264 TEE POPULAR SCIENCE MONTHLY 



I 



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 val- 

 leys 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, Eicardo 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 incom- 

 plete, 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 unsatisfac- 

 tory 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 fur- 

 nishes 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 sep- 

 arated 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. Eelated 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. 



