Isthmus Barriers Separating Fish Faunas 271 



A few others invade brackish or fresh waters and may pos- 

 sibly have found their way, in one way or another, across the 

 Isthmus of Nicaragua. Of fishes strictly marine, strictly lit- 

 toral, and not known from Asia or Polynesia, scarcely any 

 species are left as common to the two sides. This seems to 

 show that no waterway has existed across the Isthmus within 

 the lifetim.e, whatever that may be, of the existing species. 

 The close resemblance of genera shows apparently with almost 

 equal certainty that such a watenvay has existed, and within 

 the period of existence of the groups called genera. How long 

 a species of fish may endure unchanged no one knows, but we 

 know that in this regard great differences must exist in dif- 

 ferent groups. Assuming that different species crossed the 

 Isthmus of Panama in Miocene times, we should not be sur- 

 prised to find that a few remain to all appearances unchanged ; 

 that a niuch larger number have become ' ' representative ' ' 

 species, closely related forms retaining relations to the envi- 

 ronment to those of the parent form, and, finally, that a few 

 species have been radically altered. 



This is exactly what has taken place at the Isthmus of 

 Panama with the marine shore fishes. Curiously enough, the 

 movement of genera seems to have been chiefiy from the At- 

 lantic to the Pacific. Certain characteristic genera* of the 

 Panama region have not passed over to the Pacific. On the 

 other hand, most of the common generaf show a much larger 

 number of species on the Atlantic side. This may be held to 

 show their Atlantic origin. 



Of the relatively small number of genera which Panama has 

 received from Polynesia, t few have crossed the Isthmus to ap- 

 pear in the West Indian fauna. 



Views of Earlier Writers on the Fishes of the Isthmus of Panama. — 

 The elements of the problem at Panama may be better under- 

 stood by a glance at the results of previous investigations. 



* Hoplopagrits, Xenidilliys, Xcnislius, Xcnocys, Microdcsmus, Ccrdale, 

 Cratiniis, Azevia, Microlepidotus, Orthoslcechus, Isaciclla, etc. 



t Hmnulon, Anisotremus, Gerrcs, Centra poimis, Galcichlliys, llypoplcctrus, 

 Mycteroperca, UlcEma, Stellifer, Micropogon, Bodianus, Micros paihodon. 



% Among these are perhaps Teiithis (Acanilmrus) , llisha, Salarias, Myri- 

 pristis, Thaiassoma. Some such which have not crossed the Isthmus are 

 Cirrhitus, Sectator, Sebastopsis, and Lophiomus. 



