The reason for the differences in variability is partly given in our previous remarks. We have seen, when a people is descended from a small uniform group, that then its variability will decrease; while on the other hand, when a group has a much-varied origin, or when the ancestors belong to entirely distinct types, the variability may be considerably increased. In all cases which have been investigated, even in those in which the variability is small, there still remain considerable differences in bodily appearance among the individuals constituting a tribal or national or other social unit; and what is true of the physical traits is obviously no less true of the mental traits, but it is difficult to express mental characteristics in numerical terms of variability. An acquaintance with tribes which are apparently on the simplest level socially, however, shows the existence of certain individuals of most diverse types of disposition and intellect; and as might be expected, if we consider mental condition as dependent upon bodily form, the great variability, even in the most uniform group, of the structure of the body, particularly of the brain, makes it plausible that very large differences in individual disposition may be expected. What we have said before in regard to the overlapping of variations among different races and types, and the great range of variability in each type, may also be expressed by saying that the differences between different types of man are, on the whole, small as compared to the range of variation in each type. The importance of these observations in the domain of mental development will be the subject of our future discussions. ----- Footnote 2: These generations are not completely known. The values here given are the maximum values which would be found provided the unknown individuals had had no “loss of ancestors.” ------------------------------------------------------------------------ IV. THE MENTAL TRAITS OF PRIMITIVE MAN AND OF CIVILIZED MAN IN the preceding chapters we have discussed the biological conditions of various races and of different social groups, in so far as they form the basis of mental activity. We must now turn our attention to the psychological characteristics of mankind under the varying conditions of race and environment.