PROBLEMS OF DISTRIBUTION AND THEIR SOLUTION. 171 



quadrupeds and of their distribution is not now difficult of solution. 

 We pass backwards in imagination to the Triassic and Oolitic times 

 to behold, then, the dawn of mammalian life. We see the Marsupial 

 tribes representing, in the ancient Palsearctic region, the fulness of the 

 quadruped life that was afterwards to dawn. No higher form of 

 mammalian existence was then to be seen. The carnivora and 

 rodents, the bats and apes, the hoofed quadrupeds, and the variety 

 of mammalian life that marks our day, was as yet unknown. But 

 Australia at this period is in geographical connection with the 

 Asiatic continent. Over a continuous land surface, these earliest 

 quadrupeds pass to people the Australian territory. Next comes the 

 separation of Australia from Asia. The Malay Archipelago repre- 

 sents the broken and divided land-connection, first severed probably 

 at the Straits of Lombok. The higher tribes of quadrupeds are 

 evolved from the lower tribes in the ancient Europeo-Asiatic con- 

 tinent. The defenceless lower Marsupials are worsted in the 

 " struggle for existence " that ensues. The higher " tooth and claw " 

 exterminate the lower races in the Palaearctic region ; but in Australia 

 the isolated, these Marsupials, free from the irruption of later carni- 

 vores with tooth and claw, and protected by the intervening sea from 

 the inroad of the higher quadruped- races, flourish and grow. As 

 time passes, the original species of Marsupials that is, the first 

 emigrants to Australia vary, and, through variation, produce new 

 races and species of these quadrupeds. Australia in due time 

 develops a quadruped population of its own, which repeats the 

 varied features of mammalian existence elsewhere. Thus again there 

 is presented to our view an illustration of the double work of land 

 alteration and specific or biological change, in developing a strange 

 and wondrous population on the surface of the earth. 



Last of all, the history of the opossums and their distribution, 

 now limited to the New World, falls under the sway of the same 

 efficient explanation, supported by every fact of life and by all the 

 details of geological science. 



Commencing their existence in the Palsearctic region their fossil 

 remains occurring, for example, in the Eocene rocks of France the 

 opossums represent a race which never at any period of their exist- 

 ence have dwelt in Australian territory. Their occurrence in 

 America is explicable, not on any theory of possible connection 

 between America and Australia, but on the plain hypothesis of their 

 migration to the New World by a continuous land surface in the 

 middle or towards the end of the Tertiary period, from Europe or 

 from Northern Asia as a centre. Their earliest fossils, in the New 

 World, occur in the American Post-Pliocene that is, long after 

 their first appearance in European formations. Passing thus to the 

 New World, the opossums migrated southwards, where they flourished 



