On the Nature of Viruses, Genes and Life 315 



continuity of structures and say that all above this size are living and all below 

 are non-living. There appears to be a gradual transition with respect to size and 

 complexity of structure as one goes from things that are normally considered to 

 be alive to things that are generally considered to be non-living. One is reminded 

 of the quotation attributed to Aristotle over 2000 years ago to the effect that 

 Nature makes so gradual a transition from the animate to the inanimate that the 

 boundary Hne between the two is doubtful and perhaps non-existent. Much 

 scientific knowledge has been accumulated since Aristotle's time but the essence 

 of his statement is as true to-day as it was when he made it. But does this mean 

 there is really no difference between the animate and the inanimate ? I do not 

 believe that it does. However, we must be willing to define what we mean by Ufe 

 and then we must be willing to accept as living any structure possessing properties 

 fulfilling such a definition. 



The essence of Hfe 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 semi-order or even from chaos all of the 

 component parts ofthat pattern with the perpetuation ofthat pattern with time. 

 This is life. Now there is another very basic property which seems to be charac- 

 teristic 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 responsible for the whole 

 evolutionary process and thus for the myriads of kinds of hfe we have on earth 

 but, most importantly for mankind, it permits one to dare to aspire. It is presum- 

 ably 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 hfe 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 abihty to repHcate and in the last analysis this abüity to re- 

 produce remains to-day 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 abuity 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 crystaUizable nucleoprotein 15 by 300 m/i in size, many other viruses have 

 been obtained in pure form and characterized in part by their chemical and phy- 

 sical properties. My colleagues in the Virus Laboratory at the University of 

 California in Berkeley, Arthur Knight, Robley WiUiams 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 hpid, carbohydrate and in some cases a limiting membrane in addition 

 to nucleic acid and protein. Whereas some viruses, hke tobacco mosaic, are 

 crystaUizable nucleoproteins which have the usual molecvilar properties, other 

 viruses, such as vaccinia, have a degree of morphological differentiation which 



