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A summary, properly so called, of the materials of this paper is precluded by the fact that the various sections are in themselves summary reviews of researches carried out upon the most diverse materials. But all these studies have this in common: they are attempts to determine by quantitative methods whether natural selection be a reality, and if so, to measure its intensity. In conclusion, stress may be laid upon two points.

The first of these is a matter of fact. Evidences of the occurrence of natural selection for many characteristics are rapidly accumulating. That mortality is not random, but differential, and that the intensity of the selective death rate is a problem open to quantitative treatment, are propositions supported by large bodies of sound scientific evidence. Nevertheless, neither the complexity of the phenomena nor the difficulties of the collection or of the analysis of the data can be underestimated. As yet, only the surface has been touched. The results are all subject to such revision as may be rendered necessary by wider data and narrower analyses.

The second of these is a question of interpretation. The demonstration of the existence of a selective death rate in a given case is by no means equivalent to proof that evolutionary change is taking place in the character under consideration. Natural selection may only maintain a characteristic at the stage already attained. Or the force of natural selection may be offset by that of some other factor. Or, again, the variations dealt with may be of a kind not inherited; and without inheritance selection is powerless to effect any change. Indeed, first-hand experience in quantitative work on organic evolution must convince any one that the problem of the methods by which it has taken place is far more recondite than biologists have been wont to consider it. This great complexity demands an attitude of extreme caution in generalization. For the present, we must be content to attempt to measure one possible factor after another in as wide a series of organisms as possible. Having done this, we may hope in time to form a fairly trustworthy conception of the resultant of these forces as they may be combined in nature.