VUl PEEFACE 



which I began in 1898 and have been pursuing at intervals for eleven years, 

 namely, the comparison of the new and old world life. 



It is thus an exposition and explanation of two presidential addresses 

 of mine delivered before the New York Academy of Sciences in February, 

 1899 and 1900, entitled "Correlation between Tertiary Mammal Horizons 

 of Europe and America," also of a paper published recently by the United 

 States Geological Survey, entitled " Cenozoic Mammal Horizons of Western 

 North America." In the Introduction I have drawn freely on several popu- 

 lar addresses of mine, " The Rise of the Mammalia in North America " 

 (1893), " Ten Years' Progress in the Mammalian Palaeontology of North 

 America" (1905), "The Present Problems of Palaeontology " (1905), and, 

 finally, "Palaeontology," an article prepared for the forthcoming edition 

 of the Encijclopedia Britannica. 



The stimulus to put these studies into the present collected form was 

 given by the generous foundation of the Harris Lectureship in Northwestern 

 University by Mr. Norman Waite Harris in 1906. As originally delivered 

 in December, 1908, to the students of that institution, the lectures were in 

 general popular form. It has required a year to verify and expand them, 

 so that the volume is practically of date December 31, 1909. The oral style 

 appropriate to the lecture has given way necessarily to the written style ; 

 there is a greater fullness and I trust a greater clearness. 



In gathering the materials for the preparation of these addresses and 

 of this volume, my foremost acknowledgments are due to the profound and 

 accurate researches of my friend, Professor Charles Deperet of the University 

 of Lyons, as well as of my former student and present colleague in the 

 American Museum of Natural History, Dr. William Diller Matthew. The 

 very precise data which they have brought together, coupled with my own 

 researches and observations on the mammals of the Old and New Worlds, 

 have furnished the chief material for the broad comparisons and generaliza- 

 tions which I have attempted to make. I have also reviewed the general 

 literature of the subject, and I desire to acknowledge the aid of my former 

 student and present research assistant, Mrs. Johanna Kroeber Mosenthal, 

 who has been intrusted with a large part of the reading, translation, and 

 collation of facts derived from the foreign and American sources. 



The reader will observe that the collections in the American Museum 

 of Natural History as well as our observations in the field are very largely 

 drawn upon. These have been gathered and planned during the past twenty 

 years under my direction, and the fullest acknowledgments are due to the 

 able and energetic explorers who have helped to bring these rare treasures 

 of the past together, especially to Dr. J. L. Wortman, Dr. W. D. Matthew, 

 Mr. J. W. Gidley, Mr. O. A. Peterson, Mr. Walter Granger, Mr. Barnum 

 Brown, and Mr. Albert Thomson. The necessity for great precision in field 

 records, especially for recording the exact levels on which specimens are 

 found, I have impressed constantly upon the minds of these explorers. Such 

 precise records have important bearing on the question of time as well as 



