MAMMALIA. 



generally numerically the same as the upper incisors ; but in the Marsupials, 

 where there may be four pairs, they are generally less numerous than the 

 upper. Behind the lower canine come the premolars, which are followed by 

 the molars, the latter being distinguished from the last premolar by having 

 no deciduous predecessors. 



A very important subject connected with the study of Mammals is their 

 geographical distribution on the surface of the globe, but to understand this 

 thoroughly, it is essential to have a knowledge of the extinct 

 forms, and to be acquainted with the changes in the form of •f^os'^aphical 

 the continents and islands which have taken place during ^Jistribution. 

 earlier epochs of the earth's history. Every traveller is aware that the 

 Mammals of different regions of the globe diflfer more or less markedly, but 

 this difference is by no means co-extensive with the distance of the various 

 regions from one another. Thus, whereas the Mammals of Japan are very 

 similar to those of Europe, while there is a marked resemblance between the 

 former and those of North America, when we pass from the Malayan Islands 

 to Australia, there is a very sudden and remarkable change. 



As a whole. Mammals are a comparatively modern group, which have only 

 attained their present great development during the Tertiary, or latest epoch, 

 of the earth's history. It is true that they existed during the preceding 

 Secondary Epoch, or the one in which the chalk, oolites, lias, etc., were de- 

 posited ; but all tlie forms were then small, and occupied a subordinate posi- 

 tion in the fauna of the world, the continents being then peopled by various 

 strange, and frequently gigantic, kinds of Reptiles, while, in the oceans, the 

 place of the modern whales, porpoises, and Sirenians was taken by the 

 Reptilian Ichthyosaurs and Plesiosaurs, It is in consequence of this com- 

 paratively late development that the geographical distribution of Mammals 

 differs widely from that of Reptiles and Amphibians, although it coincides to 

 a certain degree with that of Birds, which are likewise a relatively modern 

 group. 



From the evidence of Mammals alone, the globe may be divided into three 

 main zoo-geographical realms, two of which may be further sub-divided into 

 regions. These three primary divisions are respectively named the Notogseic, 

 Neogseic, and Arctogaeic realms, or Notogeea, Neogsea, and Arctogsea. The 

 first of these includes Australia, New Guinea, New Zealand, and the islands 

 as far east as New Ireland and New Caledonia, together with Celebes, 

 Lombok, and the islands between these and Australia. From Borneo and 

 Java, Celebes and Lombok are separated by a deep channel, and whereas all 

 the islands to the west of this channel, which is known as Wallace's 

 line, have their Mammalian fauna of an Oriental or Indian type, and 

 are without any Marsupials, those on the west of the same show a 

 more or less marked Australian type. Marsupials making their first 

 appearance in Celebes, and becoming more numerous as we approach 

 Australia and Papua, where that group is the dominant one. 



The Austro-Malayan islands, as Celebes and the adjacent islands may be 

 called, form, indeed, a transition, so far as Mammals are concerned, between 

 the regions to the west and Australia and New Guinea, although it is on the 

 whole most convenient to include them in the Notogfeic realm. The typical 

 part of that realm, as represented by Australia, New Guinea, and the 

 adjacent islands, is characterised by the great preponderance of Pouched 

 Mammals, or Marsupials, while it is here only that the Egg-Laying Mammals, 

 or Monotremes, are met with. In Australia itself, in addition to Bats, which 



