Isthmus Barriers Separating Fish Faunas 273 



Still others (e.g., Trachurus, Carangus, Diodon sp.) are cosmo- 

 politan in the tropical waters. Most of the others (e.g., Gobius, 

 Gerres, Centropomus, Galeichthys sp., etc.) often ascend the rivers 

 of the tropics, and we may account for their diffusion, perhaps, 

 as we account for the dispersion of fresh-water fishes on the 

 Isthmus, on the supposition 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 Isthmus exactly alike in all respects. These 

 differences in some cases seem worthy of specif c value, giving us 

 "representative species" on the two sides. In other cases the 

 distinctions are very trivial, but in most cases they are appre- 

 ciable, especially in fresh specimens. 



Further, I expressed the belief that "fuller investigation 

 will not increase the proportion of common species. If it does 

 not, the two faunas show no greater resemblance than the simi- 

 larity of physical conditions on the two sides would lead us to 

 expect." This similarity causes the same types of fishes to 

 persist on either side of the Isthmus while through isolation or 

 otherwise these have become different as species. 



This conclusion must hold so far as species are concerned, 

 but the resemblance of the genera on the sides has a signifi- 

 cance of its own. 



In 1880* Dr Giinther expressed his views in still stronger 

 language, claiming a still larger proportion of the fishes of trop- 

 ical America to be identical on the two sides of the continent. 

 He concluded that "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 similar to that of the Antilles; 

 but as the reef-building corals flourished chiefly north and east 

 of these islands and were absent south and west of them, reef 

 fishes were excluded from the Pacific shores when the com- 

 munications were destroyed by the upheaval of land." 



* Introduction to the "Study of Fishes," 1880, p. 280. 



