When we find that a type of kinship terminology is determined by exogamy or matrilineal descent, we have, indeed, given a cultural explanation of a cultural fact; but for the ultimate problems how exogamy or maternal descent came about, we may be unable to give a solution. Very often we cannot ascertain an anterior or correlated cultural fact for another cultural fact, but can merely group it with others of the same kind. Of this order are many of the parallels that figure so prominently in ethnological literature. For example, that primitive man everywhere believes in the animation of nature seems an irreducible datum which we can, indeed, paraphrase and turn hither and thither for clearer scrutiny but can hardly reduce to simpler terms. All we can do is to merge any particular example of such animism in the general class after the fashion of all scientific interpretation. That certain tendencies of all but universal occurrence are characteristic of culture, no fair observer can deny, and it is the manifest business of ethnology to ascertain all such regularities so that as many cultural phenomena as possible may fall into their appropriate categories. Only those who would derive each and every trait similar in different communities of human beings from a single geographical source can ignore such general characteristics of culture, which may, in a sense, be regarded as determinants of specific cultural data or rather, as the principles of which these are particular manifestations. Recently I completed an investigation of Plains Indian societies begun on the most rigorous of historical principles, with a distinct bias in favor of the unique character of cultural data. But after smiting hip and thigh the assumption that the North American societies were akin to analogous institutions in Africa and elsewhere, I came face to face with the fact that, after all, among the Plains Indians, as among other tribes, the tendency of age-mates to flock together had formed social organizations and thus acted as a cultural determinant.