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 faune 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 
