EVOLUTION AND VIRUSES 



accumulating and reflecting on all sorts of facts which could 

 possibly have any bearing on it" — as he tells us in the 

 Introduction to The Origin, did he even allow himself to 

 speculate, and he then spent another 16 years checking facts 

 before publishing. Even then he did so only because Wallace 

 had arrived at similar general conclusions, and he apologises 

 for the inadequacies and imperfections of his abstract, which, 

 he thought, really needed "two or three more years to com- 

 plete". What a man! 



In Darwin's time the existence of viruses was not even 

 realized, but the general principles he annunciated hold no 

 less with them than with the higher organisms he studied. 

 Indeed, I am happy to be talking on systems that can, perhaps, 

 produce evidence for variation and the effects of selection on 

 variants more readily and rapidly than any other. There is a 

 simple reason for this, namely that viruses achieve population 

 figures far greater than those of any organisms. In this context 

 it is enough to say that a single bacterium may contain some 

 hundreds of virus particles, and a single tobacco leaf fully 

 infected with tobacco mosaic virus will contain thousands of 

 millions. 



The non-committal conjunction "and" in the title of my 

 contribution makes my subject embarrassingly wide. It commits 

 me to discuss, not only the evolution of viruses themselves, 

 both how they are changing now and what they have evolved 

 from, but also their effects on the evolution of other biological 

 systems. Then, too, though I do not think that viruses are 

 relevant to the fascinating problem of how life originated on 

 earth. Other people do, so I must at least mention it. Almost 

 every symposium on the origin of life has papers from virus 

 workers, and the most recent one of which I know, at Moscow 

 in 1957, had at least four. To understand how this comes 

 about, and to see that the logical reasoning is not too firmly 

 based, we had better take a little time to describe the discovery 

 of viruses and to summarize present knowledge about them. 



Definition of a Virus 



The discovery of viruses dates from 1892, when Ivanowski 

 showed that the cause of tobacco mosaic virus passed through 

 a bacteria-proof filter and that a seemingly sterile filtrate 



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