GEOLOGICAL HISTORY OF THE PRIMATES 6oi 



is none too clear ; but one is inclined to think that the con- 

 nection of Africa with the main land-mass was closer and later 

 than the similar connection of South America. 



During the Cainozoic Era there have also been separations 

 between Europe and North America. But such separations 

 were of relatively brief duration, and for the purposes of this 

 general survey Europe, Asia, * and North America may be 

 regarded as a single land-mass, constituting the Mainland. 



It remains to be stated that Africa was reunited to the 

 Mainland at the end of the Oligocene, and that the reunion of 

 South America occurred in the Pliocene. In the Miocene we 

 find the Proboscidea wandering all over Eurasia and North 

 America. At the same time, also, there were doubtless great 

 southward migrations of Perissodactyla, Artiodactyla, true 

 Carnivora, rodents, and other animals into Africa. During the 

 long period of South American isolation there occurred a re- 

 markable and of course independent evolution of Placentalia in 

 that continent. Three orders of herbivores were evolved, the 

 Toxodontia, Pyrotheria, and Litopterna, certain of the last- 

 named group being famous for the extraordinary parallelism 

 which they display to the horses of the Mainland. Apart 

 from these three extinct orders, the American edentates, the 

 Xenarthra, flourished and produced a number of families — 

 sloths, armadillos, and others. Although not known from 

 Paleocene rocks, the Xenarthra appear to have been evolved 

 in that period — probably from the Paleocene Taeniodonta — 

 because an aberrant armadillo is known from the Eocene of 

 North America, long after the separation of the two continents. 

 In the absence of carnivorous placentals, a peculiar group of 

 rapacious marsupials, the so-called Borhyenas, survived and 

 prospered in South America during the period of isolation. 

 The presence of these great groups in pre-Pliocene South America 

 is much what we should expect ; the groups are either very 

 ancient and archaic or peculiar to the continent ; the only 

 surprise is the existence of Xenarthra in North America as 

 well, and the inference which we have to draw regarding the 

 antiquity of that group. But there are two other elements in 

 the ancient fauna whose presence there is very startling. One 

 of these is the Hystricomorph (Porcupinish) rodents. The 

 other group is the monkeys of the existing family Cebidse 

 These latter problems are perhaps the most difficult and puzzling 

 questions which arise out of mammalian distribution. 



As already stated, South America was reunited to the Main- 

 land in the Pliocene, and the greatest of all migrations then 

 took place. The united faunae of the Mainland and Africa had 



^ This does not apply to the southern peninsulas of Asia, which weie 

 largely insular during the earUer Cainozoic. 



