The Geographical Distribution of Fishes 95 
or from rock to rock, till, in many cases, the same species is 
found in the Red Sea and in the tide-pools or sand-reaches of 
Japan. In the North Pacific, the presence of a range of half- 
submerged volcanoes, known as the Aleutian and the Kurile 
Islands, has greatly aided the slow movement of the fishes of 
the tide-pools and the kelp. To a school of mackerel or of 
flying-fishes these rough islands with their narrow channels 
might form an insuperable barrier. 
Temperature the Central Fact in Distribution.—It has long 
been recognized that the matter of temperature is the central 
fact in all problems of geographical distribution. Few species 
in any group freely cross the frost-line, and except as borne by 
Fie. 70.—Japanese file-fish, Rudarius ercodes Jordan and Snyder. Wakanoura, 
Japan, Family Monacanthide. 
oceanic currents, not many extend their range far into waters 
colder than those in which the species is distinctively at home. 
Knowing the average temperature of the water in a given region 
we know in general the types of fishes which must inhabit it. 
It is the similarity in temperature and physical conditions 
which chiefly explains the resemblance of the Japanese fauna 
to that of the Mediterranean or the Antilles. This fact alone 
