78 POPULAR SCIENCE MONTHLY. 



Zealand, 108; Hawaii, 144; West Indies, 299, and the Panama region, 

 256. 



Common to Japan and the Mediterranean are 79, all but two being 

 of wide distribution; to Japan and the Eed Sea, 111; to Japan and 

 Hawaii, 82; to Japan and Australia, 135; to Japan and the West 

 Indies, 113; to Japan and Panama, 91. To the Mediterranean and 

 the Eed Sea, 40 genera are common, all of wide distribution; to the 

 West Indies and the Mediterranean, 70, 59 being of wide distribution; 

 to the West Indies and Panama, 179, only 101 being of wide dis- 

 tribution. 



It is evident from an analytical table that the warm-water fauna 

 of Japan, like that of Hawaii, is derived from that of the East Indies 

 and Hindostan ; that the fauna of the Eed Sea is derived from the same 

 source; that the Mediterranean fauna bears no special resemblance to 

 that of Japan rather than to that of the other parts of Eastern Asia 

 with like conditions of temperature and no greater than is borne by the 

 West Indies; that the fauna of the two sides of the Isthmus of Suez 

 J^ave relatively little in common, while those of the two sides of the 

 Isthmus of Panama show a remarkable degree of identity. 



When the fishes of Panama were first described, it was claimed 

 that their species were almost entirely identical with those of the West 

 Indies; this statement was followed by speculations on the relation of 

 the depression of this Isthmus to the Gulf Stream, and to the glacial 

 epoch. Further investigations by Jordan, and by Evermann and Jen- 

 kins showed the fallacy of this claim of identity. Of about 1,400 

 species now known from the two sides of the Isthmus, only 70 are 

 identical, or five per cent, of the whole, and about 10 of these are 

 almost cosmopolitan in the tropics. Dr. Paul Fischer finds about 

 three per cent, of the mollusks identical on the two coasts. 



Dr. E. T. Hill goes on to show that there is neither geological nor 

 biological evidence of the submergence of the Isthmus of Panama since 

 Tertiary times, and that such a barrier existed as far back as Jurassic 

 times. There is, however, evidence of a brief connection in Tertiary 

 time at the end of the Eocene period. 



Assuming this to be true, the actual facts of distribution seem to 

 be in accord with it. The period of depression was before the life-time 

 of most of the present species. It was, however, not earlier than the 

 period of most of the present genera. It was relatively shallow, but 

 wide enough to permit the infiltration from the Caribbean Sea to the 

 Pacific of species representing most of the genera of sandy bays, rocky 

 tide pools and brackish estuaries. Since the channel was closed, the 

 species left on either side have undergone modification in varying 

 degrees, mostly retaining generic identity, while losing some of their 

 specific characters. 



