DISTRIBUTION OF ORDERS. 235 



Distribution of Orders 



1. Monotremata. Consists of only three genera, limited to part of the Australian 

 region. See p. 188. 



No fossil forms are found elsewhere, so that the place of origin of this order is unknown. 



2. Marsupialia. Consists of seven families (comprising thirty-six genera), of which only 

 one, the Opossum family (including three genera), occurs outside the Australian region. 

 The Opossums are Neotropical and Nearctic. 



Fossil opossums occur in the Pleistocene of America, and in much older European 

 deposits (Eocene to Miocene). The Secondary rocks of Europe contain a number of 

 small forms, which probably resemble the ancestors of the Australian Marsupials. We 

 may therefore suppose that this order originated in the Palaearctic region, and then 

 extended into what is now Australia (at that time united by land with Asia), isolation 

 occurring soon after, followed by specialization in various directions. The Opossums 

 seem first to have existed in Europe, from whence they spread into America by former 

 northerly land-connections. 



3. Edentata. This order is now chiefly limited to S. America, but Orycteropus is 

 peculiar to the Ethiopian region, while Manis is found both in that and the Oriental 

 region. 



The geological evidence is in favour of considerable development in Africa, whence the 

 order would spread north to the Oriental and Palsearctic regions, and thence on to 

 America. The competition with higher forms has caused this extinction in most areas, 

 and Edentates appear to be most abundant in S. America, because the competition with 

 other animals is there comparatively small. The peculiar burrowing or climbing habits of 

 most of the genera also tend to preserve them, and these habits no doubt represent 

 attempts to escape from the severe competition with higher forms. The size of existing 

 Edentates is insignificant compared with that of Pleistocene S. American and European 

 genera. 



4. Ungulata.(a) Artipdactyla. Non-rumniantia. Swine are only represented in 

 America by peccaries (Dicotyles) ; true swine are found in all the other regions but 

 only extend into the Australian as far as New Guinea. These animals are first known in 

 the European Eocene, and during Miocene and Pliocene times were as common in N. 

 America as Europe, but since then have almost entirely disappeared from the former 

 area. 



The hippopotamus is now limited to the Ethiopian region, but fossil forms occur in 

 Europe (Pliocene and Pleistocene) and India (Miocene). 



Ruminantia. Recent Camelidce are only found in the Neotropical and Palsearctic'regions, 

 but numerous forms occur in the Miocene and later deposits of N. America, where the 

 group originated. 



Tragulidce or mouse-deer have also a discontinuous area of distribution W. Africa 

 (Hyomoschus) and Oriental region (Tragulus). This is accounted for by the presence of 

 Miocene forms in Europe, whence the family extended south. 



Deer occur in all the regions except the Ethiopian, but do not extend far into the 

 Australian region. They appear to have taken origin in the Old World, from whence they 

 reached N. America in Miocene times, and afterwards passed to S. America. 



Giraffes are at the present time confined to the Ethiopian region, but fossil forms are 

 known from S. Europe and India, and a northern temperate origin is probable. 



Bovidce (oxen, sheep, antelopes, &c.) are present in all the regions except the Neotropical, 

 though they only just pass into the Australian, and are scarce in the Nearctic. The 

 family appears to have originated in the Palasarctic and Oriental regions during Miocene 

 times. 



(&) Perissodactyla. Tapirs present a striking example of discontinuous distribution, 

 being found, on the one hand, in the Malay peninsula, Sumatra, and Borneo, and on the 

 other, in S. and Central America. True tapirs occur in the W. of Europe as far back 

 as Miocene times, but in America are not found further back than the Pleistocene. 

 Migration from the Palsearctic region is thus indicated. 



Rhinoceroses are now only Ethiopian and Oriental, but they appear to have originated 

 in the Palsearctic region, where they extend back to the Miocene period. In Pliocene 

 times they also ranged into N. America. 



The genus Equus (horse, ass, zebra) is now limited to the Ethiopian and Palsearctic 

 regions. It appears to have originated in the latter area during Miocene times, and then 

 migrated not only into the Ethiopian, but also into the Oriental, Nearctic, and Neotropical 

 regions, as proved by fossil forms. 



5. Sirenia.See p. 203. 



