The Geographical Distribution of Fishes 24 i 



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 



Fig. 173. — Japanese file-fi.sh, liudarius ercodes Jordan and Snyder, ^\'akanoura, 

 Japan. Family M onacanlhida: . 



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 iVntilles. This fact alone 



