Ch. X.] 



FOKMER VICISSITUDES OF CLIMATE. 



175 



X 





V 



I 



i V 



m 





\ 



i 



■ 



- 







¥ 



f 



perature recognised in the older formations were regarded 

 with the more favour because they seemed to lend support to 

 the hypothesis of the primeval igneous fusion of the planet, 



■ 



the mass of which, while it radiated heat into the surround- 

 ing atmosphere and ocean, had gradually cooled, and had been 

 constantly acquiring a thicker crust. 



Since I first attempted, in the year 1830, to account for 

 vicissitudes of climate by reference to changes in the physical 

 geography of the globe,* our knowledge of the subject has 

 greatly increased, and the problem to be solved has assumed 

 a somewhat new aspect. More extended observations have 

 shown that in times past the climate of the extra-tropical 

 regions has by no means been always hotter than now, but, 

 on the contrary, there has been at least one period, and one 

 of very modern date geologically speaking, when the tempe- 

 rature of those regions was much lower than at present. It 

 will be desirable, therefore, before entering into a discussion 

 of the probable causes of the changes of temperature which 

 have been experienced since the earliest of the fossiliferous 

 rocks were formed, to lay before the reader a brief account 

 of the evidence by which the reality of such changes has been 

 established. 



At first sight it may seem to be the simplest way of dealing 

 with this subject to begin with a description of the proofs 



i from organic rema 

 times, and then to ] 



manifested by the 

 }h a method is impr 



fauna and flora of later epochs. 



Lcticable, for not only are all the 

 animals and plants found fossil in the oldest rocks specifically 

 distinct from those now living, but a large part of the genera 

 and even the orders to which they belong have for ages ceased 

 to exist. Consequently, when we attempt to make 



such a 



determini 



climates which prevailed at two distant periods, we find it 

 almost impossible to apply the rules derived from the study 

 of the present state of the animate world to another which 



from 



The thread of induction seems 



become 



* Principles of Geology, 1st edition. 1830. 



