4 



GEOGRAPHICAL DISTRIBUTION 



of the existing system of marine species, and only after the emerging 

 could land-shells be supplied. 



In the course of our voyage, some other instances were observed, 

 where the origin of species did not seem strictly contemporaneous : — 

 Tufts of sea-weed floating in mid-ocean, but originally detached from 

 coasts, are accompanied by marine animals, that neither belong to 

 coasts, nor are properly oceanic. It is true, these accompanying 

 species occur equally around floating animal substances; as around the 

 central cartilage of velellas, around bunches of ova of various kinds, 

 but not around living animals. 



Further : marine plants in general do not occur upon animal sub- 

 stances; but various diminutive species are attached to the tufts of 

 floating sea-weed. Some of these species may prove distinct from any 

 growing on coasts; and if so, we shall be at a loss for a contempora- 

 neous basis, unless we resort to the rarely met with floating pumice 

 of the Mineral Kingdom. 



The Extinction of species, seems more comprehensible than the 

 Creation of new ones, and may sometimes be accounted for. There 

 are good reasons for believing, that the clumsy bird called the dodo, has 

 become extinct within two or three centuries : and, among the modi- 

 fications of the Earth's surface induced by human occupancy, other 

 species of animals and plants may disappear at no distant day, espe- 

 cially those restricted to small ocean islands. But this is a different 

 question from the extinction of a whole system of species. 



While the land and w^ater surface of the globe has remained essen- 

 tially the same as at present, a system of species has been created, 

 maintained by generations throughout a very long period of time, 

 extinguished, and our existing system substituted. 



But there has been a period when the Configuration of the land 

 and water was entirely different; when the larger portion of what is 

 now land, to the tops of the loftiest mountain-chains, was the bottom 

 of the sea; and when parts of the present bottom of the sea may 

 have been dry land. At this ancient geological Period, there was 

 yet a Third System of species. Even with the ensuing disturbance 

 of the rock-strata, it may be difficult to account for a simultaneous 

 extinction of every aquatic species. 



