ment 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
