358 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 7 



Now I suppose there is no doubt that, of the four topics, life is the 

 one most people would consider to be of the greatest importance. 

 One would think that the nature of life would be easy to define since 

 we are all experiencing it. However, just as life means different 

 things to different people, we find that in reality it is extremely diffi- 

 cult to define just what we mean by life or by a living agent in its 

 most simple form. There is no difficulty in recognizing an agent as 

 living or nonliving so long as we contemplate structures such as man, 

 cats, and dogs, or even small organisms such as the bacteria, or, at the 

 other extreme, structures such as a piece of iron or glass, an atom of 

 hydrogen, or even a molecule of water, sugar, or of our blood pigment, 

 hemoglobin. The former are examples of animate or living agents 

 whereas the latter are examples of inanimate or nonliving things. 

 But what is the true nature of the difference between a man and a 

 piece of iron, or between a bacterial organism and a molecule of hemo- 

 globin? The ability to grow or reproduce and to change or mutate 

 has long been regarded as a special property characteristic of living 

 agents. Certainly mankind and bacteria have the ability to assimilate 

 and metabolize food, respond to external stimuli, and to reproduce 

 their kind — properties not shared by bits of iron or by molecules of 

 hemoglobin. Now if viruses had not been discovered, all would have 

 been well. The organisms of the biologist would have ranged from the 

 largest of animals, whales and elephants and the like, all the way 

 down to the smallest of the bacteria which are about 200 m/i 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 nut in size. Life and living agents would 

 have been represented solely by those structures which possessed the 

 ability to reproduce themselves and to change or mutate, and all of 

 these were about 200 m/i 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 living and 

 nonliving things and would have provided ample justification for con- 

 sidering life as something set distinctly apart and perhaps unap- 

 proachable 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-producing agents are characterized by their 

 small size, by their ability to grow or reproduce within specific living 

 cells, and by their ability to change or mutate during reproduction. 

 Their inability to grow or reproduce on artificial or nonliving media 

 did not cause too much concern and their reproductive and mutative 

 powers were enough to convince most people that viruses were merely 



