SELECTION IN PLANT BREEDING 203 



One may summarize by saying that two important points cover the 

 whole role of selection. The first point is that nature continually 

 causes variations to appear in plants. The majority of these variations 

 are simply accelerations or retardations of development of the whole or 

 of certain parts of the plant due to good or bad environment at critical 

 stages of the plant's growth. These variations are not inherited because 

 the reproductive or germ cells are not affected. Other variations, how- 

 ever, are being constantly produced by nature — though much more 

 rarely — which do affect the reproductive cells and are transmitted to the 

 plant's progeny. These variations are the basis of selection. They are 

 constant from the beginning and remain so unless changed by a second 

 variation affecting the same constituent in the reproductive cells that 

 is due to develop the character in question. 



The second point to be remembered is that the whole aim and action 

 of selection is to detect the desired heritable variants among the useful 

 commercial plants and through them to isolate a race with the desired 

 characters. When this is accomplished, selection can then do nothing 

 until nature steps in and produces another desirable variation. 



In other words, the results of selection are not continuous. Selec- 

 tion does not gradually perfect a character. The production of herit- 

 able variations is intermittent and the intermissions may be long. If 

 the practical results seem to be parts of a continuous process, it is 

 because of the imperfect methods at hand to isolate the desirable varia- 

 tions from their combinations with undesirable characters formed by 

 natural hybridization. 



