360 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



not believe that it does. However, we must be willing to define what 

 we mean by life and then we must be willing to accept as living any 

 structure possessing properties fulfilling such a definition. 



The essence of life is the ability to reproduce. This is accomplished 

 by the utilization of energy to create order out of disorder, to bring 

 together into a specific predetermined pattern from semiorder or 

 even from chaos all the component parts of that pattern with the 

 perpetuation of that pattern with time. This is life. Now there is 

 another very basic property which seems to be characteristic of living 

 things and that is the ability to mutate, to change or to respond to 

 a stimulus. I do not believe this property is absolutely necessary 

 for life, but it certainly lends grandeur to life, for not only is it re- 

 sponsible for the whole evolutionary process and thus for the myriads 

 of kinds of life we have on earth but, most importantly for mankind, 

 it permits one to dare to aspire. It is presumably responsible for 

 man, his conscience and his faith. It is obvious that I believe that 

 mutation merits much, much study. 



The discovery of viruses has permitted us to contemplate the nature 

 of life with a new understanding. It has enabled us to appreciate 

 in a new light the inherent potentialities of chemical structure, 

 whether that of a single molecule or that produced by the interaction 

 of two or more molecules. Viruses were discovered by virtue of their 

 ability to replicate and in the last analysis this ability to reproduce 

 remains today as the only definitive way in which they can be recog- 

 nized. We may purify and isolate preparations from virus-diseased 

 tissues but it is only when a reasonably pure material is obtained and 

 units of this are found to possess the ability to reproduce themselves 

 that we are privileged to refer to the material as virus. Since the 

 isolation of tobacco mosaic virus in the form of a crystallizable nu- 

 cleoprotein 15 by 300 m/x in size, many other viruses have been ob- 

 tained in pure form and characterized in part by their chemical and 

 physical properties. My colleagues, Arthur Knight, Eobley Wil- 

 liams, and Howard Schachman, have made major contributions to 

 the biochemical, electron microscopical, and biophysical knowledge of 

 viruses. Until two years ago all viruses studied had been found to 

 be at least as complex as a nucleoprotein. However, some appear to 

 have lipid, carbohydrate, and in some cases a limiting membrane in 

 addition to nucleic acid and protein. Whereas some viruses, like 

 tobacco mosaic, are crystallizable nucleoproteins which have the usual 

 molecular properties, other viruses, such as vaccinia, have a degree of 

 morphological differentiation which can hardly be called molecular in 

 nature and which is rather more organismal or cell-like in nature. 

 Some of the bacterial viruses have a very complex morphology, with 

 a head and a tail somewhat similar to the sperm of higher organisms. 



