Isthmus Barriers Separating Fish Faunas 261 



the final lodgement of like types, but the resemblance would 

 be general, the genera and species being unlike. Without doubt 

 part of the resemblance between Japan and the Mediterranean 

 is due to similarity of temperature and shores. Is that which 

 remains sufficient to demand the hypothesis of a former shore- 

 line connection? 



Effects of Direction of Shore-line. — We may first note that a 

 continuous shore-line produces a mingling of fish faunas only 

 when not interrupted by barriers due to climate. A north and 

 south coast-line, like that of the East Pacific, however unbroken, 

 permits great faunal differences. It is crossed by the different 

 zones of temperature. An east and west shore-line lies in the 

 same temperature. In all cases of the kind which now exist 

 on the earth (the Mediterranean, the Gulf of Mexico, the Car- 

 ibbean Sea, the shores of India), even species will extend their 

 range as far as the shore-line goes. The obvious reason is 

 because such a shore-line rarely offers any important barrier to 

 distribution, checking dispersion of species. We may, there- 

 fore, consider the age and nature of the Isthmus of Suez and 

 the character of the faunas it separates. 



Numbers of Genera in Different Faunas. — For our purposes 

 the genera must be rigidly defined, a separate name being used 

 in case of each definable difference in structure. The wide- 

 ranging genera of the earlier systematists were practically cos- 

 mopolitan, and their geographical distribution teaches us little. 

 On the other hand, when we come to the study of geological 

 distribution, the broad definition of the genus is the only one 

 usually available. The fossil specimens are always defective. 

 Minor characters may be lost past even the possibility of a 

 guess, and only along broad lines can we achieve the classifica- 

 tion of the individual fossil. 



Using the modem definition of genus, we find in Japan 483 

 genera of marine fishes; in the Red Sea, 225; in the Mediter- 

 ranean, 231. In New Zealand 150 are recorded; in Hawaii, 

 171; 357 from the West Indies, 187 from the Pacific coast of 

 tropical America, 300 from India, 450 from the East-Indian 

 islands, and 227 from Australia. 



Of the 483 genera ascribed to Japan, 156 are common to the 

 Mediterranean also, 188 to the West Indies and Japan, 169 to 



