46 THE AGE OF MAMMALS 



mals we should not limit ourselves to mammals, but should appeal to as 

 many classes of facts as possible, facts of climate, of geology, of physiog- 

 raphy, of migration and colonization, and the rise, dominance, and decline 

 of certain kinds of animals and plants. 



In comparing the mammals of the two regions we look for the following 

 tests : 



1 . Presence of similar species. — Those classic or time-honored bases 

 of comparison in establishing percentages through the presence or absence 

 of similar genera and species lead us to most interesting results, because 

 they prove that the mammals of the Old and New Worlds were alter- 

 nately brought together and separated. In other words, there was an 

 alternating convergence and divergence of the faunas. The resemblances 

 will first be very numerous and close, then there will come an estrangement 

 when they will be very few, then the resemblances will suddenly increase 

 again. It is obvious that only during the periods of faunal resemblance 

 are we able to use the following or second method of comparison. 



2. Similar stages of evolution. — This second method of comparison is 

 based upon the similarity in the stages of development of like phyla of the 

 mammals on the two continents, as expressed in the detailed changes in 

 the grinding teeth (molars and premolars), in the numerical reduction of 

 the digits, etc. For example, the different transformations of the pre- 

 molars, or anterior grinding teeth in the horses, rhinoceroses, and tapirs 

 during the Eocene and Oligocene Epochs afford very exact data for corre- 

 lation purposes. 



3. Simultaneous appearance or introduction of new mammals. — The 

 sudden appearance both in the Old and New Worlds of mammals which 

 have no known ancestors in lower horizons and have apparently originated 

 elsewhere is of great value in correlation. These coincident immigrations 

 from unknown northern regions (Eurasia) or from southern regions (Africa) 

 in several cases give us very exact datum points; for example, certain 

 kinds of modernized mammals simultaneously appear in Europe and in 

 North America in Lower Eocene and again in Oligocene times. 



4. Intermigration periods. — These periods are those in which con- 

 spicuous interchanges of mammals took place, as between North and 

 South America in the Pliocene. The horse (Equus) being unquestionably 

 derived from North America, its earliest appearance in North America 

 must antedate its first appearance in South America. 



5. Predominance of certain kinds of mammals. — Many related families 



of mammals seem to go through a cycle of gradual ascent until they 



attain a stage of world-wide predominance at aliout the same periods. 



For example, the climax of the odd-toed ungulates (Perissodactyla) is 



in the Middle and Upper Eocene of Europe and North America, while 



the climax of the even-toed ungulates (Artiodactyla) comes at a later 

 period. 



