DISTRIBUTION OF MAMMALIA IN TIME. 649 



which is illustrated more completely by the Mammalia than by 

 any other extinct animals. An examination, namely, of the 

 facts of the geological distribution of Mammals leads to the 

 striking generalisation that " the present distribution of organic 

 forms dates back to a period anterior to the origin of existing 

 species" (Lyell). In other words, though the extinct Mam- 

 mals of the later geological deposits of any given country differ 

 specifically from those now existing in the same country, they 

 are nevertheless referable to the same orders, and are in every 

 respect more closely allied to the present Mammalian fauna 

 than to that of any other country. A few examples will render 

 this perfectly clear. 



Australia at the present day is an altogether peculiar zoolo- 

 gical province, characterised by the abundance and variety of 

 Marsupials which inhabit it. In the Post-tertiary deposits of 

 Australia, however, we are presented with proofs that Marsu- 

 pials were just as characteristic of Australia during late geolo- 

 gical epochs as they are now. In the Post-pliocene period we 

 know that Australia was occupied by Kangaroos, Kangaroo- 

 rats, Wombats, Phalangers, and Carnivorous Marsupials, in 

 every way representing the living Marsupials in zoological 

 value, but specifically distinct, and generally of gigantic size. 



In the same way, South America at the present day is espe- 

 cially characterised by a Mammalian fauna, containing many 

 peculiar forms, the Edentata being especially conspicuous, and 

 having a larger representation than in any other region. Simi- 

 lar but distinct forms, however, are found to have existed in 

 South America anterior to the creation of any existing species. 

 Thus, the modern Sloths of South America are represented by 

 the colossal Mylodon, Megalonyx, Scelidotherium, and Megathe- 

 rium. The little armour-plated Armadillos are represented by 

 the equally colossal Glyptodon. The Llamas representing in 

 South America the Camels of the Old World are represented 

 by the curious extinct genus Macrauchenia. The Platyrhine 

 Monkeys have their extinct representatives. Fossil Tapirs 

 take the place of the two existing species ; and the Peccaries 

 are represented by at least five extinct species of Dicotyles. 



Similarly, India is at present the only country in which four- 

 horned Antelopes occur ; and it is in the Siwalik Hills that 

 there have been found the two gigantic four-horned Antelopes 

 which constitute the genera Sivatherium and Bramatherium. 



In Europe, again, the Mammalian fauna of the later Tertiary 

 periods is much more closely allied to that now characterising 

 the Old World, than to that of the New. We have the Lion, 

 Bear, Wolf, Fox, and other well-known Carnivora. Elephants, 



