30 CHAPTERS ON EVOLUTION. 



lineal descendants of the first mammalian population which Australia 

 obtained from the Triassic period. Thus left unopposed, until the 

 advent of the colonists, the marsupials have lived and flourished 

 in Australia, which still retains the main features of its Triassic and 

 Oolitic life. For in its seas swims the Port Jackson shark, elsewhere 

 known only by fossil representatives from the Oolitic rocks. In its 

 rivers lives the curious fish Ceratodus, whose teeth occur fossil in 

 Triassic and Oolitic formations. The cycads and araucarias, repre- 

 senting a typical and universal plant vegetation of the Oolitic times, 

 still flourish in Australian soil, though elsewhere scanty or non- 

 existent j and even the shell-fish on the shores of Australia belong 

 to types which flourished in our own Oolitic seas, but which have 

 since practically died out over the world, save the Australian shores. 

 Thus, Australian life of to-day is merely the survival of the general 

 life which prevailed over the world in the Trias and Oolitic periods. 

 The history of the kangaroo points out clearly enough that only on 

 the theory of evolution having given rise to new species from the 

 ancient and original Triassic stock, can we account for the persistence 

 in a corner of our existing world, of the otherwise lost and extinct 

 population of the first quadrupeds. 



Lastly, the opossums which, as a family of marsupials, we should 

 have expected to find in Australia are discovered, as already 

 remarked, in America. " How came they, then, to inhabit the New 

 World?" is a question worth answering, along with that which 

 inquires into the distribution of the kangaroo. The opossums, 

 firstly, represent a family which never entered Australia. They 

 were plentifully existent in Europe and elsewhere in the Oolitic 

 period; and even nearer our own day namely, in the Eocene 

 and Miocene formations the opossums lived in the Old World. 

 These facts are accurately told us by the history of their fossil 

 remains. Thence their range extended to the New World; and, 

 when a subsequent irruption of higher quadrupeds killed off 

 the opossum-race elsewhere, these animals continued to flourish and 

 grow in the New World, presumably because the struggle for exist- 

 ence was and is less severe in the latter region. As the kangaroos 

 are survivals of a quadruped-life, world-wide in Triassic and 

 Oolitic times, so the opossums are survivals in their turn of later 

 marsupials than the Australian animals. Finding in the New World, 

 to which they migrated, a suitable home, the opossums, distanced in 

 the competition in the Old World and now extinct therein, have 

 flourished apace across the sea, and have extended their bounds even 

 into the northern part of the American continent 



The deep water of the narrow "Wallace's Line" between Bali 

 and Lombok, therefore, indicates a channel of great antiquity, which 

 severed Australia from the nearest land, and which, presenting an 



