Page:Popular Science Monthly Volume 84.djvu/145

Rh characteristics of seedlings in their relation to survival are at hand for garden beans, Phaseolus vulgaris. From a lot of about 238,000 seedlings germinated in the greenhouse in sand, somewhat over 9,000 abnormal and normal plantlets were transferred to the field under as nearly as possible identical conditions. The diagram shows most clearly that the death rate, though very low, is unquestionably selective. Solid dots and lines represent the mortality of the atypical individuals in sixteen arbitrary but logical classes. The actual numbers of deaths in these classes is not large. The death rates consequently fluctuate widely. Yet in every case but one the mortality of the atypical is higher than that of the typical individuals. The solid bar gives the rate for all atypical seedlings while the broken line smoothes the circles connected by broken lines in the same way.

Seedlings are thrown only into two classes, typical and atypical. The latter is highly heterogeneous, comprising a very wide range of structural variations. Possibly some of these are more fit for survival than are the normal individuals, while others are far less so. Only the collection of far wider series of data will settle the question. On an average the variations from type are clearly inferior. This is precisely the condition which one would expect if natural selection has been a factor of weight in the development of the structural characteristics of the seedling, for the most fit type would be the one preserved.

It is important to remember that this selective mortality is found in seedlings germinated under as favorable conditions of substratum and temperature as we could give them, and then transplanted to fairly good garden conditions. In nature, a considerable part of the seedling death rate doubtless occurs in the early stages of germination where the nascent root and shoot are subjected to a substratum far less favorable to growth than those of the seed pan. Again, the transplanted seedlings were practically free from the inter-specific and intra-specific competition which must be intense in nature. The detection of a conspicuous selective death rate under such optimum conditions can leave little doubt as to the force of natural selection under the severe conditions in which plants grow in nature.

Pigmentation in Man in Relation to Selection. —The problem of