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 relativel 
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 7. 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 
usual 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 haye discovered where the group 
came from. 
* The migratory habits of birds are probably due to old changes in 
physical geography of this kind. 
