DR. GUNTHER ON THE FISHES OF CENTRAL AMERICA. 399 



middle of Sweden was submerged, and the Baltic was a great gulf of the Glacial Ocean, 

 and not connected with the German Ocean. By the gradual elevation of the Scandi- 

 navian continent, the Baltic became disconnected from the Glacial Ocean, and the great 

 lakes separated from the Baltic. 



The Isthmus of Suez appears to have been a much more permanent barrier between 

 the faunas of the Mediterranean and the Red Sea. R. A. Philippi has di-awn up a list 

 of species of shells common to both faunas ; but it was founded on a collection made by 

 Ehrenberg, in which the shells from both seas had been mixed' ; and P. Fischer- has 

 lately shown that the two faunas are quite distinct. As regards the fishes, I have men- 

 tioned (on former occasions) a few occurring in both seas (Sargus noct, Sargus rondeUtii) ; 

 but the number is so small that one might be tempted to account for it by the tempo- 

 rary existence of an artificial communication between the two seas. 



Looking at the results of the separation of the Baltic from the Glacial Ocean on the 

 one hand, and of that of the Pacific from the Atlantic on the other, we find them very 

 different. As soon as the continuity of the Baltic with the Glacial Ocean was inter- 

 rupted, the amount of fresh water earned into the former by rivers exceeded the quantity 

 lost by evaporation of its surface, and the salt water gradually changed into brackish, 

 and in the northern parts into fresh water. By far the greater part of the animals 

 became extinct ; but a few survived^ however, in spite of the greatly altered physical con- 

 ditions, loithout altenng their specific characters, still agreeing loith the typical fm-ms in 

 every point, except in size, remaining smaller, leaner, almost starved. The same thing 

 might happen if by a rising of the chain of the West- Indian islands the Gulf of Mexico 

 or the Caribbean Sea were at a future time converted into inland seas with narrow out- 

 lets into the open ocean. 



The separation of the Atlantic and Pacific Oceans was, of course, not accompanied by 

 a change of the water ; and any diff"erence that existed in the physical conditions of both 

 seas, as, for instance, the formation of corals on the Atlantic side, and their total absence 

 on the Pacific, existed already before the communication between the oceans was closed ; 

 so that the life of species was not in any way affected by the discontinuance of this 

 communication. Let us for argument's sake assume that the part of the isthmus 

 between the Lake of Nicaragua and Panama was once an island, apeupres of the form 

 of Cuba, inhabited, like Cuba, on its northern and southern coasts by a certain species of 

 fish. The only effect of a gradual rise of the land on the life of tliis species would be to force 

 it to retreat further and further from the original coast, and to accommodate itself to the 

 new one — an eff"ect to which, if felt at all, the indi\aduals on the northern and southern 

 coasts would be equally exposed. Thus there is in this case no apparent external cause 

 for an alteration of the species ; and, indeed, the specinwns examined hy me from opposite 

 coasts of the isthmus are absolutely identical, and there is not the slightest indication that 

 one of them has heen modified or degenerated into a climatic or local variety. I trust that 



' Martens, in ' Zoolog. Record,' ii. p. 237. " Journ. C'unchyl. xiii. 1S65, pp. 241-248. 



^ Seven or eight species of the northern part of the Baltic are believed to be of Arctic origin. 



