144 Pure-line Experiments 



much of the fact that selection fails to effect progress 

 in a "pure-line" — the inbred descendants of a single 

 individual. Thus Johannsen has shown that if the 

 descendants of an individual high-class bean are kept 

 apart, no amount of selection will get beyond the 

 mean of the line. There are, indeed, "fluctuations," 

 such as tall plants and short plants, but if the tails 

 are selected out and bred from, there is no establish- 

 ment of a tall race. There is nothing to choose between 

 the descendants of the tails and the descendants of 

 the shorts. The reason for this is probably that the 

 "fluctuations" in question within the pure and inbred 

 line are "modifications" or individual indents, and 

 not transmissible. There is no use trying to select 

 from non-heritable characters. 



But, apart from this, there are reasons why we 

 should not allow these pure-line experiments to hurry 

 us into a depreciation of the role of selection in natu- 

 ral wild conditions. Pure lines are not typical of wild 

 stocks, in which cross-fertilization is frequent; it is 

 dangerous to argue from brief experiments to the 

 age-long processes of Nature; it is premature to 

 deny the possibility of heritable variations or muta- 

 tions occurring in a pure or inbred line. If one did 

 occur, it might be the starting-point of a new ad- 

 vance. We have taken two modern criticisms of the 

 selectionist position, and shown that they are not 

 leading us to depart from an evolving Darwinism. 

 The same might be done all along the line, and we do 

 not believe that growing knowledge will lead to a 

 depreciation of the role of Nature's sifting. 



