MENDELISM AND SELECTION 



dreds in a single molecule. If so, it is quite 

 possible that an atom or two might be trans- 

 posed in position within the molecule without 

 wholly altering its chemical nature, and that 

 thus slight changes in the germ-plasm might 

 result, which, however, would be as permanent 

 as more profound changes. 



The argument of De Vries against any per- 

 manent effect of selection in modifying unit- 

 characters has been greatly strengthened by 

 the subsequent work of Johannsen and Jennings. 

 Johannsen has found that if one selects from a 

 handful of ordinary beans the largest seeds and 

 the smallest seeds, and plants these separately, 

 the former will produce beans of larger average 

 size than the latter. Selection here has effect. 



But if the selection is made, not from a 

 general field crop of beans, but from those 

 beans borne on one and the same homozygous 

 mother plant, then the progeny of the selected 

 large seed will be no larger than that of the 

 selected small seed. Selection here is without 

 effect. 



The different result in the two cases may 

 be explained, according to Johannsen, on 



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