LAW OF GEMINATE SPECIES JORDAN 117 



tation they are absolutely identical. They are usually identical in habits. 

 They differ in minor regards, characters which we may safely suppose to be 

 of later origin than the ordinary specific characters in their group. Illus- 

 trations of geminate species of birds, of mammals, of fishes, of reptiles, of 

 snails, or of insects, are well known to all students of these groups, and 

 illusrations may be found at every hand. 



Each island of the West Indies, which is well separated from its neigh- 

 bors, has its own form (species or subspecies) of golden warbler (Dendroica 

 cestiva). Each island in the South Seas has its geminate forms of reptiles 

 or fishes. Each island of the Hawaiian group has its own representative of 

 each one of the types or genera of Drepanida. Each of the three groups of 

 rookeries in Bering Sea has its own species of fur seal. Callorhinus alascanus 

 on the Pribilof Islands, C. ursinus on the Commander Islands, and C. curi- 

 lensis of the Kuriles and Robben Islands, these species most clearly related 

 but showing no intergradations, because no intercrossing is possible, each 

 having its own distinct line of migrations. 



Similar conditions exist among species of fresh water fishes, the world 

 over. Dr. Ortmann has described the conditions of species forming through 

 isolation in the river cray fish (Cambarus) ; Dr. Gulick in the land snails 

 (Helix) and various writers in the river mussels (Unionidce). 



Most clearly marked cases of geminate species occur among the fishes 

 on the two sides of the isthmus of Panama. Living under essentially the 

 same conditions, but separated since the end of the Miocene Period by the 

 rise of the isthmus, we find species after species which has been thus split 

 into two. These geminate species, a hundred or more pairs in number, were 

 at first regarded as identical on the two shores of the isthmus. Later one 

 pair after another was split into recognizable species. The latest authority 

 on the subject, Mr. C. T. Regan, seems to doubt if any species of shore fishes 

 are actually identical on the two sides of the isthmus. 



To make this clear, though at the risk of being tedious, I give below a 

 partial list of these genuine species about the isthmus of Panama: 



Atlantic Coast Pacific Coast 



Harengula humeralis Harengula thrissina 



Clupanodon oglinus Clupanodon libertatis 



Centropomus undecimalis Centropomus viridis 



Centropomus pedimacula Centropomus medius 



Centropomus affinis Centropomus ensiferus 



Epinephelus adscensionis Epinephelus analogus 



Alphestes afer Alphestes multiguttatus 



Dermatolepis inermis Dermatolepis punctatus 



Hypoplectrus unicolor Hypoplectrus lamprurus 



