398 DR. GUNTHER ON THE FISHES OF CENTRAL AMERICA. 
Central America'!, we cannot account for this fact by resorting to such occasional means 
of dispersal as the accidental transmission of spawn from one shore to the other by birds 
or water-spouts, or even the close proximity of the sources of rivers flowing in opposite 
directions. If we do not adopt the view that’ species were created at the spot where we 
find them now, similar creations being produced under similar physical conditions, we 
have but one way of explaining the partial similarity of these marine fish-faunas, 
namely, by assuming that the Isthmus did not form a continuous barrier between the 
two oceans at a former period, but that one or more open channels existed. I am not 
aware that geology has, up to this time, furnished us with proof positive that this is 
really the fact; but considering the-volcanic nature of Central America, and the absence 
of all fossiliferous strata, it does not appear too bold an hypothesis to assume that North 
and South America were formerly connected by a chain of islands similar to that of the 
Antilles, and that subsequently an elevation (as in other parts of the globe) took place, 
resulting in the final continuity of dry land: the long-continued activity of the 
numerous voleanoes may have been another, though secondary cause in filling up the 
channels on the Pacific side. If such a bodily elevation of Central America has taken 
place, it is easy to show where some of the broadest channels existed, namely, where 
we find the greatest depressions running from one ocean to the other. The northern- 
most of these depressions exists between Tehuantepec and the river Coatzalco; the 
second is indicated between Puerto Cabello and the Gulf of Fonseca; the third by the 
Lake of Nicaragua (the remnant and deepest part of a very broad channel); a fourth 
between Chagres and Panama. (See map, Pl. LXIII., where these supposed former 
depressions are coloured green.) As far as I have been able to ascertain, the greatest 
elevation of the first of these lines of depression would be 1500, of the fourth 287 feet 
only*. If we presume that only one of the channels was open at a period when the 
present marine fauna was already in existence, it will fully explain the existence of 
identical species on both sides of the isthmus, especially if the difference of the tides 
was as great as it is now’, causing strong currents from one ocean to the other. 
Such an instance of a disconnexion of a marine fauna by elevation of land as I am 
inclined to assume in the case of Central America does not stand quite alone. We owe 
to the researches of Prof. 8. Lovén and Dr. Malmgren‘ the knowledge of the fact that 
marine animals (Crustacea, Annelids, and Fishes) inhabiting the glacial ocean are found 
in the great freshwater lakes of Sweden and in the Bothnian Gulf, and that this is to 
be explained only by the former continuity of the Baltic with the Glacial Ocean. 
During the second half of the glacial period the greater part of Finland and of the 
' Mr. Darwin (‘ Origin of Species,’ 3rd edit. p- 378) was not acquainted with this fact, which by no means mili- 
tates against his argument, but merely modifies it. 2 M. Wagner, J. ¢. p. 87. 
3 At Chagres the mean elevation is 1:16 foot, while at Panama the highest flow is 22 feet. (Seemann, Voy. 
of HLM. ‘ Herald, i. p. 236.) 
* TLovén, Skand. Naturforsk.-Sillskap. forst. offentl. mote d. 9 Juli 1863; Stockholm, 1864, Malmgren, 
‘ Kritisk Ofversigt af Finlands Fiskfauna,’ sce ‘ Zool. Record,’ i. pp. 186-138. 
