DARWINISM AND MODERN GENETICS 



and fuse into an intermediate blend in the offspring. Darwin 

 himself pointed out one of the difficulties that would arise for 

 the theory of evolution if blending inheritance were the gen- 

 eral rule. In each generation the hereditary differences be- 

 tween two parents would unite in a blend, so that variation 

 would continually be disappearing from the population unless 

 some other process created new variation as fast as it was lost. 

 The only source from which such new variation could arise 

 would appear to be the environment. Thus, if inheritance is 

 of a blending nature, we are forced to conclude that the en- 

 vironment can engender new hereditary variation. This is the 

 theory of Lamarck. Darwin appears to have felt himself driven 

 in this direction, but was reluctant to embrace the theory whole- 

 heartedly. He acknowledged that this question of the loss of 

 variation under blending inheritance was the major weakness 

 in his theory of evolution. Geneticists responsible for the first 

 developments of Mendelian theory immediately recognized that 

 the whole difficulty disappears if hereditary potentialities are 

 carried by disci ete unit genes which do not blend. The point 

 was made very clearly by Bateson (1909) as early as 1908. 



The early Mendelians, however, conducted their experi- 

 mental work with hereditary characters that were clear-cut and 

 easily recognizable variations from the normal. These, of course, 

 present the easiest experimental material to handle. The situa- 

 tion led, however, in the hands of authors such as Bateson and 

 De Vries, to the unfortunate suggestion that evolution has nor- 

 mally depended on sudden large-scale alterations. This view 

 was quite different to that of Darwin, who had emphasized the 

 imperceptible small steps with which evolutionary processes pro- 

 ceed. Biologists who, like Darwin, were impressed with the 

 gradualness of evolution at first tended to reject the Mendelian 

 theory as a general explanation of biological inheritance and 

 as the underlying mechanism of evolutionary change. How- 

 ever, within a comparatively few years the Mendelians had 

 demonstrated that even very slight, hardly perceptible, heredi- 

 tary variations can be controlled by Mendelian unit genes, and 

 that where we are confronted with a continuous range of vari- 

 ations the phenomena find their explanation, not in blending 

 inheritance, but in the existence of numerous genes, each having 



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