DISTRIBUTION OF SPECIES 



"■fhtf "',.'>' 



RACCOONS 



Figure 111. Diagrammatic Representation of the Pleistocene Filter Bridge 

 Across the Bering Strait. ( From Simpson, /. Washington Acad. Sci., V. 30, 1940. ) 



Australia was probably colonized from Asia via a filter bridge. At pres- 

 ent, the Malay Archipelago extends toward Australia in a long arc from 

 the Malay Peninsula. But during the Mesozoic Era, when the primitive 

 mammals first arose, there was a continuous or near continuous land con- 

 nection, and via this the monotremes and marsupials reached Australia. 

 As this bridge was broken before the origin of more modern mammals 

 (placentals), these did not reach Australia, with the exception of bats 

 and small rodents which probably reached Australia by a sweepstakes 

 route. 



Another rather thoroughly known filter bridge is the Central American 

 bridge between North and South America, which has functioned at least 

 twice. In the Cretaceous and earliest Paleocene, the most ancient mam- 

 mals of the South American fossil beds reached that continent from North 

 America via this filter bridge. These included a wide variety of marsupials, 

 many of which evolved in South America to form the various carnivorous 

 types. The placental mammals which entered South America at this time 

 were very primitive, including ferungulates which had not yet become 

 clearly differentiated into herbivorous and carnivorous types, but were 

 omnivores, and edentates, such as the armadillos, anteaters, ground sloths, 

 and tree sloths. In South America, ferungulates evolved primarily along 

 herbivorous lines, perhaps because the carnivorous niches were already 

 filled by marsupials. Selection favors diversification. During most of the 

 Paleocene, however, this intercontinental connection was completely sub- 

 merged, and so evolution proceeded entirely independently on the two 

 Americas. During the late Eocene and Oligocene, the connection again 

 rose, but only enough to form a chain of more or less widely separated 



317 



