VIRUSES, CANCER, GENES, AND LIFE — STANLEY 359 



still smaller ordinary living organisms. However, around 1930 the 

 sizes of different viruses were determined with some precision, and 

 it was found that some viruses were indeed quite small, actually 

 smaller than certain protein molecules. Then in 1935 the first dis- 

 covered virus, tobacco mosaic, which is a middle-sized virus, was 

 isolated in the form of a crystallizable material which was found to 

 be a nucleoprotein, that is, a substance composed of nucleic acid and 

 protein. This nucleoprotein molecule was found to be 15 m/* in cross 

 section and 300 mju, in length and to possess the unusually high molec- 

 ular weight of about 50 million. It was, therefore, larger than any 

 molecule previously described, yet it was found to possess all the 

 usual properties associated with larger protein molecules. The same 

 material could be obtained from different kinds of mosaic-diseased 

 plants such as tomato, phlox, and spinach plants, whereas plants 

 diseased with different strains of tobacco mosaic virus yielded slightly 

 different nucleoproteins. Many tests indicated that the new high 

 molecular weight nucleoprotein was actually tobacco mosaic virus and 

 it was concluded that this virus could, in fact, be a nucleoprotein 

 molecule. Here, therefore, was a molecule that possessed the ability 

 to reproduce itself and to mutate; hence, the distinction between 

 living and nonliving things which had existed up to that time seemed 

 to be tottering and soon a full-scale intellectual revolution was in 

 progress. 



Today the revolution is past and we know that the gap between 20 

 and 200 m/i has been filled in completely by the viruses — so much so 

 that there is actually an overlapping with respect to size at both ends. 

 Some larger viruses are larger than certain well- accepted living or- 

 ganisms whereas some small viruses are actually smaller than certain 

 protein molecules. We have, therefore, a continuity with respect to 

 size as we go from the electrons, mesons, atoms, and molecules of the 

 physicist and the chemist, to the organisms of the biologist and on, 

 if you please, to the stars and galaxies. Nowhere is it possible to 

 draw a line in this continuity of structures and say that all above 

 this size are living and all below are nonliving. There appears to 

 be a gradual transition with respect to size and complexity of struc- 

 ture as one goes from things that are normally considered to be alive 

 to things that are generally considered to be nonliving. One is re- 

 minded of the quotation attributed to Aristotle over 2,000 years ago 

 to the effect that Nature makes so gradual a transition from the 

 animate to the inanimate that the boundary line between the two is 

 doubtful and perhaps nonexistent. Much scientific knowledge has 

 been accumulated since Aristotle's time but the essence of his statement 

 is as true today as it was when he made it. But does this mean there 

 is really no difference between the animate and the inanimate? I do 



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