ORGANIC AGENCIES. 129 



AlUghanian; 3. Canadian or boreal ; 3. Rocky Mountain ; 

 and, 4. Calif ornian. The limits of these are shown in 

 Fig. 09. 



Marine Faunas. 



Conditions are far less diverse in the sea than on land, 

 and the limitation of fauna is less marked, but the same 

 laws hold. 



Temperature Regions in Latitude. Fauna are here 

 also arranged in zones determined by temperature. In a 

 north and south coast-line, where the temperature changes 

 gradually, the fauna will also change gradually by the 

 substitution of one species for another; but if for any 

 cause there is a more sudden change of oceanic tempera- 

 ture, there will be a correspondingly rapid change of 

 fauna. For example, 011 our Atlantic coast, the Gulf 

 Stream hugs the southern coast as far as Cape Hatteras 

 (Fig. 69, a), and then turns away and runs at a greater 

 distance. This makes a great change of temperature at 

 this point. Again, the Arctic current, c, coming out of 

 Baffin's Bay, hugs the coast of New England as far as 

 Cape Cod, ~b, and then goes down. Thus Arctic condi- 

 tions prevail in coast waters to this point. Thus there 

 are three very different marine faunas along the coast of 

 the United States viz., a Southern, a Middle State, and 

 a Northern ; and these change somewhat suddenly at 

 Capes Hatteras and Cod. 



Distribution in Longitude. By far the larger num- 

 ber of marine species inhabit along shore. For these the 

 deep sea is a barrier no less impassable than the land. 

 Therefore, the species inhabiting the two shores of an 

 ocean like the Atlantic are as completely different as those 

 inhabiting along the two coasts of a continent, as America. 



Special Cases. There are many species which live 

 in the open sea and form a distinctive Pelagic fauna. 

 Again, there are others which are conditioned by 



LE CONTE, GEOL. 9 



