412 Mr. H. G. Seeley on the Laws which have determined 



24. If depression of a land-surface is going on, then the spe- 

 cies are converging in space and becoming numerous relatively 

 to their area, while they also ascend the mountains. If while 

 the land sinks to the north it rises to the south, the fauna and 

 flora come to occupy a more southern area"^. 



25. There being much reason for thinking that the deep 

 waters of the ocean have a comparatively uniform temperature, 

 it follows that the distribution of life in those regions will be less 

 dependent on temperature than it is at the surface of the earth. 

 Therefore the life of deep-sea limestones will have a wide range. 



26. No elevation of land can take place without (as was seen in 

 § 9) the deposits that were forming being continued over each 

 other out at sea. Thus s is the sand formed near to the shore, and 

 c the clay further out at sea ; by elevation s^ is formed over s 

 and c, and c^ is formed over c and /. By further elevation, s^ 



is formed over s^ and over c^, and of course a c^ would be formed 

 over c^ and /', so that the s, s^, s"^, s^, &c. would appear to the 

 observer of sections to form one deposit (for the divisions here 

 marked would not exist in nature) extending uniformly over 

 another deposit, c, c^, c^, which would therefore appear to be 

 an older one; and as this deposit would extend over the / 

 series, it would be inferred to be newer than that group. But, 

 although that inference is correct in regard to the vertical 

 section, obviously the s is older than the c', and much older 

 than the c^, though it appears to rest on the top of those 

 deposits. And since by elevation the sea-area is changed, the 

 fauna and flora continue to move in the direction of least resist- 

 ance, which in this case being determined by uniformity of con- 

 ditions, it happens that the fauna of s will migrate into s\ 

 and similarly will afterwards move into s^ ; so that it will be 

 impossible to identify the ages of these beds by fossils in the 

 iisual rough-and-ready way, or by superposition. Here identi- 

 fication of the strata can only be accomplished by the method 

 given in § 13. 



Often by elevation a fauna is compelled to migrate ; and then 

 the extension of a group of life assists greatly in connecting 

 deposits in an adjacent area with those formed under other 

 physical conditions, when we have discovered where the group 

 came from. 



* The migratory habits of birds are probably due to old changes in 

 physical geography of this kind. 



