236 THE DOCTRINE OF DESCENT. 



zoologically wide as the poles asunder; while Australia, 

 with its dry winds, its open plains, its stony deserts, and 

 its temperate climate, yet produces birds and quadru- 

 peds which are closely related to those inhabiting the 

 hot, damp, luxuriant forests which everywhere clothe the 

 plains and mountains of New Guinea." 



Wallace gives the most specific proofs that, as the 

 parts of this Archipelago approach one another like sepa- 

 rated extremities of two continents, they bring with them 

 two entirely different fauna. Similarly, the Mediter- 

 ranean and West Indian Archipelagos are devoid of any 

 peculiar character, and are completely dependent on the 

 adjacent continents for their animal life and vegetation. 

 We have already discussed Madeira and its land snails. 

 Insular faunas therefore do not require the hypothesis 

 of more centres of creation than are oflfered , by the 

 continents; and Riitimeyer has endeavoured to trace 

 the extraction of birds and mammals to two centres of 

 derivation. A great series of animal-geographical facts 

 is explicable only on the hypothesis of the former ex- 

 istence of a southern continent, of which the Australian 

 mainland is a remnant. The present Marsupials are 

 concentrated in Australia. Their occurrence in the 

 south-western portion of the Malay Archipelago, includ- 

 ing New Guinea, seems like a radiation from that 

 centre. No single token makes it appear that the 

 Marsupials existing in former periods in the northern 

 hemisphere, from the Jura forwards, had migrated to 

 meet those which were pressing on from the southern 

 continent towards the equator. Only as to the oppossum, 

 so widely extended in South America, could a question 

 arise, which is however solved by the examination of a 



