EVOLUTION AND VIRUSES 



reason to assume bacteria are unique and the potentialities are 

 obvious. By gaining the property of transmissibiHty, a nucleic 

 acid or nucleoprotein in one species has the opportunity of 

 being transferred to another. Now we recognize this kind of 

 transfer only with viruses, but there seems no a priori need 

 for transmissibility always to be linked with pathogenicity. 

 Indeed, many of the agents we recognize as viruses are not 

 pathogenic in all the species they can infect, but they have to 

 be pathogenic in something for us to recognize that they are 

 transmissible. With the development of better techniques of 

 testing for specific substances, we may expect to end this 

 impasse. Then, viruses may emerge as being only the patho- 

 genic individuals among a rich collection of transmissible 

 factors, some of which may have been more beneficial to the 

 organisms they have invaded than their pathogenic relatives 

 have been harmful. Variation is a random process; selection 

 is the directional one, and it is unlikely that selection of 

 infective nucleoproteins has been solely for pathogenicity. 



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