CURRENTS AND DISTRIBUTION 1049 



and variable air currents often greatly influence the migration of 

 aerial organisms, even the strongest flyers being carried away by 

 them. But the greatest influence exerted by air currents on organ- 

 isms is in the dispersal of seeds of plants, which thus may become 

 widely scattered. Air currents are the chief cause of the persistent 

 ocean currents, which are of such importance in the distribution of 

 life. In fresh water lakes temporary currents may be set up by the 

 winds, but these are of minor significance. The important currents 

 in fresh waters are those due to gravity, as exemplified in every 

 stream and river. That river currents are of great importance 

 in the distribution of fluvial life, as well as in the transportation of 

 terrestrial animal and plant life, is a matter of common knowledge. 



By far the most important currents affecting the life districts 

 of the earth are the ocean currents, for they not only aid in the dis- 

 tribution of organisms, but they are also instrumental in imparting 

 to sea and land characters which they would not otherwise possess. 



The characteristics of the ocean currents of the present geologic 

 period (Holocenic) may be taken as an illustration, especially since 

 it is highly probable that similar conditions prevailed throughout 

 most of Tertiary time. The depth of the moving bodies of water 

 constituting the oceanic currents is from 50 to 100 fathoms ; and 

 the direction of motion corresponds to that of the prevailing winds. 

 As already discussed at length in Chapter V, each of the great 

 oceans has its own eddy-like current, moving slowly around it and 

 leaving the central portion in a relative state of quiescence. In 

 the northern hemisphere the currents move in the direction of the 

 hands of a clock, i. e., clockwise ; in the southern hemisphere, coun- 

 ter-clockwise. As a result, the motion of the waters is westward 

 at the equator, both north and south of it, and eastward in the 

 polar ends of the north and south oceans, respectively. Hence 

 dispersals of organisms dependent on ocean currents would be 

 from east to west in the equatorial regions, as from the west coast 

 of Central America to the east coast of Asia, and from the west 

 coast of South America to Polynesia and Australia. It is true 

 that the equatorial counter-current sets eastward between the 

 north and south equatorial currents, but this is a relatively weak 

 current. The comparatively warm drift across the north Pacific 

 (Kuroshiwo drift) is an aid to eastward migration and dispersal 

 across the north Pacific, and this would probably be much more 

 effective if the cold water from the Arctic were shut off by the 

 closure of Behring strait. Such conditions existed during part of 

 Tertiary time, and thus may have greatly aided migration of ma- 

 rine organisms in this direction. The cold west-wind drift or 



