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.

Before illustrating these types, let us settle one or two matters of terminology.  In the changes, the development and extinction, of species we must remember that such expressions as “a new species,” or as “a species becoming extinct,” are each commonly and indiscriminately used to express totally different and opposite meanings.  Of course the “new” species is not new in the sense that its ancestors appeared later on the globe’s surface than those of any old species tottering to extinction.  Phylogenetically, each animal now living must necessarily trace its ancestral descent back through countless generations, through aeons of time, to the early stages of the appearance of life on the globe.  All that we mean by a “new” species is that from some cause, or set of causes, one of these ancestral stems slowly or suddenly develops into a form unlike any that has preceded it; so that while in one form of life the ancestral type is continuously repeated and the old species continues to exist, in another form of life there is a deviation from the ancestral type and a new species appears.