and the Elements in which they live. 173 



developed in former geological periods, or so connected with ex- 

 tinct types as to show that this connection has influenced their 

 development. 



Among Reptiles we find the largest in the family of Turtles 

 among their marine representatives ; among the Lizard-like in 

 the fluviatile Crocodiles ; among Batrachians in their aquatic 

 families. 



In Birds, the aquatic families, Pelicans, Geese, Ducks, &c. bear 

 a much larger proportion of heavy bulky forms than any terres- 

 trial families; and if the Ostrich should at once occur as a 

 striking exception, let us not forget that the giants of this family 

 are known in a fossil state, exceeding far their living represen- 

 tatives. 



Among Mammalia, we have the Whales as the largest class ; 

 and if we should be reminded of the great size of terrestrial 

 Pachyderms, let us not forget that Pachyderms were the promi- 

 nent type of Mammalia during the tertiary period. In connection 

 with these facts it might be shown that natural families through- 

 out the animal kingdom are constructed within limits of size 

 which do not admit of great differences. A comparison of Ceta- 

 ceans with Rodents, of Ruminants with Bats, of Passerine with 

 Gallinaceous Birds, of Sharks with Herrings, of Cod-fishes with 

 Blennoids, of Cuttle-fishes with Pteropods, of Crabs with Ento- 

 mostraca,&c., might easily satisfy the most sceptical that there are 

 natural limits assigned to certain combinations of structure and 

 the material bulk of the animals in which they are manifested. 



After this digression let us return to our consideration of the 

 natural connection of the secondary groups of Vertebrata with 

 the elements in which they live. 



Though the class of Fishes is entirely aquatic, we have among 

 these animals a greater number of marine types, and some which 

 are partly marine and partly fluviatile, or, at periods, marine, or, 

 at periods, fluviatile ; and others which are entirely fluviatile or 

 almost so. And though, at present, it is not plain that fluviatile 

 types on the whole are superior to the marine types, we should 

 not lose sight of the circumstance, that the only living Sauroids, 

 which have so many characters by which they may be connected 

 with the class of Reptiles, and considered as the highest among 

 Fishes, are entirely fluviatile ; both Lepidosteus and Palyptems 

 occur only in fresh waters ; some of the Lepidostei only are known 

 to reach the mouths of rivers emptying into the sea. And though 

 the families of Sharks and Skates are chiefly marine, numbers 

 of them, especially of those types of Skates which have nume- 

 rous fossil representatives during the tertiary period, such as 

 Myliobatis, are known to ascend freely the rivers in tropical re- 

 gions. Among Cyclostomes, the lowest type, Branchiostoma, is 



