162 MEANING OF THE SYSTEM. 



but in past ages, when the divisions of land and water were 

 different, they must have been, for many forms of life, easily 

 surmountable. The expression "centre of creation," which has 

 long been used in the sense of a tolerably denned district of dis- 

 tribution or better still, Riitimeyer's word, " centre of distribution" 

 has as a fundamental idea the endemic appearance of definite 

 groups of typical species and their gradual extension "' towards 

 the boundaries of the said region, a conception which harmonizes 

 well with the theory of the origin of species through gradual 

 alterations. 



The same laws apply also to the distribution of the inhabitants of 

 the sea. Great seas studded with islands which serve to confine the 

 land animals may favour the migration of marine species, while 

 extended continents, which allow their inhabitants to wander freely 

 over them, confine the sea animals within limits which cannot be 

 passed. A great number of sea animals live only in the shallow 

 water round the coast, and their distribution thus often coincides 

 with that of the land animals ; whereas the animals found on the 

 opposite coasts of great continents are very different. For example, 

 the sea animals of the east and west coasts of South and Central 

 America differ to such a degree that, with the exception of a series 

 of fishes, which, according to Giinther, are found on both sides of 

 the Isthmus of Panama, only a few forms are common to the two 

 coasts. In the same way we find that the marine inhabitants of 

 the east insular district of the Pacific differ completely from those of 

 the west coast of South America. If, however, we advance to the 

 west of this part of the Pacific till we come to the coast of Africa 

 in the other hemisphere, we find that the fauna of this extensive 

 district cannot be so sharply distinguished. Many species of fish 

 are found from the Pacific to the Indian Ocean. Numerous Mollusca 

 of the South Sea Islands live also on the east coast of Africa, almost 

 beneath the opposite meridian. In this case the limits of distribu- 

 tion are not impassable, as numerous islands and coasts afford a rest- 

 ing place to wandering inhabitants of the sea. In respect of the 

 different haunts of the inhabitants of the sea, we must make a dis- 

 tinction between the littoral animals, which are distributed along the 

 coasts, and live under different conditions and at different depths on 

 the bottom of the sea, and the pelagic animals, which swim on the 

 surface. 



* Compare Riitimeyer's Essay, "Ueber die Herkunft unserer Thierwelt." 

 Basel and Genf. 1867." 



