118 THEORY OF THE EARTH. 



racters ; and such geologists as are disinclined to 

 adopt the successive epochs which I have endea- 

 voured to establish in regard to fossil bones, may 

 for many years draw from thence an argument 

 against my system, so much the more convenient as 

 it is contained in my own work. Even allowing that 

 these epochs are liable to some objections, from 

 such as have slightly considered some particular 

 fact, I am not the less satisfied that those who shall 

 take a comprehensive view of the phenomena, will 

 not be checked by inconsiderable and partial dif- 

 ficulties, but will be led to conclude, as I have 

 done, that there has at least been one succession, 

 and very probably two, in the class of quadrupeds, 

 before the appearance of those races which now 

 inhabit the surface of our globe. 



30. Proofs that the extinct Species of Quadrupeds 

 are not Varieties of the presently existing Species. 



The following objection has already been start- 

 ed against my conclusions. Why may not the pre- 

 sently existing races of mammiferous land-quadru- 

 peds be mere modifications or varieties of those 

 ancient races which we now find in the fossil state, 

 which modifications may have been produced by 

 I change of climate and other local circumstances, 

 and since raised to the present excessive diffe- 

 rence, by the operation of similar causes during a 

 Jong succession of ages ? 



