1562 THE STORY OF THE UNIVERSE 



the lowest links of the ascending chain. Here, also, we 

 observe organic development proceeding with more 

 rapidity, and raising structures of greater dimensions 

 in aquatic than in terrestrial animals. The order 

 Cetacea comprises by far the largest animals which 

 inhabit the globe. Whatever may have been the 

 magnitude of those huge monsters which once moved 

 in the bosom of the primeval ocean, or stalked with 

 gigantic strides across antediluvian plains, and 

 whose scattered remains bear fearful testimony of the 

 convulsions of a former world, certain it is that, at 

 the present day, the whales of the northern seas are 

 the most colossal of the living animal structures ex- 

 isting on the surface of this planet. 



A cursory survey of the organization of the tribes 

 belonging to this semi-amphibious family will Im- 

 press us with the resemblance they bear to fishes ; 

 for they present the same oval outline of the body, 

 the same compact form of the trunk, which is united 

 with the head without an intervening neck; the same 

 fin-like shape of the external instruments of motion 

 and the same enormous expansion and prolongation 

 of the tail, which is here also, as in fishes, the chief 

 agent in progression. With all this agreement in 

 external characters, their internal economy is con- 

 ducted upon a totally different plan; for although 

 constantly inhabiting the ocean, their vital organs 

 are so constructed as to admit of their breathing only 

 the air of the atmosphere, and the consequences 

 which flow from this difference are of great im- 

 portance. The necessity of aerial respiration com- 

 pels them to rise, at short intervals, to the surface 



