Larger Aspects of Natural Selection

Abstract

THE brilliant researches of Harrison and Garrett, wherein they succeeded in producing heritable modifications in geometrid moths by the use of lead nitrate and manganese sulphate, suggest some reflections on the relation of these phenomena to natural selection. In 1908 I wrote (Popular Science Monthly, Dec. 1908, p. 547): “Speaking philosophically, progressive or orthogenetic evolution— the existence of which no naturalist has any ground for doubting—must have a cause external to itself. All probability favours the idea that this did not operate once for all, but has continued in action throughout the ages. It may be found, perhaps, in the susceptibility of the hereditary mechanism to environmental influences of particular kinds, the nature of which remains for the present obscure. These reactions would fall under the operation of natural selection from the very beginning; thus a too susceptible organism would quickly be thrown out of gear and would perish; a too conservative one, unless adapted to practically unchanging types of life, would equally perish. There would be a certain optimum susceptibility, which would be preserved, and would differ for different groups. More than this, certain kinds of susceptibility would be favoured, and being once developed might, like bad habits, become harmful through the accumulation of results, resulting in extinction.”