CONTINENTAL DISPERSAL OF MAMMALIAN ORDERS From the studies of palaeontologists and mammalogists emerge the following among other general results bearing on geographic dispersal of the mammalian orders: (I) At various times during the Age of Mammals pathways were opened by which a given group of mammals, acquiring its special characters in regional isolation, could pass from central Asia westward to Europe or eastward to North America, or from either Europe or America to Asia. Among the orders and families of placental mammals that may have originated in the northern realm (including Europe, Asia, North America) may be mentioned the perissodactyls and artiodactyls, the insectivores, the fissipede carnivores, the rodents, the edentates, the lemuroids, catarrhine primates, anthropoids and probably man. The families of rhinoc eroses, lophiodonts, titanotheres and camels afford examples of the more or less free intercontinental commerce at certain periods.
(2) South America was early in contact with North America; then the contact was broken and for millions of years the country was left to develop its own fauna of litopterns, toxodonts, ground sloths and other strange beasts. In late Tertiary times the way was again opened and mastodons, camels, tapirs, deer and other animals streamed in from the north, while South America sent platyrrhine monkeys, ground-sloths and other animals north ward.
(3) Africa, especially in the north, was frequently in contact with the northern land mass but in late Eocene and early Oligo cene times the Fayum district in Egypt may have been an outlier of some central African region which seems to have produced the Proboscidea, arsinoitheres, hyracoids and sirenians. The most
primitive known cetaceans (Pappocetus, Protocetus, Prozeuglo don) are also found there. Madagascar, possibly part of a broad archipelago formerly connected with India, was the centre of peculiar families of carnivores (viverrids) lemuroids, insectivores (centetoids). Broadly, Africa's mammalian fauna to-day repre sents Europe and Asia of Miocene and Pliocene times.
(4) Australia, receiving its original marsupials at some early date, was then cut off from Asia and developed one of the most interesting mammalian faunas of all time, in which the marsupial stock was fashioned into herbivorous, carnivorous and rodent-like mammals, strangely similar in habitus to their analogues of the placental world and yet always preserving their marsupial heritage in brain and reproductive system and in the deeper characters of the skull and skeleton. At times during this long period placental invaders from the north managed to get in; first a peculiar family of rats and much later the dingo or native dog, together with that most devastating of all placentals, man. Australia, on the other hand, succeeded in sending some marsupials into New Guinea and as far northwest as Celebes. Some hold also that from Australia by way of Antarctica came the extinct carnivorous marsupials and caenolestoids of South America. (See MARSUPIALIA.)