In spite of these arguments and the evidence of asymmetry of measurements at least at some periods of life it is to be noted that current opinion is probably contrary to this hypothesis, although, as I believe, because it has been concerned mainly with those who are not of extreme ability. For all large medium ranges of ability slight skewness might well be negligible. It is interesting to note that Galton says that “eminently gifted men are raised as much above mediocrity as idiots are depressed below it” (_159_, p. 19). Measured by intelligence quotients with the Stanford scale, Terman finds among school children that deviations below normal are not more common than those above (_197_, p. 555). Burt, following a suggestion of Cattell as to college men, however, seems to incline to the opinion that the general distribution of ability, like wages, is skewed toward the upper end. He adds, “In crude language, dullards outnumber geniuses, just as paupers outnumber millionaires” (_85_). (d) EQUIVALENT UNITS OF DEVELOPMENT WHEN THE FORM OF DISTRIBUTION IS UNCERTAIN. For our problem of units and scales of measurement, an asymmetrical distribution sets a very difficult problem. It may be that this very difficulty has been one of the main reasons for slowness in recognizing the drift of the evidence. In order to set forth the difference in the conception of measurement when distributions become asymmetrical I have presented this hypothesis in connection with the curves of development in Fig. 5. It will be noted that if the distributions of mental capacity vary in symmetry, the units of standard deviation change in significance from one form of distribution to another. Minus 2 S. D. may exclude very different portions of groups differently distributed, while it would always exclude the same proportion if the distributions had the same symmetry, or skewness. Under conditions of variable symmetry there is a sense in which the same relative physical score in units running from zero ability to the best ability would always have an equivalent objective meaning, but this might not express equivalent development conditions at different ages. For example, with shifting forms of distribution, to say that a child of six years had reached three-fifths of the best development for his age on an objective scale might give no significant indication of how nearly he was keeping pace with those three-fifths of the best ability of another age. Neither would his position in units of the deviation of ability at his age give this information without knowledge of the form of the distribution of ability at his age. With varying forms of distribution at different stages of development this would afford an insurmountable difficulty. [Illustration: FIG. 5. _Hypothetical Development Curves (Changing Forms of Distribution)_]