In general we may say, then, that there is no single principle which will explain the evolution of the family from the earliest times down to the present. Any attempt to reduce the evolution of the family to a single principle, or to show that it has been controlled by a single set of causes, must inevitably end in failure. The economic determinism of Marx and his followers, the ideological conceptions of Hegel, the geographical influences of Buckle and his school, and like explanations, are all found wanting when they are applied to the actual history of the family. It is not different with the theories of recent sociologists, who would strive to explain all social changes through a single principle. Professor Giddings' principle of "Consciousness of Kind" and Tarde's principle of "Imitation" will not go further in explaining the changes in the family life than some of the older principles that we have just mentioned. Human life is, indeed, too complex to be explained in terms of any single principle or any single set of causes. The family in particular is an organic structure which responds first to one set of stimuli and then to another. Now it is modified by economic conditions, now by religious ideas, now by legislation, now by imitation, and so on through the whole set of possible stimuli which may impinge upon and modify the activity of a living organism. So it is with all institutions. The Influence of Christianity upon the Family.--While we cannot study further the evolution of the family in any detail, still it is necessary, in order to avoid too great discontinuity, to notice in a few sentences the influence of Christianity upon the family in Western civilization.