312 TROPICAL NATURE, AND OTHER ESSAYS. 



offshoots towards the South Pole. This singular arrange- 

 ment of the land surface into what is practically one 

 huge mass with diverging arms, offers great facilities for 

 the transmission of the varied forms of animal life over 

 the whole earth, and is no doubt one of the chief causes 

 of the essential unity of type which everywhere charac- 

 terises the existing animal and vegetable productions of 

 the globe. 



There is, moreover, good reason to believe that the 

 general features of this arrangement are of vast 

 antiquity ; and that throughout much of the Tertiary 

 period, at all events, the relative positions of our con- 

 tinents and oceans have remained the same, although 

 they have certainly undergone some changes in their 

 extent, and in the degree of their connection with each 

 other. This is proved by two kinds of evidence. In 

 the first place, it is now ascertained by actual measure- 

 ment that the depths of the great oceans are so vast 

 over wide areas, while the highest elevations of the land 

 are limited to comparatively narrow ridges, that the 

 mass of land (above the sea-level) is not more than ^Vth 

 part of the mass of the ocean. Now we have reason to 

 believe that subsidence and elevation bear some kind of 

 proportion to each other, whence it follows that although 

 several mountain ranges have risen to great heights 

 during the Tertiary period, this amount of elevation 

 bears no proportion to the amount of subsidence required 

 to have changed any considerable area of what was 

 once land into such profound depths as those of the 

 Atlantic or Pacific Oceans. In the second place, we find 

 over a considerable area of all the great continents fresh- 

 water deposits containing the remains of land a,nimals 



