234 TEK YEARS^ PROGRESS IN VERTEBRATE PALEONTOLOGY 



of the Tertiary epoch and the formations which contain them in Xorth 

 America. Such correlation between horizons and countries where mam- 

 mals alone are found has certain peculiar advantages : first, that the trend 

 and rate of development is fairly uniform the world over; second, that 

 large faunal communities in similar stages of development generally 

 imply geographic intercourse ; third, that the cycles of physiographic and 

 climatic change are broadly similar in the two halves of the nort'iern 

 hemisphere ; fourth, that faunal community implies land connections and 

 geographic continuity or continuity of migration areas. 



General Contributoes of the past Decade 



Among the general contributors of the past decade to correlation and 

 migration in the northern hemisphere are the following: Matthew (1906, 

 1908),^ Osborn (1900, 1909, 1910),^ Weeks (1902),^ Woitman (1903).« 



Correlation of American Tertiaries 



Marsh and Cope treated the great formations of the American Eocene 

 as units, even where 1,000 to 2,000 feet in thickness. The greatest prog- 

 ress which has been made in the past decade is due to the breaking up of 

 these formational units into sub-units, or life zones, usually marked off 

 more or less clearly, by geologic discontinuity. This splitting up of the 

 formations has been accompanied by new standards of precision in record- 

 ing the succession of mutations and of species on certain levels which have 



3 W. D. Matthew : Hypothetical Outlines of the Continents in Tertiary times. Bulletin 

 of the American Museum of Natural History, vol. xxii, art. xxi, October 25, 1906, pp. 

 353-384. 



Mammalian Migrations between Europe and North America. American Journal of 

 Science, vol. xxv, January, 1908, pp. 68-70. 



* H. F. Osborn : The Geological and Faunal Relations of Europe and America during 

 the Tertiary Period and the Theory of the Successive Invasions of an African Fauna. 

 Science, n. s., vol. xi, no. 276, April 13, 1900, pp. 561-574. 



Correlation between Tertiary Mammal Horizons of Europe and America. An introduc- 

 tion to the more exact Investigations of Tertiary Zoogeography. Preliminary study, with 

 third trial sheet. Annals of the New York Academy of Science, vol. 13, no. 1. July 21, 

 1900, pp. 1-72. 



Cenozoic Mammal Horizons of Western North America, with Faunal Lists of the Ter- 

 tiary Mammalia of the West, by W. D. Matthew. Publications of the U. S. Geological 

 Survey, Bulletin No. 361. 



The Paleontologic Correlation through the Bache Fund. Science, n. s.. vol. xxxi, no. 

 794, March 18, 1910. pp. 407-408. 



Correlation of the Cenozoic through its Mammalian Life. Journal of Geologj-, vol. 

 xviii, no. 3, April-May, 1910, pp. 201-215. Outlines of geologic history. Svo, Chicago 

 University Press, July 10, 1910, pp. 251-264. 



The Age of Mammals in Europe, Asia, and North America. 8vo, The Macmillan Com- 

 pany, New York, October 25, 1910, pp. 635. 



5 F. B. Weeks : North American Geologic Formation Names. Bibliography, Synonym, 

 and Distribution. Bulletin of the U. S. Geological Survey, no, 191, 1902. 



•* J. L. Wortman : Origin of the Primates. American Journal of Science [4], xv, June, 

 1903. 



