THE DISTRIBUTION OF ANIMALS. 377 
sea. The land forms are restricted by climate and food, 
the marine by shallows or depths, by cold currents, by 
a sandy, gravelly, or mud bottom. The Bivalves are also 
found on every coast and in every climate, as well as in 
rivers and lakes, but do not flourish at the depth of much 
more than two hundred fathoms. The fresh-water Mus¬ 
sels are more numerous in the United States than in 
Europe, and west of the Alleghanies than east. The sea- 
sliells along the Pacific coast of America are unlike those 
of the Atlantic, and are arranged in five distinct groups: 
Aleutian, Californian, Panamic, Peruvian, and Magel¬ 
lanic. On the Atlantic coast, Cape Cod and Cape Hatte- 
ras separate distinct provinces. Of land Snails, Helix has 
an almost universal range, but is characteristic of North 
America, as Bulimus is of South America, and Achatina 
of Africa. The Old World and America have no species 
in common, except a few in the extreme north. 
The limits of Insects are determined by temperature 
and vegetation, by oceans and mountains. There is an 
insect-fauna for each continent, and zone, and altitude. 
The Insects near the snow-line on the sides of mountains 
in the temperate region are similar to those in polar lands. 
The Insects on our Pacific slope resemble those of Europe, 
while those near the Atlantic coast are more like those of 
Asia. Not half a dozen Insects live in the sea. 
The distribution of Fishes is bounded by narrower lim¬ 
its than that of other animals. A few tribes may be called 
cosmopolitan, as the Sharks and Herrings; but the species 
are local. Size does not appear to bear any relation to 
latitude. The marine forms are three times as numerous 
as the fresh-water. The migratory Fishes of the northern 
hemisphere pass to a more southern region in the spring, 
while Birds migrate in the autumn. 
Living Beptiles form but a fragment of the immense 
number which prevailed in the Middle Ages of Geology. 
