314 WENDELL M. STANLEY 



the smallest of the bacteria which are about 200 m/t, or a few millionths of an 

 inch in diameter. There would have been a definite break with respect to size, 

 since the largest molecules known to the chemist were less than 20 m/x in size. 



Life and hving agents would have been represented solely by those structures 

 which possessed the abiUty to reproduce themselves and to change or mutate 

 and all of these were about 200 m/t or larger in size, thus more than ten times 

 larger than the largest known molecule. This would have provided a comfortable 

 area of separation or discontinuity between hving and non-hving things and would 

 have provided ample justification for considering life as something set distinctly 

 apart and perhaps unapproachable and unexplainable by science. 



Then around 1900 came the discovery of the viruses — first the plant virus of 

 tobacco mosaic, then foot-and-mouth disease virus of cattle and then the first 

 virus affecting man, namely, yellow fever virus. These infectious, disease pro- 

 ducing agents are characterized by their small size, by their ability to grow or 

 reproduce within specific Hving cells and by their abihty to change or mutate 

 during reproduction. Their inability to grow or reproduce on artificial or non- 

 Uving media did not cause too much concern and their reproductive and mutative 

 powers were enough to convince most people that viruses were merely still 

 smaller ordinary Hving 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 I had the good fortune to isolate the virus first discovered by Ivanovskiï, 

 namely, tobacco mosaic, which is a middle-sized virus, in the form of a crystal- 

 Hzable material which was found to be a nucleoprotein, that is, a substance com- 

 posed of nucleic acid and protein. This nucleoprotein molecule was found to be 

 15 m/x in cross-section and 300 m/x in length and to possess the unusually high 

 molecular weight of about fifty milHons. It was, therefore, larger than any mole- 

 cule previously described, yet it was found to possess all of 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 sHghtly 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 abihty to reproduce itself and to 

 mutate; hence the distinction betv^'een Hving and non-living things which had 

 existed up to that time seemed to be tottering and soon a full-scale intellectual 

 revolution was in progress. 



To-day 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 actuaUy 

 an overlapping with respect to size at both ends. Some larger viruses are larger 

 than certain well-accepted Hving organisms 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 Hne in this 



