lative, but Beeton and Pearson have succeeded in obtaining more definite evidence.
From extensive researches on inheritance in man, Pearson and his associates have shown that duration of life gives much lower correlations—both parental and fraternal—than the substantial values found for other physical and psychical characters. If the duration of life of an individual were absolutely determined by physical and mental fitness, then one would expect it to show a correlation as high as that found for other characters. The fact that the values are regularly and conspicuously lower evidences for the existence of a non-selective death rate. The relative amount of the selective and non-selective death rate may be roughly estimated from the reduction in correlation as one passes from the inheritance of characters in general to that of longevity. By this means Beeton and Pearson calculated that from fifty to eighty per cent, of the death rate in civilized man is selective.
C. The Fitness of Organs
Except among the lowest forms of life, every animal or plant is made up of a large number of parts which are differentiated in form and function. The fitness of such a complex organism for self preservation and perpetuation probably depends not merely upon the degree of development of its several component members, but also upon the nicety with which they are coordinated.
Undoubtedly the proper way of taking up the study of Natural Selection is to compare by means of the measurement of particular organs series of individuals which survive with series which perish, but after this is done in a large number of cases we shall have considered only the first part of our problem.
This first phase consists in finding out whether variations in the form, size or other property of an organ affects its efficiency to such an extent as to prejudice the chances of survival of the individual possessing it.
Involving, as it does, questions of structural characteristics and functional efficiency this is at bottom a problem on the boundary line between morphology and physiology. For several years it has seemed tc me that we might, in the long run, make better progress in the study of the problems of evolution if we turned our backs on some of its more
- Beeton, Mary, and K. Pearson, "A First Study of the Inheritance of Longevity and the Selective Death-rate in Man," Proc. Roy. Soc. Lond., Vol. LXV., pp. 290-305, 1889. Beeton, Mary, and K. Pearson, "On the Inheritance of the Duration of Life, and on the Intensity of Natural Selection in Man," Biometrika, Vol. I., pp. 50-89, 1901.
- In actual work one is at once confronted by the difficulty that variations in the organ he is studying may have no real influence upon the chances of survival but merely an apparent significance due to its correlation with other organs.