THE ORIGIN OF SPECIES 



mucli of this connection was submerged, leaving only the islands of the 

 Bering Strait. In warmer times, during the Mesozoic Era and until some- 

 time in the Eocene, a very extensive exchange of floras and faunas oc- 

 curred across this north Pacific bridge. How complete tliis exchange was 

 is indicated by the fact that no less than 156 living genera of plants are 

 known to be common to the two regions at present. In some cases, such 

 as Symplocarpus foetidus (skunk cabbage), the species appear to be iden- 

 tical and even the races are closely similar. The exchanged genera are by 

 no means confined to such rapid migrants as herbs, for many genera of 

 trees are common to the two areas, as Acer (maple), Catalpa, and Mag- 

 nolia. Many known as fossils in both areas are now living in only one. 

 Thus Castanea, the chestnut tree, still survives in eastern Northern Amer- 

 ica, while Ginkgo, the very primitive maiden hair tree, still survives in 

 Asia. This problem has been studied less in animals, but parallel examples 

 are known. Thus, the alligator is known only from the United States and 

 China, and the salamanders Triturus and Cnjptobranchus are also found 

 in these remote places. The present isolation of these forms was accom- 

 plished by geologic and climatic changes. Geologically, the north Pacific 

 corridor was largely submerged, and the mountains of western North 

 America were elevated, thus making climate and topography unfavorable 

 for the former inhabitants of this area. Further, the climate of the entire 

 northern part of the world became colder, with the result that these tem- 

 perate and subtropical organisms became extinct over much of their 

 former range, leaving the distributions as they are found today. 



Filter Bridges. The main characteristic of a filter bridge is that it does 

 filter out many of the organisms of the connected regions, while permit- 

 ting the passage of others. Also, a filter bridge is typically of brief duration, 

 while a corridor lasts for periods which are long even on the geological 

 time scale. But, while genuine corridors between continents appear to 

 have been rare, perhaps only those described above having existed, filter 

 bridges have been fairly common. The Bering Strait had this character 

 repeatedly in the Pleistocene (Figure 111). Its filtering action has prob- 

 ably been due in part to the fact that the land connection may not have 

 been completely continuous. Any organisms for which small expanses of 

 salt water form an impassable barrier would have been unable to cross. 

 Also, because of its short duration, it woidd be crossable only by plants 

 and animals capable of migrating fairly rapidly. But a more important 

 factor concerns the location of the bridge and the prevailing climatic con- 

 ditions. In this example, the location was just below the Arctic Circle, and 

 the climate was cold, for it was during the Pleistocene that the great glaci- 

 ation occurred. Thus, only those Palearctic and Nearctic animals which 

 were adapted to cold could make the crossing. Temperate and tropical 

 animals were excluded simply because they were unable to approach the 

 bridge. Mainly mammals seem to have made the crossing, perhaps be- 

 cause of their superior powers of locomotion. Animals such as bears, cats, 

 bison, deer, and mammoths seem to have crossed from Asia to North 

 America, while dogs, horses, and camels crossed in the other direction 

 (Figure 111). 



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