382 OUTLINES OF EVOLUTIONARY BIOLOGY 



when transported there by human agency, and it is equally 

 impossible to believe that such animals as sheep and rabbits, to 

 which the Australian climate appears to be pre-eminently suited, 

 were specially created in Europe and Asia but never in Australia. 

 The existing condition of the Australian fauna is, however, easily 

 explained on the supposition that it was originally derived from 

 Asia at the time when marsupials and monotremes flourished in 

 the north, and that the island continent became separated from 

 the mainland before the more recent mammalian types, such as 

 sheep and rabbits, had arisen on the latter. Divergent evolution 

 within the limits of this isolated area is then quite sufficient to 

 account for the immense variety of marsupials occurring there at 

 the present day. 



It is very instructive in this connection to contrast the con- 

 dition of the fauna of a comparatively recently separated 

 continental island, such as Great Britain, which is not far 

 removed from its parent continent, with that of the fauna of a 

 typical oceanic island which has never formed part of a continent 

 at all and is very widely separated from any other land. The 

 native or indigenous population of continental islands always 

 exhibits a close relationship with that of the adjacent mainland, 

 from which it was originally derived and with which it is still 

 able to keep up a certain amount of intercourse. Such an island 

 will contain indigenous quadrupeds, and the great majority of the 

 species of plants and animals found in it will be identical with 

 those of the mainland. True oceanic islands, on the other hand, 

 such as St. Helena and the Sandwich Islands, are peopled 

 entirely by waifs and strays which have gained access to them at 

 rare intervals in one or other of the ways discussed in the earlier 

 part of this chapter. They never contain large quadrupeds and, 

 owing to their more or less complete isolation, the animals which 

 do occur almost always belong to peculiar species found nowhere 

 else in the world. 



|^ (3) Palaeontological investigations have demonstrated that the 

 present animal population of any tolerably isolated area is closely 

 related to the population of the same area in comparatively 

 recent geological periods. Thus in Australia we not only find 

 that at the present day marsupials are by far the most charac- 

 teristic features of the fauna, but also that the remains of extinct 

 marsupials, many of which .belong to genera and species different 

 from any now living, are very abundant in the tertiary deposits 



