Page:Popular Science Monthly Volume 86.djvu/268

264 extent variation in physical conditions in the Mohave has therefore been related to stages in the life of our great ranges. The latest period in the history of the mountains is the stage in which the peaks and valleys were modeled to their present form through gradual wearing down by ice, water and chemical decay. The clearly visible evidences of this last epoch mark for us a period longer than the full span of human history. In the story of the mountains, the earlier stages standing in relation to the history of life on the Mohave are observed only through study of a complicated geologic problem, but the measure of these early stages in time is far longer than that of the latest epoch.

The Barstow, Ricardo and Manix faunas present three stages in the life history of the Mohave area within the extent of a long period marked by many great physical changes. The records of these faunas are incomplete, and should be considered only as imperfect pages from a volume that has passed through fire, flood, earthquake and decay incident to the passage of almost limitless time. As fragmentary and unsatisfactory as the story is, it opens to us a wide vision of previously unknown life history in this region; it offers significant evidence regarding the origin, evolution and migration of important mammal groups; it furnishes information concerning the climatic history of the Mohave; and it contributes largely to our knowledge of the chronology of great crustal movements in western North America. If this were the only record known in the world, from it alone we could gather evidence that the life of the earth is very old, that this life has completely changed from time to time, and that in each successive fauna there was a nearer approach to the life types now in existence. We might not be able from the Mohave story to demonstrate the fact of evolution, as the fragments are small, and represent periods so widely separated that the suggestion of continuity is indistinct. Taken in connection with the great volume of records now available from other regions of the world, the Mohave story serves in a modest way to fill gaps in the previously known history; and in its close relationship to faunas remotely separated from it geographically, it illustrates the faunal unity of the world as a whole when the broader outlines of evolution are followed through long periods.

The story of the Mohave read alone cannot do less than impress one with the magnitude of faunal changes and with their apparent definite trend toward the life of to-day. Related to other records, it becomes a part of the great world-scheme of life growth or evolution leading up through the ages to the present living world of which we are a part.