390 Agassiz on the Relations between Animals 
structural and embryological evidence, agrees most fully with the 
gradation of the eleinents in which they live. Among Batrachi- 
ans we have chiefly fluviatile and terrestrial families. The Ich- 
thyodes, or Batrachians with permanent branchie, are all aquatic, 
and acknowledged the lowest in the class. Some of their lowest 
representatives occur even in brackish swamps, and, as soon as 
attention is called to this subject, it cannot fail to be perceived 
that the Frogs with their more or less palmate fingers, and their 
more aquatic habits, rank lower than the Toads with their divided 
fingers and terrestrial mode of life. Among Ophidians we have 
chiefly terrestrial. families, and only a few marine and aquatic 
ones; but who can fail to perceive that the marine serpents with 
their flattened tail, are inferior to the terrestrial genera, and that 
among these it is a well known fact there are some with rudi- 
mentary posterior extremities which assigns them a superior rank. 
Some objections might be drawn from the consideration of the 
Saurians, among which, the highest type, the Crocodiles, are chief- 
ly fluviatile ; but it has elsewhere been shown that Crocodiles are 
n@ truly Saurians of the same type with our Lizards, but modern 
representatives of a large family which was very numerous in 
former geological periods, when their first representatives were 
marine types provided with fins instead of distinct fingers; so 
that, far from being an exception, the Crocodiles of our days which 
are either fluviatile or terrestrial, must be considered as the high- 
est representatives of that almost extinct type of Reptiles, the 
earliest forms of which were marine, followed by fresh water. 
inally, among Chelonians the gradation in connection with the 
natural elements in which they live is most striking, for the infe- 
riority of marine Turtles is as plain as it can be, not only in the 
form of their organs of locomotion, but even in the peculiarity of 
many of their internal organs especially of their ovaries, which 
contain eggs almost as numergus as those of Fishes.~ Next we 
place the fresh water Turtles with palmate fingers, and highest, 
terrestrial Testudines with their short undivided fingers. So that 
we have in this class with its various marine and fresh water and 
terrestrial types, not only a full illustration of these laws, but so 
intimate a connection between gradation of structure and mode o 
living in various elements, as to lead to the conviction that the 
mere mode of living might in many instances be almost as safe a 
uide to ascertain the natural gradation of types, as the study of 
their internal structure. . w 
Ever since the class of Birds has been the object of regular 
investigation, their aquatic types have been considered as inferior 
to the terrestrial ones, and among the former, those which live 
entirely an aquatic life are decidedly the lowest. They are s0, 
not only on account of the more imperfect development of their 
_ which preserve throughout their embryonic form, but 
a’ 
Pe 
7 
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