98 



feet all the tropical plants would disappear, as there would be 

 only a temperate zone within the tropics ; if, on the contrary, 

 the land was depressed so as to be entirely below 3000 feet, or 

 say mean temperature 75°, there would be only the productions 

 of the hot regions : therefore the oscillations of the land must 

 considerably influence the width and parallelism of the zones. 

 In the former there would be no tropical band on dry land, and 

 in the latter case it would be much augmented ; consequently 

 they can never be definitely marked, or permanently preserve 

 their parallelism. 



When we compare the productions of the southern hemisphere 

 with the north, in corresponding zones of equal mean tempe- 

 rature, we find a great difference. Among the tallest of living 

 trees the Norfolk Island pine is found in south latitude 30°, 

 but is not known in the north temperate zone. Tree ferns, 

 which require abundance of moisture, as well as equalization of 

 heat, are found in Van Diemen^s Land and New Zealand in 

 south latitude 45° : there are none in a corresponding latitude in 

 the northern hemisphere. The distribution of plants cannot 

 therefore be explained solely by the influence of climate, for it 

 frequently happens that similar climates are found in different 

 parts of the globe without identity of productions. 



In ascending from the vegetable to the animal world, and from 

 one rank of animal existence to another, the most admirable 

 order is manifest. Each description of zoophyte has its place 

 of residence determined by the temperature and nourishment 

 required for its support. The zones of depth below the level of 

 the sea may be governed by a law similar to those above. 



Professor E. Forbes, in his admirable observations on the 

 Mollusca and Radiata of the iEgean Sea, states, " It was found 

 to be a law, that the extent of the range of a species in depth is 

 correspondent with the extent of its geographical distribution. 

 On the other hand, species having a very limited range in depth 

 were found to be either peculiar Mediterranean forms, or such 

 as are extremely rare in the ^Egean, but abundant in more 

 northern seas. The Testacea of the ^Egean are for the most 

 part dwarfs, as compared with their analogues in the ocean, and 

 the number of Medusae and Zoophytes are comparatively small. 

 Below the fourth region in depth the number of animals dimi- 



