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 drawn 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, Sarqus rondeletit) ; 
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 carried 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 afew survived’, however, in spite of the greatly altered physical con- 
ditions, without altering their specific characters, still agreeing with the typical forms 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 difference 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, a@ pew prés 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 this species would be to force 
it to retreat further and further from the original coast, and to accommodate itself to the 
new one—an effect to which, if felt at all, the individuals 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 specimens examined by me from opposite 
coasts of the isthmus are absolutely identical, and there is not the slightest indication that 
one of them has been modified or degenerated into a climatic or local variety. I trust that 
1 Martens, in ‘ Zoolog. Record,’ ii. p. 237. * Journ. Conchyl. xiii, 1865, pp. 241-248, 
3 Seyen or eight species of the northern part of the Baltic are believed to be of Arctic origin. 
