210 GEOGRAPHICAL DISTRIBUTION. 



— ;i 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 littoral faunas, 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 subse- 

 quent variations after their isolation through submergence, has been the 

 main agent in the distribution of the existing terrestrial fauna 1 , we must ac- 

 knowledge a similar agency to currents in the distribution of marine faunas; 

 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 distribution at different geological epochs. The 

 greater the batliymetrical range of littoral species, the longer will such spe- 

 cies remain unaffected, while deep-sea species may early become isolated 

 and remain as outliers as it were, — mementoes of a former condition of 

 currents, or even of a previous geological period. The careful analysis of 

 the fauna of a given point, its comparison with other faunae, and accurate 

 batliymetrical data, would go far towards reconstructing the Natural History 

 of the sea in former ages, and showing its relation to the present and past 

 times. 



The representative species of Echinus, Echinocardium, Brissopsis, Schi- 

 zaster, in the Arctic and Antarctic boreal zones would be considered as the 

 living representatives of a cosmopolitan fauna existing at the time when tin- 

 great equatorial current flowed unbroken round the globe, sending branches 

 north and south along Eastern North and South America, along Eastern 

 Japan and Australia, and the Eastern Coast of Africa ; while the tropical 

 species of the genera Diadcina. Clypeaster, Echinoneus, Echinolampas, etc., 

 existing at that time, had a more limited equatorial geographical distribu- 

 tion. The subsequent period of isolation of Atlantic and Pacific currents is 

 shown by the existence of truly Atlantic and Pacific species ; while as we go 

 down in depth we go back also in time, and find at first representatives of 

 the genera found in our Tertiaries, while at greater depth the species are 



