CHAPTEE III. 



DISTRIBUTION AS AFFECTED BY THE CONDITIONS AND CHANGES OF 

 THE EARTH'S SURFACE. 



The distribution of animals over the earth's surface, is evidently 

 dependent in great measure upon those grand and important 

 characteristics of our globe, the study of which is termed physical 

 geography. The proportion of land and water ; the outlines and 

 distribution of continents; the depth of seas and oceans; the 

 position of. islands ; the height, direction, and continuity of moun- 

 tain chains ; the position and extent of deserts, lakes, and forests ; 

 the direction and velocity of ocean currents, as well as of prevalent 

 winds and hurricanes ; and lastly, the distribution of heat and 

 cold, of rain, snow, and ice, both in their means and in their 

 extremes, have all to be considered when we endeavour to 

 account for the often unequal and unsymmetrical manner in 

 which animals are dispersed over the globe. But even this 

 knowledge is insufficient unless we inquire further as to the 

 evidence of permanence possessed by each of these features, in 

 order that we may give due weight to the various causes that 

 have led to the existing facts of animal distribution. 



Land and Water. — The well-known fact that nearly three - 

 fourths of the surface of the earth is occupied by water, and but 

 a little more than one-fourth by land, is important as indicating 

 the vast extent of ocean by which many of the continents and 

 islands are separated from each other. But there is another fact 



