Isthmus Barriers Separating F'ish Faunas 269 



around the southern point of Africa. To pelagic and deep-sea 

 fishes the Cape of Good Hope has offered no barrier whatever. 

 To ordinary fishes it is an obstacle, but not an impassable one. 

 This the fauna itself shows. It has, however, not been passed 

 by many tropical species, and by these only as the result of 

 thousands of years of struggle and point-to-point migration. 



Relations of Japan to Mediterranean Explainable by Present 

 Conditions. — We may conclude that the resemblance of the 

 Mediterranean fish fauna to that of Japan or India is no more 

 than might be expected, even had the present contour of the 

 continents been permanent for the period of duration of the 

 present genera and species. An open channel in recent times 

 would have produced much greater resemblances than actually 

 exist. 



The Isthmus of Panama as a Barrier to Distribution. — Con- 

 ditions in some regards parallel with those of the Isthmus of 

 Suez exist in but one other region — the Isthmus of Panama. 

 Here the first observers were very strongly impressed by 

 the resemblance of forms. Nearly half the genera found on 

 the two sides of this isthmus are common to both sides. Taking 

 those of the Pacific shore for first consideration, we find that 

 three-fourths of the genera of the Panama fauna occur in the 

 West Indies as well. 



This identity is many times greater than that existing at 

 the Isthmus of Suez. Moreover, while the Cape of Good Hope 

 offers no impassable barrier to distribution, the same is not 

 true of the southern part of South America. The subarctic 

 climate of Cape Horn has doubtless formed a complete check 

 to the movements of tropical fishes for a vast period of geologic 

 time. 



Unlikeness of Species on the Shores of the Isthmus of Panama. 

 — But, curiously enough, this marked resemblance is confined 

 chiefly to the genera and does not extend to the species on the 

 two shores. 



Of 1400 species of fishes recorded from tropical America 

 north of the Equator, only about 70 are common to the two 

 coasts. The number of shore fishes common is still less. In 

 this 70 are included a certain number of cosmopolitan types 

 which might have reached either shore from the Old World. 



