LIFE ON THE EARTH. 163 



reduction of the surface temperature by only 10 

 is so vast and even inconceivable, that the adoption 

 of the hypothesis to that extent seems to require 

 something more than courage. But even ten degrees 

 of added warmth in lat. 60 would not be sufficient 

 to meet the case of the Corals in the sea, or the 

 Palms, Ferns, or Cycadacese on the land. 



These objections have been urged as fatal to the 

 opinion that the internal heat of the earth, now 

 hardly sensible among the elements of surface climate, 

 was formerly a real and efficient, if not the principal, 

 cause of the superior mean temperature of the sea 

 and lands in extra-tropical countries. They have 

 even been urged by geologists, who accept the same 

 heat as real and very influential in that general 

 metamorphosis of the lowest rocks, which is observed 

 in every country, and in those disturbances of the 

 strata, which are equally universal, and of various 

 ages. It appears to me, however, that in these ob- 

 jections one thing is forgotten the state of the at- 

 mospheric mantle which envelopes the terraqueous 

 globe, mitigates solar heat and stellar radiation, and, 

 like the clothing of a steam cylinder, prevents ex- 

 cessive waste of the warmth treasured within. For 

 nothing can be advanced to justify the supposition 

 that this important element in the economy of nature 

 was always of the same total weight, always identical 



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