388 Agassiz on the Relations between Animals 
succession in geological times, as the respective types of any 
given group are the larger in the present period, whether terres- 
trial or aquatic, for being representatives of families which had 
humerous 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 sec- ~ 
ond marine and fluviatile, the third entirely fluviatile ; but the 
three types are either exclusively representatives of families largely 
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, &e., bear 
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 
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 shoul 
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 
shes, are entirely fluviatile; both Lepidosteus and Palypterus 
occur only in fresh waters; some of the Lepidostei only are 
en 
