ii8 NATURAL SCIENCE. apr.l. 



are only two. Tliere is the Central African region, with Victoria 

 Nyanza and other equatorial sheets ; and there is the somewhat 

 similar cluster in North America, including Lake Superior. Another 

 group once occupied most of the Mediterranean region and extended 

 far into Asia ; it is now represented by the Caspian Sea and other 

 more or less isolated sheets of brackish water, but the western 

 part has again become connected with the ocean and has lost its 

 peculiar fauna. A fourth district to which we shall refer is now the 

 arid region, or Great Basin, of North America.' This country was 

 occupied not long since, geologically speaking, by magnificent inland 

 seas, which now are represented by a few shrunken and saline 

 remnants, such as the Great Salt Lake. No doubt other extensive 

 lakes can be recognised by studying earlier geological history, but 

 our present purpose is not to trace the former recurrence of lacustrine 

 conditions ; it is rather to study the history of the two great groups 

 of freshwater lakes which still exist. 



It is evident that, for the purpose of study, it will be most 

 convenient to select lakes which still remain, but which formerly had 

 a greater extent, and have thus left their ancient history written in 

 the marginal sediments now accessible to our exploration. The two 

 regions which most thoroughly answer to these conditions have been 

 carefully studied, and they represent two distinct types. The 

 Mediterranean has been alternately connected with the ocean or 

 converted into a chain of brackish-water lakes. The Great Basin of 

 North America, on the other hand, has contained lakes changing 

 often in size, sometimes becoming almost dried up, sometimes filled 

 to overflowing, but never forming part of the ocean. Thus we find 

 that the lakes of the Mediterranean basin, even when nearly fresh, 

 contain seals and other marine animals ; while the Great Basin is 

 characterised, and apparently always has been characterised, by the 

 poverty of its fauna and the absence of marine forms. 



Let us now trace the past history of these two continental basins, 

 as far as our knowledge will permit us. We shall then be in a 

 position to understand the probable mode of origin of the great lakes of 

 Africa and America. We shall then also be able to realise the bygone 

 conditions which led to the formation of such vast sheets of fresh 

 water, and to their peopling with the animals and plants that now 

 inhabit them. 



In the long-settled and carefully-studied region between Europe 

 and Africa, we find, at the present day, the western part occupied by 

 the extensive inland sea, known as the Mediterranean. To the east, 

 this is connected with other seas somewhat less salt, known as the 

 Black Sea and the Sea of Azov. Still further east we find the 

 isolated Caspian Sea and Sea of Aral. There may not appear at 



1 The maps illustrating this paper are all drawn to the same scale, so as to 

 indicate the relative importance of the Basins. The figures on the lakes show the 

 height of their surface above the sea. 



