DR. WHEWELL's views CONSIDERED. 129 



The point at which extraordinar}- causes have to 

 be supposed is e^•ideIltly quite arbitrary, resting 

 exactly on the limits of the knowledge existing at 

 any time, and always flying fiirther and further 

 back, in proportion as our knowledge increases. 

 Had Dr. Whewell been writing fifty years ago, he 

 would of course have included among his palaetio- 

 logical sciences, the formation of strata, and the 

 intrusions of the granitic and trappean among the 

 aqueous rocks, which ingenuity has since explained 

 by existing causes ; — for there is not a single argu- 

 ment for his considering the formation of globes 

 and origin of species as palaetiological, which 

 would not have applied with equal force to these 

 phenomena before the days of Pallas and Hutton. 

 Against a theory of mere assumption — a reasoning 

 jfrom ignoran.ce to ignorance — such considerations 

 form serious objections. But let us come to closer 

 argument. Let us inquire how the idea of a dif- 

 ferent set of causes for the more important of these 

 phenomena, agrees with such exact knowledge as 

 we have attained respecting them. 



" According to the nebular hypothesis," says 

 Dr. WheweU, " the formation of this our system of 

 Sim, planets, and satellites, was a process of the 

 same kind as those which are still going on in the 



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