554 



SCIENCE. 



[N, S. Vol. XIV. No. 354. 



other hand, most of the common genera * 

 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, 

 few t have crossed the Isthmus to appear 

 in the West Indian fauna. 



GiJNTHEB ON THE ISTHMUS OF PANAMA. 



The elements of the problem at Panama 

 may be better understood by a glance at 

 the results of previous investigations. 



In 1869 Dr. Giinther, after enumerating 

 the species examined by him from Panama, 

 reaches the conclusion that nearly one third 

 of the marine fishes on the two shores of 

 tropical America will be found to be iden- 

 tical. He enumerates 193 such species as 

 found on the two coasts ; 59 of these, or 31 

 per cent, of the total, being actually iden- 

 tical. From this he infers that there must 

 have been, at a comparatively recent date, a 

 depression of the Isthmus and intermingling 

 of the two faunas.l 



OBSERVATIONS IN 1885. 



In an enumeration of the fishes of the 

 Pacific coast in 1885, § the present writer 

 showed that Dr. Giinther's conclusions were 

 based on inadequate data. 



In my list, 407 species were recorded from 

 the Pacific coast of tropical America — 

 twice the number enumerated by Dr. Giin- 

 ther. Of these, 71 species, or 11^ per cent., 

 were found also in the Atlantic. About 800 

 species are known from the Caribbean and 

 adjacent shores, so that out of the total 



* Exmulon, Anisotremus, Gerres, Centropomus, Gal- 

 eichthys, Hypoplectrus, Mycteroperca, Ulsema, Stellifer, 

 Micropogon, Bodianus, Microspathodon. 



t Among these are perhaps TeM^Ais {Acanthurus), 

 Ilisha, Salarias, Myriprisiis, Thalassoma. Some such 

 ■which have not crossed the Isthmus are Cirrhites, Sec- 

 tator, Sebastopsis and Lophiomus. 



t ' Fishes of Central America,' 1869, 397. 



^Proc. U. S. Nat. Mus., 1885, 393. 



number of 1,186 species, but .71, or 6 per 

 cent, of the whole, are common to the two 

 coasts. This number does not greatly ex- 

 ceed that of the species common to the "West 

 Indies and the Mediterranean, or even the 

 West Indies and Japan. It is to be noted 

 also that the number 71 is not very defin- 

 itely ascertained, as there must be consider- 

 able difierence of opinion as to the bound- 

 aries of species, and the actual identity in 

 several cases is open to doubt. 



This discrepancy arises from the compar- 

 atively limited representation of the two 

 faunae at the disposal of Dr. Giinther. He 

 enumerates 193 marine or brackish- water 

 species as found on the two coasts, 59 of 

 which are regarded by him as specifically 

 identical, this being 31 per cent, of the 

 whole. But in 30 of these 59 cases, I re- 

 gard the assumption of complete identity 

 as erroneous ; so that taking the number 

 193, as given, I would reduce the percentage 

 to 15. But these 193 species form but a 

 fragment of the total fauna, and any con- 

 clusion based on such narrow data is cer- 

 tain to be misleading. 



Of the 71 identical species admitted in 

 our list, several (e. g., Mola, Thunnus) are 

 pelagic fishes common to most warm seas. 

 Still others (e. g., Trachicrus, Garangus, Dio- 

 don sp.) are cosmopolitan in the tropical 

 waters. Most of the others (e. g., Gohius, 

 Gerres, Centropomus, Galeichthys sp., etc.) 

 often ascend the rivers of the tropics, and 

 we may account for their difi'usion, perhaps, 

 as we account for the dispersion of fresh- 

 water fishes on the isthmus, on the supposi- 

 tion that they may have crossed from marsh 

 to marsh at some time in the rainy season. 



In very few cases are representatives of 

 any species from opposite sides of the Isth- 

 mus exactly alike in all respects. These 

 differences in some cases seem worthy of 

 specific value, giving us ' representative 

 species ' on the two sides. In other cases, 

 the distinctions are very trivial, but in 



