GEOGRAPHICAL DISTRIBUTION AND DESCENT. 



137 



those of the Old World, but the forms belong- 

 ing to the two sides of the ocean show a 

 certain parallelism. It may be presumed 

 that the three forms mentioned reached 

 North America by migrating from the Polar 

 regions. 



The Hollow-horned Ruminants are alto- 

 gether absent from South America, and are 

 but feebly represented in the northern division 

 of the continent. The curious forms of the 

 Rocky Mountains, the pronghorn antelope, 

 the big-horn, and the white goat, together 

 with the musk-ox, are the only characteristic 

 forms of North America. The American 

 bison approaches too closely to that of the 

 Old World for us to be able to consider 

 these two forms as anything else than varieties 

 descended from the same species, which has 

 immigrated into both continents from the 

 Polar regions. In the Old World the Goats 

 and the Sheep appear to be exclusively of 

 Eurasian origin. The Barbary wild sheep 

 makes only an apparent exception; the 

 Mediterranean zone forms, in fact, a whole 

 by itself, which belongs to the Palaeozoic 

 region. On the other hand, the big-horn 

 of the Rocky Mountains might perhaps be 

 only an immigrant from the opposite peninsula 

 of Kamchatka. Oxen and Antelopes inhabit 

 the whole of the Old World. These two 

 groups present in part very characteristic 

 genera and species for the two continents of 

 Eurasia and Africa. Among the antelopes, 

 for example, the saiga and the chamois for 

 Europe and Asia, the gnus for Africa, 

 among oxen the yak and anoa for Asia, 

 while other groups are found everywhere in 

 the Old World, though almost entirely absent 

 in the New, for North America possesses 

 only the forms already mentioned and South 

 America none at all. 



The Giraffes are exclusively African types. 

 The Llamas are confined to South America, 

 and the Camels belong originally to Central 

 Asia, whence they have been introduced into 

 Africa. 



Vol. II. 



The present geographical distribution of 

 the Artiodactyla differs widely from that 

 which immediately preceded in the Quater- 

 nary period, and corresponds exactly to the 

 facts from which palaeontologists have inferred 

 certain lines of descent. 



In comparing the faunas of the Quaternary 

 period with the present distribution we 

 observe remarkable transpositions in two' 

 opposite directions, and in connection with 

 that fact we have to note some rather striking 

 instances of the extinction of certain species. 

 Let us now, then, examine the various groups 

 from this point of view, and trace them out 

 to the point where their characters become 

 clearly prominent. 



In the Quaternary period the Hippopota- 

 muses inhabited the rivers of Southern and 

 Central Europe, and extended as far as 

 Ireland. A larger species than the present 

 hippopotamus (^Hippopotamus major), but 

 otherwise very little different, lived within 

 these wider limits, while in Sicily and the 

 valley of the Arno has been found a species 

 i^H. minor) which was no larger than a pig, 

 and may have been more closely allied to the 

 hippopotamus of Liberia. Now still older 

 hippopotamuses have hitherto been found 

 only in Algeria in the Pliocene, and in India 

 in the Miocene of the Sewalik Hills, where 

 there existed hippopotamuses with six incisors 

 and poorly developed canines, and in addition 

 to these a genus, Merycopotamus, which 

 through the structure of its skeleton and its 

 dentition forms the transition to the pigs. 

 At the present day thei^e are no longer any 

 members of the family in India. This new 

 type, for it is new, since it appears first in 

 the Upper Miocene, must accordingly have 

 migrated from its home to its present African 

 domain after it had peopled a part of Europe 

 in the Quaternary period. 



The Hog Family shows two parallel primi- 

 tive stocks of much greater antiquity, the one 

 belonging to the Old World, the other to 

 America. True Pigs (Sus) are found already 



60 



