The best test of the difficulty of an inquiry is the number of theories which its solution has evolved. If we test the subjectmatter of the present communication in this way, we shall be assured that, second only perhaps to the Loess, which has occupied us previously, it may claim pre-eminence for the difficulty of the problems which surround it. We only venture upon a tentative solution. Such a solution is in effect the only one which is available in science at all; for to-morrow's fiercer light is certain to throw out of perspective some of to-day's most cherished convictions; and finality is outside the pale of all living and fertile inquiry.