DARWINISM AND MODERN GENETICS 



against the capacity to respond in this manner. We are, there- 

 fore, certainly not dealing with the inheritance of an acquired 

 character, but with the results of selection for, or against, a 

 certain capacity for developmental modification. 



Although this mechanism is quite different from the La- 

 marckian one, or that supported by Lysenko, its evolutionary 

 consequences will be very similar. In considering evolution 

 we may always think in terms of many generations. There is 

 plenty of time available in which natural selection can act. In 

 the experiments with the missing cross-vein in Drosophila, strong 

 artificial selection was applied. In each generation the popu- 

 lation was propagated only by individuals which acquired the 

 character of the missing cross-vein. After only about twelve 

 generations of intense selection, the character had become genet- 

 ically assimilated; that is to say, it appeared in individuals which 

 had been reared under normal temperatures and to which no 

 environmental stress had been applied. Under natural condi- 

 tions selection would usually be less stringent, but one must 

 expect that similar results would be achieved over a longer 

 period. We have then a mechanism by which, on the evolu- 

 tionary time scale, acquired characters could be converted into 

 inherited ones. 



It is interesting to remember that at the period when Dar- 

 win, and still more Lamarck, discussed the inheritance of ac- 

 quired characters, the basic problems of evolutionary theory 

 were not clearly distinguished from one another, and no such 

 science as population genetics had been contemplated. In that 

 period the concept of inheritance was capable of two quite dif- 

 ferent interpretations. We usually discuss Lamarck's views as 

 though he were using the word "inheritance" with its present 

 sense, to mean the transmission of a character from one pair of 

 parents to their immediate offspring. This is the elementary 

 phenomenon studied by genetics. It is only fairly recently, 

 however, that it has been clearly distinguished from a com- 

 pletely different phenomenon. The word "inheritance" might 

 also be used to mean that, if the individuals in a population 

 acquire a character from their environment, the frequency with 

 which this character occurs will be greater in some evolutionary- 

 derived population many generations later. This is the type 



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