Though changes in method and ideals cannot reduce the differences between individuals in the intellectual field to any marked extent, such changes can raise the level of achievement of the whole group. For instance, more emphasis on silent reading may make the reading ability of a whole school 20 per cent better, while leaving the distance between the best and worst reader in the school the same. Granting that heredity, original nature, is the primary cause of individual differences in intellect (aside from those sex differences mentioned) there remains for environment, education in all its forms, the tremendous task of: First, providing conditions favorable for nervous health and growth; second, providing conditions which stimulate useful capacities and inhibit futile or harmful capacities; third, providing conditions which continually raise the absolute achievement of the group and of the race; fourth, providing conditions that will meet the varying original equipments; fifth, assuming primary responsibility for development along moral and social lines. Concerning those individual differences of which heredity is the controlling cause, two facts are worthy of note. First, that human nature is very highly specialized and that inheritance may be in terms of special abilities or capacities. For instance, artistic, musical, or linguistic ability, statesmanship, power in the field of poetry, may be handed down from one generation to the next. This also means that 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.