EVOLUTION AND VIRUSES 



host cells and infect other cells unaided. These are the complex 

 particles of protein and nucleic acid. Only the nucleic acid 

 seems to enter newly infected cells; usually the nucleic acid 

 not only multiplies but also produces the protein to go with 

 it, and the classical sequence of infection and lysis with the 

 liberation of new bacteriophage ensues. But sometimes the 

 nucleic acid seems to become associated with the genetical 

 mechanism of the host bacteria in a manner that allows it to 

 survive without producing the characteristic bacteriophage 

 protein. This is the condition in lysogenic bacteria: bacteria 

 that are not already lysogenic sometimes become so by infec- 

 tions with appropriate bacteriophages, so it again seems more 

 reasonable to assume that natural lysogeny also results from 

 past infections rather than from an ab initio creation in the 

 lysogenic bacterium. Similar conditions to those in lysogenic 

 bacteria have yet to be identified with certainty in other 

 organisms, though the ability of some seemingly normal insects 

 to produce virulent viruses when subjected to some stimuli 

 could be a similar phenomenon. 



We are thus in a rather odd position in theorising about the 

 origins of virus. It is clear that every living cell has the 

 potentialities of producing viruses for they all can make nucleic 

 acids and proteins, which are the basic materials of viruses. 

 It also seems that the main requirement of any nucleic acids 

 and proteins to acquire the distinguishing features of viruses 

 is to become transmissible, that is to be able to survive extra- 

 cellularly for at least the time needed to spread from cell to 

 cell and become incorporated into other cells. We have, too, 

 in some organisms the apparent phenomenon of viruses 

 suddenly occurring when cells are subjected to appropriate 

 stimuli, but all these are adequately explained as consequences 

 of previous infections. Although we have the pointers to the 

 possibilities of any cell producing something acting as a virus, 

 either in itself or in some other organism, we lack any con- 

 clusive evidence to show that any of the now recognized 

 viruses has, in fact, derived from non-pathogenic or non- 

 transmissible nucleic acids. 



Variation and Selection 



Whatever were the origins of the agents we now call 

 viruses, their variability and adaptability are so great that no 



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