DISTRIBUTION OF MAMMALS 157 



neously brought about slowly changing climates. The draining 

 of shallow seas had modifying effects on the temperature and 

 moisture of the air above and around them. The elevation of 

 large mountain ranges, particularly if directed north and south, 

 not only produced colder climates on their own slopes and 

 crests but by altering prevailing winds modified profoundly 

 the rainfall and temperature of the lowlands on either side. 

 The reverse of these processes also took place: The heights of 

 other mountain ranges already elevated were gradually reduced 

 by weathering and erosion, again with the effect of modifying 

 local climates. Such changes take place on islands as well as on 

 continents. 



From time to time during the earth's history periods of lower 

 temperatures have, by cumulative effect, resulted in "ice ages," 

 when vast amounts of water from the oceans slowly accumulated 

 as snow or ice at or near the earth's poles. Those huge polar 

 ice-caps — smaller ones exist today on Greenland and Antarctica 

 — ^had, at their greatest development, pronounced climatic ef- 

 fects on the regions near them. The grinding movement of the 

 ice also caused important local changes in the rocks and soils 

 which it overspread. 



The climate affects mammals largely indirectly, through its 

 effect on vegetation. Damp climates favor forests, relatively 

 dry climates lead to spread of grasslands, while desert climates 

 result in the development of leafless water-storing plants which 

 often produce great quantities of seeds after the occasional 

 rains. Few mammals live with equal advantage in these various 

 climates ; in all environments there are species suited to the 

 special conditions of food and shelter. The presence of these is 

 an important factor in keeping out the less well adapted 

 species. 



CONTINENTAL AND OCEANIC ISLANDS 



Two classes of islands are commonly recognized. Continental 

 islands are those which stand on the continental shelf, sepa- 



