302 Comparative Evolution of Biomes 



groups. According to one interpretation of this continental drift 

 hypothesis, all the continents formed a single land mass as recently 

 as middle Cenozoic time, and since then this mass has fractured 

 and drifted apart to form the present continents (Wegener, 1924). 

 If this were true, dispersal in the past would have been essentially 

 inter-regional. More recent views concerning continental drift, how- 

 ever, express the opinion that little drifting has occurred since 

 Cretaceous time, that only a small amount of continental drifting 

 occurred in the northern hemisphere since Permian time, but that 

 considerable drifting occurred in the southern hemisphere during 

 the Mesozoic (Irving, 1959; Runcorn, 1959). 



It seems, therefore, that whatever the outcome of these contro- 

 versies, dispersal patterns for Cenozoic and late Mesozoic times 

 can be reckoned on the basis of the present positions of the con- 

 tinents. Prior to late Mesozoic time the picture of intercontinental 

 biotic interchange is clouded by uncertainties regarding the crustal 

 history of the earth. Because of these circumstances the discussion 

 in the remainder of this chapter concerns the later Mesozoic- 

 Cenozoic eras. The general ideas expressed would hold for any 

 period of changing land bridges, but the examples would be dif- 

 ferent if the juxtaposition of the land masses were different. 



With the exception of Antarctica the continents now form three 

 definitely north-to-south systems: North and South America, west- 

 ern Eurasia and Africa, and eastern Eurasia and Australia. The 

 equator being at right angles to these axes, the climates of the world 

 are stratified like rings, the warmest being at the equator, the cold- 

 est at the poles. If a member of the tropical Pacific littoral biome 

 dispersed at the present through natural waterways to the Atlantic 

 tropical littoral biome, it would have to go around either the 

 southern tip of South America or the northern part of North Amer- 

 rica, or island-hop in the Bering Sea or North Atlantic. If this 

 species could not exist outside tropical conditions, it would not be 

 able to disperse through the cold waters of either route. However, 

 the Central American land bridge which separates the parts of this 

 marine biome connects the tropical rain forest and other conti- 

 nental biomes of southern North America and northern South 

 America. This situation exemplifies the complementary action of 

 land bridges— they have an opposite result on the terrestrial and 

 marine biomes. If a land bridge rises, it connects land biomes and 

 divides marine biomes; if it sinks, it divides the land biomes and 

 connects marine ones. 



