74 ESSAY ON CLASSIFICATION. 



The class of Fishes has its largest representatives in the 

 sea ; fresh water fishes are on the whole dwarfs in com- 

 parison to their marine relatives, and the largest of them, 

 our Sturgeons and Salmons, go to the sea, The same 

 relations obtain among Crustacea ; and to be satisfied of 

 the fact, we need only compare our freshwater Crawfishes 

 with the Lobsters, our Apus with Limulus, etc. Among 

 Worms, the Earthworms and Leeches furnish a still wider 

 range of comparison, when contrasted with the marine 

 types. Among Gasteropods and Acephala, this obtains 

 to the same extent ; the most gigantic Ampullarise and 

 Anodontee are small in comparison to certain species of 

 Fusus, Voluta, Tritonmrn, Cassis, Strombus, or to the Tri- 

 dacna. Among Eadiata even, which are all marine with 

 the exception of the single genus Hydra, this rule holds 

 good, as the fresh water Hydroids are among the smallest 

 Acalephs known. 



This coincidence, upon such an extensive scale, seems 

 to be most favourable to the view that animals are mo- 

 dified by the immediate influence of the elements ; yet I 

 consider it as affording one of the most striking proofs 

 that there is no causal connexion between them. Were 

 it otherwise, the terrestrial and the aquatic representatives 

 of the same family could not be so similar as they are in 

 all their essential characteristics, which actually stand in 

 no relation whatsoever to these elements. That which con- 

 stitutes the Bear in the Polar Bear is not its adaptation to 

 an aquatic mode of existence. That which makes the 

 Whales Mammalia bears no relation to the sea. That which 

 constitutes Earthworms, Leeches, and Eunice members of 

 one class has no more connexion with their habitat, than 

 the peculiarities of structure which unite Man, Monkeys, 

 Bats, Lions, Seals, Beavers, Mice, and Whales into one class. 



