The Geographical Distribution of Fishes 243 



current, bearing Ochotsk-Alaskan types southward as far as 

 the Santa Barbara Islands, to which region it is accompanied 

 by species of Aleutian origin. A cold return current seems to 

 extend southward in Japan, along the east shore perhaps as 

 far as Matsushima. A similar current in the sea to the west of 

 Japan extends still further to the southward, to Noto, or beyond. 



It is, of course, not necessary that the movements of a 

 species in an oceanic current should coincide with the direction 

 of the current. Young fishes, or fresh-water fishes, would be 

 borne along with the water. Those that dwell within floating 

 bodies of seaweed would go whither the waters carry the drift- 

 ing mass. But free-swimming fishes, as the mackerel or flying- 

 fishes, might as readily choose the reverse direction. To a free- 

 swimming fish the temperature of the water would be the only 

 consideration. It is thus evident that a current which to certain 

 forms would prove a barrier to distribution, to others would be 

 a mere convenience in movement. 



In comparing the Japanese fauna with that of Australia, we 

 find some trace of both these conditions. Certain forms are 

 perhaps excluded by cross-currents, while certain others seem 

 to have been influenced only by the warmth of the water. A 

 few Australian types on the coast of Chile seem to have been 

 carried over by the cross-currents of the South i\tlantic. 



It is fair to say that the part taken by oceanic currents in 

 the distribution of shore fishes is far from completely demon- 

 strated. The evidence that they assist in such distribution 

 is, in brief, as follows: 



1. The young of shore fishes often swim at the surface. 



2. The young of very many tropical fishes drift northward 

 in the Gulf Stream and the Japanese Kuro Shiwo. 



3. The fatmal isolation of Hawaii may be correlated with 

 the direction of the oceanic currents. 



Centers of Distribution. — We may assume, in regard to any 

 species, that it has had its origin in or near that region m which 

 it is most abundant and characteristic. Such an assumption 

 must involve a very large percentage of error or of doubt, but 

 in considering the mass of species, it may represent essential 

 truth. In the same fashion we may regard a genus as being 

 autochthonous or first developed in the region where it shows 



