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 development and death, which we as yet understand only in the most imperfect manner. As knowledge increases our wisdom is often turned into foolishness, and many of the phenomena of evolution which seemed clearly explicable to the learned master of science who founded these lectures, to us nowadays seem far less satisfactorily explained. The scientific men of most note now differ widely in their estimates of the relative parts played in evolution by natural selection, by mutation, by the inheritance of acquired characteristics; and we study their writings with a growing impression that there are forces at work which our blinded eyes wholly fail to apprehend; and where this is the case the part of wisdom is to say that we believe we have such and such partial explanations, but that we are not warranted in saying that we have the whole explanation. In tracing the history of the development of faunal life during this period, the age of mammals, there are some facts which are clearly established, some great and sweeping changes for which we can with certainty ascribe reasons. There are other facts as to which we grope in the dark, and vast changes, vast catastrophes, of which we can give no adequate explanation.