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N papers on "The Measurement of Natural Selection" and "On Assortative Mating in Man," which have appeared in these pages, I have endeavored to show by a review of the quantitative work already done that natural selection and sexual selection are not subjects for idle speculation and polemics, or even for inductions from comparative evidence, but that, like the other factors of organic evolution, they are open to direct quantitative investigation.

It is perhaps not too soon to list up for the convenience of those who desire to take a broad view of evolutionary research, the studies in natural selection which have appeared since the first of these papers was written.

In doing this the ideals of the earlier papers will be carefully maintained. That is, only questions of observed facts and the methods of analyzing them will be taken into account. Theories will be ignored. Again, both positive and negative results will be given impartially, for in the real advancement of science both are of importance in the direction of research and in the formulation of laws.

It will be conducive to clearness to recognize that two fairly distinct problems confront the student of natural selection. The first is to determine whether in any given case the death rate is random or selective. The second is to ascertain what physical, physiological or psychological characteristics make for fitness or unfitness for survival. The attack upon the second problem presupposes the successful solution of the first, for if there be no evidence of the selective nature of the death rate, it is obviously idle to test the selective value of individual characteristics. It is equally clear that any study which stops short of the second of these tasks is in a high degree unsatisfactory. From the standpoint of evolutionary science it is desirable that the significance for survival in various environments of each type of variation in structure or function should be worked out. But this is a task of the highest difficulty and will probably never be accomplished for more than a few selected cases. In these exceedingly difficult fields practicability must be a primary consideration. In many cases, it may