EVOLUTION AND VIRUSES 

 Introduction 



The titles of the other contributions to this symposium make 

 mine seem rather narrow and unimaginative. However, so far 

 from being embarrassed by the insignificance of my subject, 

 I am more so by its scope. This fact could, I suppose, be taken 

 to prove that I am one of those scientists who are so often, and 

 often so wrongly, condemned as narrow-minded specialists, 

 but I hope for a more charitable interpretation. The one I 

 suggest, rather than lack of imagination, is that I prefer to 

 deal with a subject in which at least some conclusions can be 

 subject to test. My outlook and experience as an experimental 

 biologist make me a sceptic, reluctant to accept anything on 

 faith and always suspicious of conclusions that cannot be 

 supported by factual evidence. Not that science is only a 

 collection of facts. Factfinding is the determinant part, but to 

 be aesthetically satisfying the facts need to be fitted into some 

 general concept. To be useful, however, scientific theories 

 must be more than mental exercises; not only must they fit 

 all known facts but they must be susceptible to further tests 

 of their validity. I appreciate how valuable are speculation 

 and imagination, and I fully understand the pleasure and 

 satisfaction to be derived from logical reasoning, but a 

 generalization or theory that cannot be checked remains for 

 me in the general category of fairy stories or detective fiction. 

 Facts are needed too, not only as a brake on speculation, but 

 because unaided imagination, however vivid, is quite unable 

 to produce such remarkable phenomena as are readily un- 

 covered by biological experiment or observation. 



We celebrate today one of the greatest generalizations of 

 all times, the theory of evolution by natural selection operating 

 on mixed populations. It was open to objective test in all 

 biological systems, and the measure of its success is that it 

 stands now even more firmly established than when Darwin 

 published The Origin of Species one hundred years ago. This 

 is no accident. The theory was not put forward as a hasty 

 deduction or an inspired guess; it was a conclusion forced on 

 Darwin, an admirably cautious and critical biologist, from the 

 wealth of factual evidence he had so tirelessly gathered. He 

 already had the idea when in 1837 he returned from his travels 

 in H.M.S. Beagle, but only after 5 years' work — ''patiently 



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