398 DE. GtJNTHEE ON THE FISHES OF CENTEAL AMEEICA. 



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 Ave 

 And 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 volcanoes may have been another, though secondary cause in filling up the 

 cliannels 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 

 T,uke 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 difierence 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 S. Loven 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. " M. Wagner, I. c. p. ST. 



■'' At Chagres the mean elevation is 1-16 foot, while at Panama the highest flow is 22 feet. (Seemann, Voj-. 

 of H.M.S. • Herald,' i. p. 236.) 



* Loven, Skand. Natui-forsk.-Siillskap. forst. oftentl. miite d. 9 Juli 1863 : Stockholm, 1864. Mabngren, 

 ■ Kritisk Ofvorsigt af Finlands Fiskfauna,' see ' Zool. Eecord,' i. pp. 130-138. 



