Page:Popular Science Monthly Volume 77.djvu/200

194 selected beans were not so extreme, however, as their parents but regressed toward the average character of the parent race. This was nothing new. Galton had discussed the matter a decade before and had interpreted the regression as due to the "pull toward mediocrity" exerted by former ancestors that must have been on the average mediocre. Johannsen was not satisfied with this interpretation and in order to investigate the subject more thoroughly introduced the individual pedigree culture method, or pure line method as he spoke of it, into his work. All of his plants under experiment were self-fertilized for successive generations, so that all of his future bean progeny were descendants of single individuals from the original commercial variety. Each pure line he found to fluctuate around a typical size just as the commercial variety had done. Some types were exactly the same as the original mixed type, but others fluctuated around averages that would have been considered more or less extreme variations in the original. He then grew extreme variants from each of his pure lines and made the discovery that no progress at all was made by repeated selections of this kind. The progeny of the high extremes and the progeny of the low extremes each were found to fluctuate around the same pure line average. It was quite evident then that in the first place he had been dealing with a mixed race. This mixture consisted of sub-races each with a heritable difference in the character size. These heritable variations, however, were obscured by size fluctuations produced by differences in moisture, sunlight and fertilizer received by the different individual plants. There was even a difference in the size of individual beans on the same plant, due probably to location of some pods in places on the plant more desirable than others for the utilization of the plant's soluble foods waiting to be stored in the seeds. These differences due to immediate environment were not inherited. They behaved exactly as the acquired characters of an animal. This made the rôle of selection clear. The only improvement that selection can achieve is to isolate a substrain if such a substrain or substrains exist in the variety under experiment. When this substrain has been isolated, selection has absolutely no effect, and even if continued for countless generations will have no effect until nature produces one of the heritable changes which are so much rarer than the fluctuations produced by environment. It is also evident that the older idea that improvements made by continued selection;—i. e., gradual isolation of a type—are inconstant, is wrong. The explanation is that since non-inherited fluctuations obscure the heritable variations, only a pure line method can absolutely isolate a pure strain; and in the German method of mass selection with poor control against mediocre pollen, the chances were overwhelmingly in favor of the selected type recrossing with the more commonly cultivated and poorer type from which it came.