two brothers may be extremely alike along some lines and extremely different along others.  Second, that there seems to be positive combinations between certain mental traits, whereby the presence of one insures the presence of the other to a greater degree than chance would explain.  For instance, the quick learner is slow in forgetting, imagery in one field implies power to image in others, a high degree of concentration goes with superior breadth, efficiency in artistic lines is more often correlated with superiority in politics or generalship or science than the reverse, ability to deal with abstract data implies unusual power to deal with the concrete situation.  In fact, as far as exact measures go, negative correlations between capacities, powers, efficiencies, are extremely rare, and, when they occur, can be traced to the influence of some environmental factor.

Individuals differ from each other to a much greater degree than has been allowed for in our public education.  The common school system is constructed on the theory that children are closely similar in their abilities, type of mental make-up, and capacities in any given line.  Experimentation shows each one of these presuppositions to be false.  So far as general ability goes, children vary from the genius to the feeble-minded with all the grades between, even in the same school class.  This gradation is a continuous one—­there are no breaks in the human race.  Children cannot be grouped into the very bright, bright, mediocre, poor, very poor, failures—­each group being distinct from any other.  The shading from one to the other of these classes is gradual, there is no sharp break.  Not only is this true, but a child may be considered very bright along one line and mediocre along another.  Brilliancy or poverty in intellect does not act as a unit and apply to all lives equally.  The high specialization of mental powers makes unevenness in achievement the common occurrence.  Within any school grade that has been tested, even when the gradings are as close as those secured by term promotions, it has been found in any subject there are children who do from two to five times as well as others, and from two to five times as much as others.  Of course this great variation means an overlapping of grades on each side.  In Dr. Bonser’s test of 757 children in reasoning he found that 90 per cent of the 6A pupils were below the best pupils of 4A grade and that 4 per cent of 6A pupils were below the mid-pupils of the 4A, and that the best of the 4A pupils made a score three times as high as the worst pupils of 6A.  Not only is this tremendous difference in ability found among children of the same class, but the same difference exists in rate of development.  Some children can cover the same ground in one half or one third the time as others and do it better.  Witness the children already quoted who, skipping a grade, were ready at the end of three years to skip again.  Variability, not uniformity, is what characterizes the abilities and rate of intellectual growth of children in the schools, and these differences, as has already been pointed out, are caused primarily by a difference in original nature.