MUSEUM OF COMPARATIVE ZOOLOGY. 305 



passing through Central Asia, Arabia, the northern part of Africa, and 

 connecting with the Pacific by a narrow strait through the Isthmus 

 of Panama. The existence of this connection in the cretaceous period 

 is placed beyond doubt by the presence of an Ananchytes, which I am 

 unable to distinguish from Ananchytes radiata, collected on the Isthmus 

 of Panama, and now in the Museum of Yale College, kindly loaned 

 me for examination by Professor Verrill. From the small number of 

 identical species, either of Mollusca, Crustacea, or Fishes, recorded on 

 both sides of the Isthmus, this connection must have been very imper- 

 fect at a comparatively recent geological period, — since the existence 

 of the present Faunas. 



The question naturally arises, Have we not in the different Faunae 

 of both sides of the Isthmus a standard by which to measure the 

 changes which these species have undergone since the raising of the 

 Isthmus of Panama and the isolation of the two Faunae ? If the up- 

 heaval of the isthmus has been gradual, it must, of course, have cut 

 off the deep-water species on both sides of the isthmus, and gradually 

 have isolated the more shallow, till the littoral species also became 

 separated. As a natural consequence, the deeper we go, the farther 

 back in time we must expect to find the representation, — a result which 

 is strikingly confirmed by the nature of the deep-water Fauna of the 

 West Indies. Unfortunately we have not, as in the case of the lit- 

 toral Fauna 1 , a standard of comparison. At the same time, with 

 the gradual closing of the Isthmus of Panama, the greater part of 

 Central Asia, of the Arabian Peninsula, and of Northern Africa was 

 emerging from the sea, reducing ■ the range of the equatorial current, 

 and thus confining the course of the currents much as they are at the 

 present time. This would thus cause a limitation in the range of the 

 species formerly having the greatest distribution, and extend that of 

 those which were more local. 



If migration on land when continents were joined together, and 

 subsequent variations after their isolation through submergence, has 

 been the main agent in the distribution of the existing terrestrial 

 Faunae, we must acknowledge a similar agency to currents in the dis- 

 tribution of marine Fauna? ; and by the submergence or rise of various 

 portions of the continents, we shall be able, if we can trace these 

 changes, to reconstruct within certain limits the altered courses of the 

 main oceanic currents, and get some idea -of the probable geographical 

 35 



