HISTORICAL ZOOGEOGRAPHY 99 



zonal climates, with accompanying uplift of the continents in adjust- 

 ment to the isostatic balance and with renewal of erosion. The uniform 

 base leveling corresponds to widespread deposits of limestones, ending 

 in coal formations. The periods of uplift are marked by barren forma- 

 tions, often red in color, indicating arid conditions, and culminate in 

 great extensions of glaciers from mountains and from the poles. 



Permanence of barriers. — Mountains have been no more perma- 

 nent than marine barriers. Some mountain ranges are younger than 

 many types, even than some species, of animals. The very ranges which 

 are now highest and most important as distributional barriers, such as 

 the American Cordillera and the Himalayas, and the whole series of 

 successive ranges from central Asia to the Alps and Pyrenees, are 

 demonstrably Tertiary. Low mountain regions, on the other hand, may 

 often be shown to be the remains of once higher ranges, whose impor- 

 tance as barriers has been reduced by long-continued erosion. 



Steppes and deserts also change; they are formed anew or are re- 

 captured for a renewed development of plant and animal life. There 

 are traces of a recent steppe period in central Europe ; 2 this was brought 

 to an end by conditions favoring extension of the forests. The ruined 

 cities of the deserts of central Asia indicate that this region was in- 

 habited at a relatively recent date. The deserts of the Great Basin in 

 North America are in part the bottoms of extinct lakes. The imperma- 

 nence of fresh-water basins and of rivers has already been discussed. 



Climatic changes have gone hand in hand with the changes in the 

 earth's surface. Should a submarine ridge arise between the British 

 Isles and the Faeroes, the Gulf Stream would be deflected from the 

 Scandinavian coast, and both the land and marine fauna of this area 

 would be powerfully affected. New mountain ranges influence the 

 amount of precipitation in adjacent areas, while their upper levels, 

 with lower temperatures, precipitous topography, less protection from 

 sunlight, and more violent winds, offer wholly new conditions. 



Changes in climate have also taken place independently of 

 topographic changes. 3 Fossil remains of dense forests, such as are now 

 to be found in the warmer parts of the temperate zone, are found in 

 the north polar regions, where the length of the winters and the dry- 

 ness of the air now make tree growth impossible. These remains indi- 

 cate a warmer and more humid polar climate in the early Tertiary; 4 

 and similar climatic change in the South Polar region is indicated by 

 the discovery of silicified tree trunks in Kerguelen Island. On the other 

 hand, there is abundant and conclusive evidence of the existence of a 

 glacial period at the close of the Tertiary, during which a large part 

 of the northern hemisphere was covered by glacial ice. In Europe the 



