With this bare skeleton of the subject of variation before us let us see how facts of this kind may have any significance for the subject of Eugenics, any bearing upon the possibility of racial improvement. When any of the varying human traits, and they all vary, is measured carefully and the results tabulated we find that they give us a curve approximating the normal frequency curve, such as we have described above and illustrated in Fig. 3. The coefficients of variability of a great many human traits are known and a few representative coefficients are given in Table I. This type of variability is given then, by measurements of physical characteristics of all kinds, and, what is of greater importance, physiological traits, including mental and moral characteristics, so far as they can be measured by present methods, vary in just the same way. Annual individual earnings give us a curve closely similar to that of a normal frequency curve with an approximate minimum limiting value. Even the tabulation of citizens according to their social standing or "civic worth" gives the same sort of thing. This has been brought out nicely in Galton's discussion of Booth's classification of the population of London. TABLE I _Coefficients of Variability of Certain Human Traits_ Adult Stature 3.6 to 4.0 Length at Birth 5.8 to 6.5 Length of Limb Bones 4.5 to 5.5 Cephalic Index 3.7 to 4.8 Skull Capacity 7.0 to 8.0 Weight (University Students) 10.0 to 11.0 Weight at Birth 14.2 to 15.7 Weight of Brain 7.0 to 10.6 Weight of Heart 17.4 to 20.7 Weight of Liver 14.3 to 22.2 Weight of Kidney 16.8 to 22.5 Lung Capacity 16.6 to 20.4 Squeeze of Hand 13.4 to 21.4 Strength of Pull 15.0 to 22.6 Swiftness of Blow 17.1 to 19.4 Dermal Sensitivity 35.7 to 45.7 Keenness of Eyesight 28.7 to 34.7 It is not so easy to answer the question whether mutations or true variations are occurring frequently in the human species. Usually it is impossible to distinguish between an extreme fluctuation and a true variation without experimental test and the observation of the behavior of the varying trait through several generations. In most instances this has been impossible with human beings. From collateral evidence it seems quite probable that man is mutating with considerable frequency, especially with respect to psychic traits.