DISTRIBUTION OF SPECIES 



Central America, and also in the Andes of Peru. The beech tree, Fagus, 

 is widely distributed in the northern hemisphere, and the closely related 

 tree Nothofagiis is found in both South America and Australia. Many 

 mosses, grasses, and sedges show comparable distributions. This bipolar 

 mirrorism cannot be the result of a corridor, because the regions con- 

 cerned do not have enough in common, and those plants that they do have 

 in common are mostly small. Very few trees are common to the areas 

 under discussion. Yet a corridor should cause a very general exchange, 

 resulting in floral unification of the areas connected. Du Reitz suggested 

 a filter bridge via the Pacific Islands. But the chain of islands extending 

 from Lower California toward South America ends in the tropics, and 

 the mirrorism begins not in the tropics but in the temperate zones. Thus 

 the facts of bipolar mirrorism are opposed both to a corridor and to a 

 filter bridge, and so the sweepstakes route seems to be the most probable 

 explanation for this type of distribution. Transport of seeds by migrating 

 birds could be a factor. 



The Australia-South America Case. Yet another difiicult problem of 

 distribution is that of the similarities of the inhabitants of South America 

 and the Australian region, including New Zealand. The zoological evi- 

 dence centers around the marsupials. Perhaps the fact that the mam- 

 malian fauna of Australia is predominantly marsupial is better known 

 than any other fact of Australian natural history. In the Tertiary period. 

 South America had a large marsupial fauna, predominantly carnivorous, 

 and paralleling the Australian marsupial carnivores rather closely. Al- 

 though most of these became extinct when the placental mammals from 

 North America arrived in the Pleistocene, there are still more species of 

 marsupials in South America than on any other continent except Australia. 

 Many genera of plants are common to the two continents. Nothofagus 

 has already been mentioned. Other examples include the sedge Carex 

 and the moss Sphagnum. Because of these facts, some biogeographers 

 have postulated a land connection in earlier ages between South America 

 and Australia. But this is highly improbable, not only because no positive 

 geological evidence favors such a corridor, but also because the biological 

 evidence itself is not consistent with a corridor. There are common ele- 

 ments in the floras and faunas of these continents, but there is no sem- 

 blance of unity between them. Further, a corridor should be freely used 

 by mammals, yet the marsupial families of the two continents are all 

 different, and none of the South American placentals reached Australia. 

 Perhaps the most likely theory is that exchanges occurred by a sweep- 

 stakes route via Antarctica and intervening oceanic islands during warmer 

 ages. This is supported by the fact that the only extant vascular plant of 

 Antarctica, the grass DescJiampsia antarctica, is represented by other spe- 

 cies of the same genus in both South America and New Zealand. It should 

 be added, however, that there is every reason to believe that the mar- 

 supials reached the southern continents by invasion from the north. 



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