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- 

 shells 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 Reptiles form but a fragment of the immense 

 number which prevailed in the Middle Ages of Geology. 



