Abstract
The tropical Indo-Western Pacific region is believed to be the center of dispersal of the Japanese soles, based upon 3 facts: 1) the center of dispersal is nothing but the center of specific differentiation in this group, 2) the 10 tropical soles, which are almost half the known members of the entire species inhabiting the waters of Japan, are found both in the Japanese waters and the tropical Indo-Western Pacific, and 3) the larger number of species and genera of this group are concentrated in the latter region.
The phylogenetic migration of the Japanese soles from the center of dispersal to our area is presumed to have taken Place through 2 main lines. One is the Kuroshio line, which has brought the tropical Western Pacific fauna with it to the southern part of Japan by the Kuroshio. Through the other line, which is herein called “Continental line”, the soles have extended their habitats northwards along the continental coast of Asia from the East Indies to Japan, passing the China Sea.
It is a srtiking evident that in the northern hemisphere, the distribution of this group coincides with a fair degree of accuracy with the surface isothermals in February. The temperature in this month is 25°C or a little higher in the center of dispersal, and it is 0°C in the northern limit of the dispersal in Japan. The isothermals of 10°, 15° and 20°C form respectively the boundaries of zoogeographic division of this group (Fig. 1).
