the inadequacy of space and food for all, certain individuals are by virtue of their characteristics better fitted to survive under their existing conditions of life than others.
For Darwin these were merely hypotheses to be conscientiously tested against all known facts. With the facts which Darwin painstakingly collected and sifted they seemed to agree so well that naturalists accepted his theory as the best explanation of the diversity of the organic world. Fifty years has furnished, it seems to me, a highly satisfactory substantiation of the first two propositions. We have forgotten that it was once necessary to convince biologists that variations do occur, and are now trying to measure the frequency and amount of variations, to determine what their proximate causes are, and to classify them. Variation and heredity are so intimately linked together that one can not be extensively investigated without considering the other. Since the pioneer work of Galton a few men have been actively engaged in the measurement of the intensity of heredity, and of recent years many more have been occupied with the experimental study of the physiological phases commonly known as genetics.
In consequence of this activity our knowledge of variation and inheritance is much more extensive than was that of Darwin—it would be to our shame if this were not true—but if biologists could all escape for a moment from the limitations of vision imposed by the tangle of post-Darwinian detail and by assumption and subsidiary qualification, and could look at the problems and the data of organic evolution as a whole and in the large I think they would be almost unanimous in regarding these two first propositions as so well established that they present no difficulty to the acceptance of the Darwinian theory.
The insecurity of the Darwinian tripod is to be seen in the weakness of the proposition that natural selection moulds the species by eliminating variations not adapted to the environment. While the first two hypotheses have been replaced by the masonry of quantitative science, the third remains largely a hypothesis, weakly reinforced by analogy and by the indirect evidence of adaptation.
To make more widely known the fact that natural selection is capable of quantitative treatment, of direct measurement, just as are variation and inheritance, is the purpose of this essay. It is not a brief for Darwinism, but a plea for direct quantitative researches into one of the more neglected problems of organic evolution.
II. The Problems of Selection
By the word selection in its most general evolutionary sense we mean merely that those individuals which leave offspring are not on the average representative of their generation, but that they differ in some regards from those which do not survive to be parents. In statistical terminology they are not a random sample of the population.