One may summarize by saying that two important points cover the whole rôle 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, however, 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. Selection does not gradually perfect a character. The production of heritable 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 variations from their combinations with undesirable characters formed by natural hybridization.
