Comparative Evolution of Biomes 303 



Biotic traffic between the Old and New Worlds apparently oc- 

 curred chiefly near the poles because elsewhere the continents are 

 far apart. The proximity of the northern portions of the northern 

 continents favors freer biotic interchange at higher latitudes by 

 various over-water dispersal methods in addition to the probability 

 that land connections existed. In the southern hemisphere some- 

 what the same relationship exists between Australasia, Antarctica, 

 and the tip of South America. 



Traffic north and south has depended greatly on the existence of 

 land connections between North and South America, between 

 southeastern Asia and Australia, and between Africa and areas 

 north or northeast of it. Biotic exchange in the regions of these 

 connections has been augmented by over-water dispersals. 



Enough geologic and biogeographic evidence has accumulated 

 to indicate that certain of these land connections have occurred 

 several times (Simpson, 1947, 1950), but when and for how long 

 these connections remained on each occasion is open to specula- 

 tion and greatly in need of critical investigation. The number of 

 times different connections occurred simultaneously is not known 

 either, a subject on which available evidence is not conclusive. The 

 only safe assumption at present is that overland dispersals between 

 the continents followed an irregular and opportunistic pattern 

 depending on the land connections in existence at any one time. 



Climatic Zonation 



At the present time the climate of the earth forms zones roughly 

 paralleling the equator, constantly warm and frost-free through the 

 tropics, becoming less warm away from the tropics, and culminat- 

 ing in frigid conditions at the polar areas. Over geologic time, 

 however, the earth's climate in the higher latitudes has fluctuated 

 over a fairly wide range. We live at the moment in a cool but not 

 the coldest part of that fluctuation. Through extrapolation from 

 the ecological tolerances of their living relatives, fossils can be 

 used to provide a thermometer for these oscillations over the last 

 500 million years (Schuchert, 1924; Durham, 1950). More re- 

 cently, measurements of certain radioactive isotopes of oxygen 

 and other elements have contributed information on these tempera- 

 ture changes (Lowenstam and Epstein, 1954; Emiliani, 1955). 

 Combined information from these sources suggests ocean tempera- 



