January 28, 1916] 



SCIENCE 



115 



tion to all the rest of the world; it is the 

 empire of marsupials and monotremes, 

 while the other continents together consti- 

 tute the empire of the placental mammals. 

 This placental empire is, in turn, very 

 unequally divided into two realms; the 

 first, called Arctogcea, comprises North 

 America, Europe, Asia and Africa and has 

 an unmistakable general similarity in its 

 mammals, in contrast to the second realm, 

 Notogcea, which includes South and Cen- 

 tral America and the West Indies. After 

 Australia, Notogsa is zoologically the most 

 peculiar region of the earth, a peculiarity 

 which is likewise due to its long separation 

 from the continents of Arctogsea. 



The frequently made and interrupted 

 communication of North America with the 

 eastern hemisphere, principally by way of 

 a land which occupied the site of the 

 present shallow Bering Sea, is reflected in 

 the geographically composite character of 

 its mammalian fauna. This connection 

 allowed intermigrations of animals between 

 the eastern and the western hemispheres, 

 each furnishing to the other elements which 

 still persist in their new homes, though 

 often becoming extinct in their places of 

 origin. For example, the horses and 

 camels have disappeared from North 

 America, although they passed through 

 the greater part of their development in 

 that continent. The last of these migra- 

 tions, which took place in the Pleistocene 

 epoch, brought in such a host of Old World 

 types, that the northern half of North 

 America, comprising the Arctic and Boreal 

 Zones of Merriam, belongs zoologically to 

 the great Holaretic Region, which includes 

 Europe, northern Africa and all of Asia 

 except its southern peninsulas and thus en- 

 circles the earth. 



The characteristic part of North America 

 is the Sonoran Region, roughly the United 

 States and the Mexican plateau, and ever 



this region contains many Old World 

 forms, immigrants which arrived here at 

 very different geological dates and, in ac- 

 cordance with the length of their stay here, 

 have become more or less extensively modi- 

 fied. With these are associated many in- 

 digenous types, derived from a long North 

 American ancestry and a very few mi- 

 grants from South America, the short- 

 tailed porcupine, probably the opossums, 

 and in southeastern Texas an armadillo. 

 These southern animals are the insignif- 

 icant remnant of a great immigration 

 which entered North America from the 

 south, but was not able to gain a lasting 

 foothold here. 



South American mammals are ob- 

 viously divisible into two radically differ- 

 ent assemblages, one of which is related to 

 the types characteristic of Arctogtea and 

 the other is entirely peculiar to Notogsea. 

 The first, or immigrant, assemblage com- 

 prises all the beasts of prey, the wolves, 

 cats, otters, skunks and one species of bear ; 

 all of the hoofed animals, the tapirs, pec- 

 caries, deer, and that remarkable section of 

 the camel family, the guanacos, llamas, 

 etc. ; among the rodents, the rabbits, squir- 

 rels, rats and mice. The second, or indig- 

 enous, series includes the opossums, the 

 highly characteristic edentates, sloths, 

 armadillos and anteaters, and a very large 

 number of peculiar rodents, all of which 

 belong to the porcupine group, tree-porcu- 

 pines, chinchillas, cavies, water-hogs, etc., 

 etc. The paleontological history of South 

 American mammals amply justifies the dis- 

 tinction of these two assemblages as immi- 

 grant and autochthonous and shows that 

 South America derived from the north a 

 much larger proportion of its modern mam- 

 mals than North America did from the 

 south. 



It is altogether probable that in the 

 Mesozoic Era all of the continents were 



