266 Isthmus Barriers Separating Fish Faunas 



elements of the East Asiatic fauna in similar conditions of tem- 

 ]ierature, and no greater than is borne by either to the West 

 Intlies; that the faunas of the sides of tlie Isthmus of Suez 

 have relativelv little in common, while those of the tAVO sides 

 of the Isthmus of Panama show large identity of genera, al- 

 though few species are common to the two sides. Of the 255 

 genera recorded from the Panama region, 179, or over 70 per 

 cent., are also in the West Indies, while 68, or more than 30 

 per cent, of the number, are limited to the two regions in ques- 

 tion. 



The Isthmus of Suez as a Barrier to Distribution. — With the 

 aid of the above table we may examine further the rela- 

 tion of the fauna of Japan to that of the Mediterranean. If a 

 continuity of shore-line once existed, it would involve the ob- 

 literation of the Isthmus. With free connection across this 

 isthmus the fauna of the Red Sea must have been once 

 practically the same as that of the Alediterranean. The pres- 

 ent differences must be due to later immigrations to one or 

 the other region, or to the extinction of species in one locality 

 or the other, through some kind of unfitness. In neither 

 region is there evidence of extensive immigration from the out- 

 side. The present conditions of water and temperature differ 

 a little, but not enough to explain the difference in faunse. 

 The Red Sea is frankly tropical and its fauna is essentially 

 Indian, much the same, so far as genera are concerned, as that 

 of southern Japan. The Mediterranean is at most not more 

 than semi-tropical and its fishes are characteristically European. 

 Its tropical forms belong rather to Gumea than to the East 

 Indies. With the Red Sea the Mediterranean has very httle 

 in common, not so much, for example, as has Hawaii. Forty 

 genera of shore fishes (and only fifty of all fishes) are identical 

 in the two regions, the Mediterranean and the Red Sea. Of 

 those, every one is a genus of wide distribution, found in nearly 

 all warm seas. Of shore fishes, only one genus in seven is com- 

 n:on to the two regions. Apparently, therefore, we cannot 

 assume a passage across the Isthmus of Suez within the life- 

 time of the present genera. Not one of the types alleged to 

 be peculiar to Japan and the Mediterranean is thus far known 

 in the Red Sea. Not one of the characteristically abundant 



