EVOLUTION AND VIRUSES 



organisms is clear, but it is also clear that, if a gene is defined 

 as something determining a single inherited factor, the viruses 

 are more complex than single genes for each contains many 

 features that are inherited independently of one another. 



Viruses, of course, differ from other nucleoproteins or we 

 would not distinguish them and place them in a separate 

 category. There are, though, only two distinguishing features 

 common to all viruses; these are ability to cause disease and 

 be transmitted from organism to organism. It is true that some 

 viruses have a characteristic morphology and differ in other 

 ways from non-infective nucleoproteins, but this is no more 

 a barrier to assuming similar ancestries than the differences 

 between giraffes and other mammals preclude their evolution 

 from a common stock. The viruses now most studied are those 

 that cause most obvious effects and are most easily transmitted, 

 and these are unlikely to have achieved their present eminence 

 and success without many adaptations. 



It is now evident what a precise piece of mechanism is the 

 bacteriophage's tail and what it does to get the virus from 

 one bacterial cell to another; such a mechanism obviously did 

 not arrive out of the blue as a single change in a "normal" 

 non-infective nucleoprotein. The particles that first behaved 

 like a virus and managed to escape from one cell and enter 

 another, were almost certainly less well equipped and found 

 the transfer much more difficult than do current bacterio- 

 phages. There seems no reason why viruses should not still 

 be originating from non-pathogenic nucleoproteins, but I think 

 it would be wrong to expect any new ones to behave like those 

 currently most studied. They are likely much more to resemble 

 "normal" nucleoproteins. and so be more difficult to isolate 

 and characterize; they are also less likely to be easily trans- 

 mitted from organism to organism or cell to cell. Indeed, I 

 would expect newly originated viruses to cause conditions 

 resembling those now described as "non-transmissible" tumours 

 of animals and "non-infectious" chloroses of plants. That is 

 to say, normal components become pathogenic, but not yet 

 adapted for transmission. In considering the origin of viruses, 

 it is relevant to stress that only a few have been isolated and 

 studied in any detail. There are many reasons for this, but 



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