TERRESTRIAL DISTRIBUTION 95 



can easily have been transported on floating timber or other similar 

 means, they are totally absent from what are known as oceanic 

 islands — that is islands arising from great depths in the ocean, 

 mainly composed of coral or volcanic rocks, and showing no signs 

 of having ever been connected with the existing continents, or the 

 larger and so-called continental islands. The obvious explanation 

 of this feature is, that from their total isolation these islands 

 have never been able to receive a mammalian fauna from the 

 great continental areas on which mammalian life was probably 

 first developed. 



As an intermediate step between these islands which are 

 practically void of mammalian life and the continents which teem 

 with such a variety of forms, are certain larger islands and portions 

 of continents containing a mammalian fauna more or less markedlj' 

 distinct from that of the whole of the other regions of the globe. 

 The best instance of this is Australia, which, with the exception of 

 one dog — the Dingo — and certain Muridce and Bats, has no mammals 

 except Monotremes and Marsupials. The latter are, moreover, per- 

 fectly distinct from those of America, which, if we exclude the islands 

 in the neighbourhood of Australia, is the only other region which now 

 possesses any Marsupials at all. Here also we have a ready and full 

 explanation which accords with all the facts ; since it is evident 

 that Australia has been isolated from the Asiatic continent from 

 some very remote geological epoch, at which period it is probable 

 that Monotremes and Marsupials were the dominant if not the sole 

 representatives of the Mammalia then existing. Consequently 

 Australia has never been able to receive an influx of the Eutherian 

 orders, which have probably swept away all the Marsupials except 

 the small American Opossums from the rest of the globe. Again, 

 the large island of Madagascar, which has a fauna of an African type, 

 but still very markedly different from that of the mainland, may 

 be considered to have been connected with the latter at a time 

 when the Eutheria had become the dominant forms, but has been 

 separated for a sufficiently long period to have enabled a large 

 number of its species and genera to have become distinct from those 

 of the adjacent continent. Similarly, there is evidence to show 

 that South America was probably cut off for a considerable period 

 from the northern half of the American continent, in consequence 

 of which its lowly organised fauna of Edentates were enabled to 

 attain such a remarkable development in the later geological 

 periods. 



In contrast to the mammalian fauna of islands of the preceding- 

 type is, or rather was, that of the British Islands, Avhich in the 

 early historic and prehistoric periods was identical with that of 

 the Continent. This leads to the inference that at a comparatively 

 late epoch there was a direct land communication between Britain 



