LECTURES IN BIOLOGICAL SCIENCES 



ing been detected and distinguished, it was omitted from fur- 

 ther consideration. This implies that the environment plays 

 no part in bringing about, or controlling, the direction of the 

 variability on which evolution depends. 



The most extreme alternative view of the role of the en- 

 vironment in evolution is to suppose that it can directly produce 

 new hereditary variation of a kind appropriate to the influences 

 which are operating. This view, of course, has a long ancestry 

 reaching back to the theories of Lamarck. In recent years it 

 has been defended by a group of Soviet biologists working under 

 the influence of Michurin and Lysenko. Their work has, how- 

 ever, not won general acceptance in the non-Soviet world. This 

 is partly on practical and partly on theoretical grounds. There 

 are, of course, very many cases, both in controlled experiments 

 and in general agricultural practice, in which animals and plants 

 have been submitted to abnormal environments and have not 

 undergone any hereditary changes, either of an appropriate or 

 an inappropriate kind. The difficulties which have been en- 

 countered in trying to produce strains of European dairy cattle 

 for use in the tropics is one example (Mahadevan, 1954, 1955). 

 Thus, the phenomena of induced hereditary change which 

 Lysenko and his followers have described must demand some 

 rather special conditions and certainly do not occur whenever 

 the environment is altered. However, if there are special con- 

 ditions in which such changes are produced, biologists outside 

 the sphere of the Soviet influence have been unsuccessful in 

 finding them. 



In addition to this negative practical evidence, most biolo- 

 gists find it difficult to envisage any mechanism by which such 

 effects would be expected to occur. Even if we suppose that 

 there is, in addition to the well-understood chromosomal system 

 of hereditary determinants, a much more elaborate cytoplasmic 

 system than at present seems probable, it is still rather difficult 

 to imagine any way in which such factors as light intensity, 

 temperature, etc., would eventually become transformed into 

 appropriate alterations in the hereditary system. Perhaps it is 

 just conceivable that something similar to, but much more far- 

 reaching than, induced enzyme synthesis could bring about such 

 results. At present, however, most geneticists concerned with 



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