{"content":[{"text":"2I4 EUGENfICS REVIEW. \n population, and indicates the extent to which its members differ \n one from another; applied to an individual it is meaningless, \n although each individual measurement is taken into account in \n calculating it. In the same way it should be clearly understood \n what we mean by a cause of variability. If we say, \"This boy \n has grown tall because he has been well fed,\" we are not merely \n tracing out cause and effect in an individual instance ; we are \n suggesting that he might quite probably have been worse fed, \n and that in this case he would have been shorter. We are, in \n fact, suggesting that existing differences of nutrition can account \n for differences of stature comparable to the standard deviation \n of stature. Now this is just what is meant when we speak \n of nutrition as a cauise of variability; we thereby mean that in \n a population absolutely uniform in regard to other causes, such \n as breeding and exercise, existing differences of nutrition would \n produce a certain variability-In fact, that a certain percentage of \n the variance must be ascribed to nutrition. \n This point of view throws a flood of light upon the meaning \n to be attached to the results of biometrical research. The state- \n ment that the correlation, in stature, between father and son is r \n may be interpreted in several ways. For our present purpose \n it means that r2 represents the fraction of the total variance of the \n sons, which is due to the fact that their fathers differ in height. A \n selected population of sons of fathers of identical height would \n have its variance reduced by this fraction. Since r is about *5 I we \n may say that 26 per cent. of the variance is due to the stature \n of the father; the remaining 74 per cent. must have some other \n cause. Now this very value for the parental correlation has been \n freely used to prove that the influence of heredity is all-important. \n It has been contrasted with correlations such as °S obtained from \n particular environmental conditions. These latter evidently make \n no sensible contribution to the total variance, but it is just as \n evident that inheritance, although making an extensive contribu- \n tion, has not been shown to furnish so much that there may not be \n other factors, equally or more important. An advocate of the \n importance of environment might easily point to a dozen causes to \n which height or shortness is commonly ascribed, such as regular \n athletic exercise, or accidental illness in childhood, and it would","bbox":{"left":0.171733,"top":0.080509,"width":0.730824,"height":0.8026500000000001}}],"images_bbox":[],"images_bbox_not_text_overlap":[]}