CHAPTER III. 
DISTRIBUTION AS AFFECTED BY THE CONDITIONS AND CHANGES OF 
THE EARTH’S SURFACE. 
THE distribution of animals over the earth’s surface, 1s 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 
