minimum size which a living organism can have 

 and exhibit the phenomena of growth and re- 

 production. Experiments with these organisms, 

 when methods of cultivating them have been 

 developed, may lead us to new views of what con- 

 stitutes life. The number of protein molecules 

 in the smallest of them cannot be a large one, and 

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