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 

 separated extremities of two continents, they bring with 

 them two entirely different fauna. Similarly, the Medi- 

 terranean 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 offered 

 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-geo- 

 graphical facts is explicable only on the hypothesis of the 

 former existence of a southern continent, of which the 

 Australian mainland is a remnant. The present Marsu- 

 pials are concentrated in Australia. Their occurrence 

 in the south-western portion of the Malay Archipelago, 

 including 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 opossum, 

 so widely extended in South America, could a question 

 arise, which is however solved by the examination of a 



