EVOLUTION AND VIRUSES 



could be infective. The germ theory of disease was then in its 

 hey-day and it was confidently thought that, for each infec- 

 tious disease, there would be found a micro-organism iden- 

 tifiable under the microscope and probably able to grow on 

 artificial media. However, in the next fev/ years, several 

 diseases were found that resembled tobacco mosaic in 

 that their causes were invisible, passed through fine filters 

 and would not multiply, as most bacteria do, on nutrient 

 media. Until then the word virus had been used more or less 

 indiscriminately for all kinds of germs, but now it became 

 increasingly restricted to these anomalous entities made all 

 the more fascinating because of the fact that their nature was 

 shrouded in mystery. Even today, we can define a virus only 

 in negative terms, as an invisible germ that multiplies only in 

 living cells; or, to say the same thing in longer words and 

 make it sound more impressive, an obligately parasitic pathogen 

 too small to be resolved by microscopes using visible light. 



Primitive or Advanced Stages in Evolution? 



Viruses show two features widely considered to be specific 

 to organisms; they multiply and vary, for offspring sometimes 

 differ from their parents. There is, then, little surprising in 

 the fact that most people were content to regard them as 

 essentially similar to bacteria, different in size and complexity 

 rather than in any more fundamental manner. Perhaps most 

 conflict for some time was between workers who regarded 

 viruses as retrograde organisms, evolved from larger ones by 

 losing characters for v/hich as obligate parasites they had no 

 use. and those who regarded them as upward stages in the 

 evolution of organisms from simpler systems. It was the second 

 view that brought viruses into discussions about the origin of 

 life: for if viruses were primitive organisms, might they not 

 resemble the form in which "life" started? This, overlooking 

 as it did the fact that viruses will not multiply even in rich 

 nutrient media, was not very realistic, for ability to live as a 

 saprophyte must surely have been the prime requirement of 

 the first forms of life. 



There was a further conflict of opinion about the nature of 

 viruses, between the majority who saw them as organisms, in 

 whichever direction they might be evolving, and the minority, 

 mainly from those working with tobacco mosaic virus, who 



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