(iKNKKAL COXCIX'SIOXS. 1>,S<) 



has iidl sulliced cvuii to produce nciw races; aiul the 

 inevitalile conclusion is tlial any possible derivation of 

 one 8i)ecies from another is i)ushed hack indetinitely, that 

 the origin of specilie types is ([uite distinct from varietal 

 niodilication, and that the hitter attains to a niaxinunu in 

 a comparatively siiort time, anil then runs on unchanged, 

 except in so far as geological vicissitudes nuiy change the 

 localities of certain varieties. This is jirecisely the same 

 conidusion at which 1 have elsewhere arrived from a 

 sinular comjjarison of the fossil tloras of the Uevoniau 

 and Carboniferous periods in America. 



A second leading point to which 1 would direct atten- 

 tion is the relative value of hind ice and water-borne ice 

 as causes of geological change in the Pleistocene. Ou 

 this subject I have constantly maintained that moderate 

 view which was that of Sir JiodericU ^lurchison and Sir 

 Charles Lyell, that the rieistocene subsidence and refrig- 

 eration produced a state of our continents in which the 

 lower levels, and at certain ])eriods even the tops of the 

 higher hills, were submerged, under water tilled every 

 season with heavy tield-ice formed on the surface of the 

 sea, as at present in Smith's Sound, and also with abundant 

 ice-bergs derived from glaciers descending from unsub- 

 merged mountain districts. These conclusions have been 

 reinforced by the recent establishment of the fact of 

 difl'erential elevation and submergence, whereby the moun- 

 tain ridges retained their elevation even when plains and 

 table-lands were submerged. I need not reiterate the 

 arguments for these conclusions, but may content myself 

 with a reference to the changes of opinion on the subject. 

 The glacier theory of Agassiz and others may be said to 

 have grown till, like the imaginary glaciers themselves, it 

 overspread the earth. All northern Europe and America 



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