12 Mr Alfred R. Wallace on some Anomalies 



The marsupial Opossums are the most remarkable case of 

 vertebrata in America having Australian affinities. It is 

 very doubtful whether these could have been introduced in 

 the same manner as the plants and insects already alluded 

 to, because the latter have to a considerable extent an ant- 

 arctic character, and do not appear in such numbers as to 

 indicate an actual continuity of land, which would have 

 been almost indispensable for the passage of mammalia, 

 and would at the same time have undoubtedly admitted 

 Australian forms of land birds, which do not exist in South 

 America. It seems more reasonable, therefore, to suppose 

 that these marsupials have inhabited America since the 

 Eocene period, when the same genus existed in Europe, and 

 the marsupial order had probably a universal distribution. 



With this one exception, the birds, the mammalia, and 

 the reptiles* of south temperate America have little or no 

 affinity either with north temperate or Australian forms, 

 but are modifications of the true denizens of the Neotropical 

 regions. They appear to have been enabled rapidly to seize 

 liold of the country, and to adapt themselves to its modified 

 climate and physical features — a remarkable instance, of 

 which is mentioned by Mr Darwin in the woodpecker of 

 the Pampas, which never climbs a tree. The tropical 

 insects, on the other hand, having become gradually spe- 

 cialized during long periods for a life amid continual verdure 

 and unvarying summer, were totally unfitted for the new 

 conditions presented to them, and only in a very few cases 

 were able to struggle against forms already adapted to a 

 more barren country and a more rigorous climate. 



This difference in the adaptive capacity of groups, com- 

 bined with an unequal power of diffusion, will cause the 

 various kinds of barriers to be sometimes more and some- 

 times less effective. For example, when a mountain range 

 has attained only a moderate elevation, it will already com- 

 pletely bar the passage of many insects, while mammalia, 

 birds, and reptiles, more capable of sustaining different 

 conditions, will readily pass over it. On the otherhand, 

 an arm of the sea, or even a wide river, will completely 



* Except the batrachians, which, show some affinities between Australia and 

 South America, a case analogous to that of Japan. 



