THE PURE LINE AND SELECTION 127 



Incidentally all the progeny from both large and 

 small parents averaged notably less in 1904 than all 

 the progeny from large and small parents in 1906, 

 a result due to a "poor year" when certain factors 

 of environment were unfavorable. Such unfavorable 

 conditions, however, are known to influence in no way 

 the hereditary qualities of the beans. Thus it appears 

 that, although the progeny of a pure line present 

 plenty of variations of the fluctuating type, due prob- 

 ably to environmental differences in nutrition, moisture, 

 etc., such variations are quite ineffectual so far as 

 inheritance is concerned, and it makes no difference 

 whether the largest or the smallest beans within a pure 

 line are selected from which to breed, the result will be 

 the same, in that there is a complete return to medioc- 

 rity or type with no "inheritance" of the parental modi- 

 fication. As a matter of fact in 1903, 1906 and 1907 

 the lighter parents gave heavier progeny than the 

 heavier parents. 



It will be seen at once that here is a discovery of 

 far-reaching importance which may require us to 

 reconstruct certain cherished ideas about the part 

 played in the evolution of species, as well as in heredity, 

 by natural selection. 



4. THE DISTINCTION BETWEEN A POPULATION AND 

 A PURE LINE 



A mixture of pure lines has been called a population 

 (Johannsen). 



It is not possible to distinguish by inspection a group 



