660 ZOOLOGY. 



insects, mollusks, and crustaceans. Different systems of riv- 

 ers have distinct sets of fluviatile animals ; for example, the 

 fishes of the Ohio and Upper Mississippi and its tributaries 

 differ from those of the Hudson River and the New England 

 rivers, and the latter from those draining the Southern At- 

 lantic States. The fresh-water mussels, so abundant and 

 characteristic of the waters of the Mississippi and its tribu- 

 taries are confined to the region lying west of the Allegha- 

 nies and east of the Great Plains. The fishes and mollusks 

 of the rivers of the Pacific slope differ from those of the 

 scanty waters of the Great Basin. 



Means of Dispersal. The most general are the alterna- 

 tions of winter and summer, leading birds and mammals to 

 migrate great distances to and from their breeding-places. 

 Ocean-currents are most important factors in the dispersal 

 of many marine and some land animals. By means of such 

 great currents as the Gulf Stream, tropical animals are borne 

 to temperate and even subarctic regions ; and, on the other 

 hand, arctic and temperate animals are borne southward, 

 and thus marine fauna? interdigitate and merge insensibly 

 into one another. By this agency also new coral islands 

 are peopled from the mainland, and peninsulas are colo- 

 nized from adjoining continents or islands ; for example, 

 the southern extremity of Florida has been visited by trop- 

 ical plants and animals borne by currents and winds from 

 the West Indies, thus lending a purely tropical aspect to 

 the southern part, a semi-tropical fauna occupying the mid- 

 dle and northern part of the State. 



Trade winds play an important part in' scattering insects, 

 and especially the minute forms of life ; whirlwinds and 

 tornadoes catch up larger forms and transport them from 

 stream to stream, pond to pond, and from lowlands to 

 highlands, and even to Alpine summits, where may some- 

 times be found, under loose stones, multitudes of insects 

 which have been borne up from below by strong gales or 

 ascending currents of air. 



The direction of the migrations of the Rocky Mountain 

 locust seems to be mainly dependent on the direction of pre- 

 vailing winds. Insects as well as birds are blown off-shore 



