240 ORIGIN OF LIFE IN AMEEICA 



Seven species of crabs and their allies are identical to the two 

 oceans, while many others are closely related. The fauna as a 

 whole implies that the union of the seas cannot have been of 

 a very recent date. Accordingly Dr. Ortmann * is of opinion 

 that there was no communication since Miocene times. 



In 1880 Dr. Giinther f declared that the genera of fishes, 

 with scarcely any excepition, are identical on the two sides 

 of Central America. Of the species found on the Pacific side, 

 nearly one half, he states, have proved to be the same as those 

 of the Atlantic, and he upholds the view of the existence o"f 

 straits and channels between the two seas until a geologically 

 recent period. Since Dr. Giirither expressed these views, the 

 fish fauna of this region has received renewed attention and 

 study, over a thousand species now being known from the 

 coasts of the two seas. Of this large (number only seventy- 

 one species, or six per cent., are common to the two oceans, 

 according to the latest researches of Professor Jordan. This 

 great discrepancy between Dr. Giinther's and Professor 

 Jordan's results arises, of course, to a large extent from the 

 different views these observers hold as to specific limits. 

 To account for the remarkable identity of genera and the 

 divergence of species. Professor Jordan ij: suggests that the 

 isthmus of Panama was depressed in or before Miocene times, 

 that the channel was shallow, and that the currents set chiefly 

 westward, thus favouring the transfer of Atlantic rather than 

 Pacific types. In the case of the Medusae, the affinity between 

 the littoral faunas of the two sides of Central America is 

 much closer according toDr.Bigelow§ than in the higher groups. 



It has been suggested by Messrs. Gill and Bransford || that 

 the occurrence of marine fishes in the great lake of Nicaragua 

 is to be looked upon as an indication that the sea once flooded 

 the area now occupied by these lakes. That these fishes are 

 true " relicts " of the past, representing the survivors of a 

 marine fauna, has recently been reaffirmed by Dr. M'eek.^[ 



* OrtmanB, A. E., "Distribution of Decapods," p. 398. 



t Gunther, A., " Study of Fishes," p. 280. 



t Jordan, D. 8., " Study of Fishes," Vol. I., pp. 272—280. 



§ Bigelow, H. B., " Medusae," p. 228. 



II Gill, Th., and J. F. Bransford, " Fishes of Lake Nicaragua," p. 179. 



U Meek, S, E,, " Figies of Nicaragua," p. 99. 



