LECTURES IN BIOLOGICAL SCIENCES 



a small effect. This was demonstrated in the first place by 

 plant breeders such as Nilsson-Ehle (1909), working on wheat 

 in Scandinavia, and East (1910) and others working on corn in 

 the United States. Hereditary mechanisms dependent upon 

 Mendelian genes may, therefore, produce gradual continuous 

 evolution. 



In characters that, within a population, vary continuously 

 over a certain range, it has always been clear that the environ- 

 ment usually plays some part in determining the precise value 

 which the character will have in a particular individual. The 

 classical studies of Johannsen (1909) on pure lines of beans, and 

 many other similar studies, have shown that these variations 

 are, in general, not inherited. It became, therefore, a major 

 preoccupation, both for the theory of evolution and for that 

 of animal and plant breeding, to find methods of estimating 

 the extent of the environmental variation, and to separate this 

 from the hereditary variation which might be passed on to later 

 generations. 



It should be remarked, in the first place, that this problem 

 has been historically very closely connected with political mat- 

 ters. It is clearly of profound sociological importance to de- 

 termine whether the variations we see between men at the 

 present day have been produced by environmental influences, 

 which may be quite changed in the next generation, or whether 

 they are the expression of relatively permanent hereditary 

 qualities. Some of the early Mendelians unfortunately regarded 

 this question in much too simple a manner, and attributed an 

 overwhelming importance to hereditary factors. In the same 

 manner, some of Darwin's early followers developed quite super- 

 ficial social theories based on the simple transference to the hu- 

 man world of such phrases as "the survival of the fittest." It 

 would be inappropriate here to attempt to summarize the whole 

 confused history of the impact of Darwin on social theory. It 

 is sufficient to point out that at the present day it has become 

 rather widely recognized that man, with his development ot 

 methods of communicating information from one generation to 

 the next through speech and writing, has produced what is, in 

 effect, a second system of inheritance, which operates on top of, 

 and in addition to, the biological mechanisms characteristic of 



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