358 PHYSIOGRAPHY. [CH. xx. 



of the animal and vegetable population of a country, or its 

 fauna and its flora. In studying the past history of the 

 basin of the Thames, as revealed by the organic remains 

 described in Chapter XVII., it is evident that this area has 

 at different times undergone great vicissitudes of climate ; 

 at one time supporting a tropical or sub-tropical vegetation, 

 (p. 288), and at another time offering a congenial feeding- 

 ground to herds of northern mammals, such as the musk- 

 sheep (p. 283). These differences of climate may be par- 

 tially accounted for by alterations in the relative distribution 

 of the masses of land and water ; but some of the climatic 

 changes appear to have been so extreme, that the geologist 

 has been led to seek their explanation in astronomical 

 causes.^ 



^ See, for example, Mr. Croll's- Climzte and Ti?ne, 1875. 



