240 
ORIGIN OF LIFE IN AMERICA 
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. Gunther f declared that the genera of fishes, 
with scarcely any exception, 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 of 
straits and channels between the two seas until a geologically 
recent period. Since Dr. Gunther 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. Gunther’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 J 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.^f 
* Ortmann, A. E., “Distribution of Decapods,” p. 398. 
f Gunther, A., “ Study of Fishes,” p. 280. 
t Jordan, D. S., “ Study of Fishes,” Yol. I., pp. 272—280. 
§ Bigelow, H. B., “Medusae,” p. 228. 
|j Gill, Th., and J. F. Bransford, “ Fishes of Lake Nicaragua,” p. 179. 
H Meek, S. E., “ Fishes of Nicaragua,” p. 99. 
