214 STRUCTURE AND MODE OF ACTION 



little raised above the level of the sea, or whether it changes 

 rapidly hi ascending in an almost vertical direction the steep 

 declivities of mountain-chains. Organic nature gives to 

 each zone of the earth a peculiar physiognomy ; but where 

 the solid crust of the earth appears unclothed by vegetation, 

 inorganic nature imparts no such distinctive character. The 

 same kinds of rocks, associated in groups, appear in either 

 hemisphere, from the equator to the poles. In a remote 

 island, surrounded by exotic vegetation, beneath a sky where 

 his accustomed stars no lon^r shine, the voyager often re- 

 cognises with joy the argillaceous schists of his birth-place, 

 and the rocks familiar to his eye in his native land. 



This absence of any dependence of geological relations on 

 the present constitution of climates does not preclude or 

 even diminish the salutary influence of numerous observa- 

 tions made in distant regions on the advance and progress 

 of geological science, though it imparts to this progress 

 something of a peculiar direction. Every expedition en- 

 riches natural history with new species or new genera 

 of plants and animals : there are thus presented to us some- 

 times forms which connect themselves with previously long 

 known types, and thus permit us to trace and contemplate 

 in its perfection the really regular though apparently broken 

 or interrupted network of organic forms : at other times 

 shapes which appear isolated, either surviving remnants of 

 extinct genera or orders, or otherwise members of still un- 

 discovered groups, stimulating afresh the spirit of research 

 and expectation. The examination of the solid crust of the 

 globe does not, indeed, unfold to us such diversity and va 



