Then we must remember that Natural Selection in man has fallen chiefly upon groups, not upon individuals, and differences in the nature and organization of human groups are determined chiefly by what we can best sum up as differences of tradition in the widest sense of the term. The later history of mankind, from a period long antedating written records, has been one of the rapid rise and equally rapid extinction, not only of one group-unit after another, but of one type of group-unit after another. It is further obvious at first glance that the group-units, the types of society which are at present dominant, are far from perfect and far from stable, and indeed that they are evolving, with speed of change hitherto unsurpassed, towards new and unknown forms. When the mammalian type first became dominant on the globe--at the transition between the Secondary and Tertiary periods--a somewhat similar history was passed through. The new type of organization gave its possessors marked advantages over other animal types: but the full potentialities of the mammal (excluding man) were not realized until well over half of the Tertiary period had elapsed, and man was being prepared in the womb of circumstance. The Pliocene sees the triumph of the perfected types of mammal: the preceding Miocene, broadly speaking, sees the first rise of these new types, while the Eocene and Oligocene show us a rapid rise and as rapid extinction of variation upon variation on the original theme.[12] With man, however, only the beginnings of a similar process have as yet come to pass. Further, we must distinguish clearly between the different ways in which progress may be operative in man. In the first place it can appear, as we have just pointed out, in the organization of the communities to which he belongs and on which natural selection seems mainly to act. Secondly, it can appear as a raising of the _average_ of certain qualities among the individuals composing those communities. And thirdly, it can appear as a raising of the _upper level_ of attainment in those qualities, in the appearance of individuals biologically higher than any that have previously existed.