26 MAMMALIAN DENTITION 



changes of alternate lowering and raising of the earth's sur- 

 face the climate changes. In general a low marshy ground 

 possesses a warm equable humid climate ; a high plateau induces 

 a cold variable and arid climate ultimately ending in glacier 

 formation. The character of the land and the nature of the 

 climate are intimately associated with each other. 



The continents have joined and separated again in the regions 

 already indicated not once but several times ; most islands also 

 have varied in their relationship to the adjacent greater land 

 masses. Britain and Japan are comparatively young islands; 

 they have not been separated from the adjacent Eurasiatic con- 

 tinent long enough for the production of marked insular 

 changes in the types of animal life occurring upon them. Mada- 

 gascar on the other hand has not been connected with Africa 

 since the life time of the early Mammals and we find in conse- 

 quence that the modern fauna of this island differs very con- 

 siderably from that of the neighboring continent. New Guinea 

 and Australia form portions of a still older island: between the 

 Australasian archipelago and the furthest extension of the 

 Oriental Region represented by the islands of Java and Borneo, 

 there lies a channel of deep water formed by the Strait between 

 the islands of Bali and Lombok and by the Macassar Strait 

 separating Borneo from Celebes. This barrier of water is 

 known as Wallace 's Line and it has been said that no land has 

 ever bridged across this channel since before the time when 

 Mammals of modern type came to be. It is doubtful how- 

 ever if this statement should be accepted in the light of modern 

 investigation. The fauna of Celebes is difficult to harmonize 

 with the view that the island has been entirely separated from 

 the Oriental Region for so long a period. Many authorities 

 now believe that Wallace's Line has never existed except in 

 theory. It is certain however that the modern type of pla- 

 cental Mammals with but feAv easily explained exceptions, 

 never penetrated Australia and the neighboring islands until 

 taken there by Man. With the foregoing exceptions the only 

 Mammals found in the great Australasian archipelago are the 



