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 isa 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 
