VIRUS PROTEINS — STANLEY 505 



lishing a difference were it not for the fact that the slime molds appear 

 to grow without well-differentiated cell walls. The absence of a cell 

 wall, therefore, need not necessarily mean that tobacco mosaic virus 

 is nonliving. Tobacco mosaic virus protein appears to possess a 

 definite solubility, and it might be expected that this might be used 

 as a distinguishing characteristic. However, if an organism were 

 sufficiently small you might expect it to be governed by the ordinary 

 laws of physics and chemistry. Furthermore, this property is rela- 

 tive, for if you move your point of reference to the inside of a whirling 

 centrifuge the virus protein loses this property and settles out as 

 would any bacterial suspension. Another point that may be con- 

 sidered is respiration, for it is well known that nobody has been able 

 to demonstrate that viruses respire, whereas it is usually possible to 

 show that living things respire. However, even this point fails us, 

 for we know that it is difficult, if not impossible, to demonstrate respi- 

 ration in the cases of some living tilings, such as certain seeds or bac- 

 terial spores. It is obvious that we can take up the supposed differ- 

 ences and eliminate them one by one when considered individually. 

 However, when we consider these differences collectively, one is not 

 quite so certain, and I think you will agree with me that there is a 

 tendency for a reasonable doubt as to whether viruses are really 

 similar in all respects to organisms. This doubt becomes even 

 stronger when we consider one additional factor, namely, the size 

 of viruses. I have already pointed out that certain viruses are 

 smaller than accepted protein molecules. We have always assumed 

 that a certain degree of complexity was necessary for life. In its 

 simplest form this complexity might conceivably be represented by 

 the interplay of a very few, perhaps even two, molecules. It may 

 appear very questionable whether or not the degree of complexity 

 characteristic of living organisms can be woven into a single molecule. 

 However, argument based on size alone is not valid over the whole 

 range, for, if we consider vaccinia virus a molecule, we know that 

 into an entity of the same size can be woven the degree of complexity 

 under discussion, for the pleuro-pneumonia organism is just such an 

 entity. Nevertheless, I think there must be a difference between 

 entities, the properties of which may in one case be ascribed to the 

 interplay of several molecules within a definite system and in the 

 other case to a peculiar type of organization within one molecule, 

 ft may be possible to weave into one large molecule a structure that 

 may be responsible for properties which are quite similar to the prop- 

 erties caused by the interaction of several different molecules in a 

 living system. I think that we may conclude, therefore, that, 

 although it is impossible to decide whether viruses are different 

 from ordinary organisms on the basis of any one of the differences, 



