218 L. Agassiz on the Natural Relations between 



But if we review the classes separately, and consider their 

 secondary groups by themselves, we find that the rule holds 

 good, but bears, at the same time, most interesting reference 

 to the order of succession in geological times, as the respec- 

 tive types of any given group are the larger in the present 

 period, whether terrestrial or aquatic ; for being representa- 

 tives of families which had numerous representatives in older 

 periods. Among fishes, we find the largest in the family of 

 Sharks and Skates, Sturgeons, and Garpikes, the first of which 

 are exclusively marine, the second marine and fluviatile, the 

 third entirely fluviatile ; but the three types are either exclu- 

 sively representatives of families largely developed in former 

 geological periods, or so connected with extinct types as to 

 shew 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 

 terrestrial 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 representatives. 



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 pro- 

 minent type of Mammalia during the tertiary period. In 

 connection with these facts, it might be shewn that natural 

 families throughout the animal kingdom are constructed 

 within limits of size, which do not admit of great differences. 

 A comparison of Cetaceans 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 Entomostraca, &c., might easily 

 satisfy the most sceptical, that there are natural limits as- 

 signed to certain combinations of structure, and the material 

 bulk of the animals in which they are manifested. 



