ALL study of the origins of social institutions must be based on what ethnology can tell us of the psychology of the lower races and on the primitive conceptions of human relations which are thus established. It is only in early modes of thought that we can find the explanation of ceremonies and systems which originated in primitive society ; and, if ceremony and system are the concrete forms in which human relations are expressed, an examination, ethnological and psychological, of human relations, is indispensable for enquiry into human institutions. It is necessary to lay stress upon this principle, for students of the history of marriage have hitherto ignored it, or rather, while using the facts of ethnology, have shown no sympathy with primitive thought. They have interpreted primitive custom by ideas which are far from primitive, which, in fact, are relatively late and belong to the legal stage of human culture. The attribution of legal conceptions to primitive thought has had the usual effect of a priori theory, and has checked enquiry.