280 FISHES. 



neck of land uniting the two American Continents, it would 

 most assuredly be regarded as a portion of the Fauna of the 

 tropical Atlantic. With scarcely any exceptions the genera 

 are identical, and of the species found on the Pacific side 

 nearly one-half have proved to be the same as those of the 

 Atlantic. The explanation of this fact has been found in 

 the existence of communications between the two oceans by 

 channels and straits which must have been open till within 

 a recent period. The isthmus of Central America was then 

 partially submerged, and appeared as a chain of islands simi- 

 lar to that of the Antilles ; but as the reef-building corals 

 flourished chiefly north and east of those islands, and were 

 absent south and west of them, reef-fishes were excluded from 

 the Pacific shores when the communications were destroyed 

 by the upheaval of the laud. 



h. The Galapagocs district received its coast fauna princi- 

 pally from the Central American district, a part of the species 

 being absolutely the same as on the coast of the Isthmus of 

 Panama, or as in the West Indies. Yet the isolation of this 

 group has continued a sufficiently long period to allow of the 

 development of a number of distinct species of either pecu- 

 liarly Atlantic genera (such as Centropristis, Bhypticus, 

 Gobiesox, Prionotus), or at least tropical genera (such as 

 Chrysophrys, Pristipoma, Holaccmthus, Caranx, Balistes). A 

 few other types from the Peruvian coast {Doydixodon), or 

 even from Japan (Prionurus), have established themselves in 

 this group of islands. A species of Cestracion has also reached 

 the Galapagoes, but whether from the south, north, or west, 

 cannot be determined. 



The presence of the Atlantic fauna on the Pacific side is 

 felt still farther west than the Galapagoes, some Atlantic 

 species having reached the Sandwich Islands, as Chcetodon 

 humeralis and Blcnnius hrcvipinnis. 



c. The Peruvian district possesses a very limited variety 



