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 markedly 
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 Muride 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, which 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 
