GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 17 
both coasts belonging to peculiar American types,—and yet 
the shores of America are connected by zones of equal tem- 
perature with the Central and Western Pacific, and with the 
eastern shores of the Atlantic. How is this peculiar Amer- 
ican character preserved? What prevents the interchange 
of species, if temperature is the great cause which limits their 
distribution? A glance at the ocean currents shows that 
none of them leave our shores without undergoing a marked 
change in temperature, and that none, from other shores, 
arrive upon them without undergoing a similar change. The 
Gulf Stream, after leaving the coast of the Southern States 
and the Bermudas, changes its temperature from 68° F. to 
60° before its southern outflow reaches the Azores, and to 
almost 50° before it arrives on the shores of Europe. The 
Atlantic equatorial current is formed off the coast of Africa 
by the union of the returning Gulf Stream, flowing from 
Southern Europe and the Azores, and the northern current 
flowing from Cape Good Hope. These currents flow di- 
rectly from temperate coasts into the torrid zone, which, by 
their influence, is narrowed down, on the western shores of 
Africa, to 20° of latitude, while on the American shores it 
extends through 60°. The antarctic current from Cape 
Horn flows northward into the warmer waters of the south- 
ern Atlantic. The antarctic polar current of the Pacific 
comes north from the frigid regions of the south into the 
temperate waters, is bent eastward against the shores of 
South America, and the principal branch flowing north along 
the coast is turned westward from Cape Blanco or Punta Pa- 
rina, and, under the equator, still retaining the low temper- 
ature of the southern waters, sweeps into the torrid regions 
beyond the Galapagos. The current, flowing from the north 
along the western shores of the United States, leaves the coast 
of California and flows southward into the tropics. The 
frigid regions of North America are, of course, excepted, and 
the arctic American partake strongly of the character of the 
arctic species of the old world. 
AMER. NATURALIST, VOL. II. 
