ORGANIC GEOLOGICAL DYNAMICS. 643 



amount of change which we suppose in the 

 sical conditions of the earth. It must be a task of 

 extreme difficulty to estimate the effect upon the 

 organic world, even if the physical circumstances 

 were given. To determine the physical condition to 

 which a given state of the earth would give rise, I 

 have already noted as another very difficult pro- 

 blem. Yet these two problems must be solved, in 

 order to enable us to judge of the sufficiency of any 

 hypothesis of the extinction of species ; and in the 

 mean time, for the mode in which new species come 

 into the places of those which are extinguished, we 

 have (as we have seen) no hypothesis which physio- 

 logy can, for a moment, sanction. 



Sect. 7. The Imbedding of Organic Remains. 



THERE is still one portion of the Dynamics of Geo- 

 logy, a branch of great and manifest importance, 

 which I have to notice, but upon which I need only 

 speak very briefly. The mode in which the spoils 

 of existing plants and animals are imbedded in the 

 deposits now forming, is a subject which has natu- 

 rally attracted the attention of geologists. During 

 the controversy which took place in Italy respect- 

 ing the fossils of the Sub-Apennine hills, Vitaliano 

 Donati 12 , in 1750, undertook an examination of the 

 Adriatic, and found that deposits containing shells 

 and corals, extremely resembling the strata of the 

 hills, were there in the act of formation. But with- 



12 Lyell, B. i. c. iii. p. 67. (4th ed.) 



TT 2 



