furnishes us with the most valuable materials for our present study. Further back still we have the Eocene period, with apparently an almost tropical climate in Europe; and here we find a clue to some of the most puzzling facts in the distribution of living animals. Our knowledge of this epoch is however very imperfect; and we wait for discoveries that will elucidate some of the mystery that still hangs over the origin and migrations of many important families. Beyond this there is a great chasm in the geological record as regards land animals; and we have to go so far back into the past, that when we again meet with mammalia, birds, and land-reptiles, they appear under such archaic forms that they cease to have any local or geographical significance, and we can only refer them to wide-spread classes and orders. For the purpose of elucidating geographical distribution, therefore, it is, in the present state of our knowledge, unnecessary to go back beyond the tertiary period of geology. The remains of Mammalia being so much more numerous and important than those of other classes, we shall at first confine ourselves almost exclusively to these. What is known of the birds, reptiles, and fishes of the tertiary epoch will be best indicated by a brief connected sketch of their fossils in all parts of the globe, which we shall give in a subsequent chapter. {110}_Historic Period._--In tracing back the history of the organic world we find, even within the limits of the historical period, that some animals have become extinct, while the distribution of others has been materially changed. The _Rytina_ of the North Pacific, the dodo of Mauritius, and the great auk of the North Atlantic coasts, have been exterminated almost in our own times. The kitchen-middens of Denmark contain remains of the capercailzie, the _Bos primigenius_, and the beaver. The first still abounds farther north, the second is extinct, and the third is becoming so in Europe. The great Irish elk, a huge-antlered deer, probably existed almost down to historic times.