history, we cannot at present say. The historian should never forget, what the highest type of scientific man is always teaching us to remember, that willingness to admit ignorance is a prime factor in developing wisdom out of knowledge. Wisdom is advanced by research which enables us to add to knowledge; and, moreover, the way for wisdom is made ready when men who record facts of vast but unknown import, if asked to explain their full significance, are willing frankly to answer that they do not know. The research which enables us to add to the sum of complete knowledge stands first; but second only stands the research which, while enabling us clearly to pose the problem, also requires us to say that with our present knowledge we can offer no complete solution.
Let me illustrate what I mean by an instance or two taken from one of the most fascinating branches of world-history, the history of the higher forms of life, of mammalian life, on this globe.
Geologists and astronomers are not agreed as to the length of time necessary for the changes that have taken place. At any rate, many hundreds of thousands of years, some millions of years, have passed by since in the eocene, at the beginning of the tertiary period, we find the traces of an abundant, varied, and highly developed mammalian life on the land masses out of which have grown the continents as we see them to-day. The ages swept by, until, with the advent of man substantially in the physical shape in which we now know him, we also find a mammalian fauna not essentially different in kind, though widely differing in distribution, from that of the present day. Throughout this immense period form succeeds form, type succeeds type, in obedience to laws of evolution, of progress and retrogression, of develop-
