18 



THE CHAIN OF LIFE. 



the following chapters, and to appreciate the actual nature of 

 the succession of life in so far as it is at present known. 



Fig. 12a.— Footprints of modern Limulus, or king-crab, in the sand, which enable us to 



interpret tho!>e in Fig. 12. 



Fiu. 13. — Currant markings on shale, resembling a fossil plant, 

 photograph (Dawson's Acadian Geology). 



Reduced from a 



Note. — It should have been stated above that, on certain theories 

 now somewhat generally accepted, respecting the nature and source 

 of solar heat, the absolute duration of geological time would be much 

 reduced below the estimate of Sir Wm. Thomson. Prof Tait has based 

 on such data an estimate of fifteen millions of years. Prof. Simon 

 Newcomb says that "on the only hypothesis science will now allow 

 us to make respecting the source of the solar heat" (the gravitation 

 hypothesis of Helmholtz) "the earth was, twenty millions of years ago, 

 enveloped in the fiery atmosphere of the sun." Dr. Kirkwood has recently 

 called attention to these results in connection with the planetary hypothesis 

 of La Place, in the Proceedings of the American Philosophical Society 

 (Sept. 1879). Should such views prove to be well founded, geological 

 calculations as to the time required for the successive formations may have 

 to be revised. 



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