SYSTEM OF NATURE. 61 
Ichthyosaurus appears, in the opinion of palaeontologists, 
to have been constructed somewhat like an Iguana, Plesi¬ 
osaurus like a chameleon, and Mosasaurus like a monitor, 
but all three differed widely from existent forms in having 
the legs converted into natant, instead of ambulant organs. 
Much stress has been laid on the supposed similarity of 
these paddles to those of dolphins and whales ; but, if we 
admit the power of nature to modify, and adapt the struc¬ 
ture of any part to the office which it has to perform, we 
shall see, in this structure of the Enaliosaurians, no more 
than that alteration which an altered mode of life absolutely 
requires : there is no more evidence of an approach to¬ 
wards the whales or dolphins than we see in the ornitho- 
rhynchus, penguin or turtle, except inasmuch as the general 
character of the reptiles more nearly approaches that of 
the fishes. The extraordinary structure of the ribs in Ple¬ 
siosaurus, commonly cited as evidence of its approach to 
the chameleon, seems to me capable of another explanation, 
namely , that they were designed to distend some covering 
analogous to that of the turtles, required to protect their 
body from the attacks of other marine animals, particularly 
the huge Crustacea, whose retreats were among the Alga? 
in which the Plesiosauri probably lurked for their living 
prey ; still this covering was less compact, and its pressure 
in deep water, if not obviated by this apparatus, might 
have been a cause of inconvenience. The explanation 
hitherto given to these sterno-costal arcs implies an useless 
freak of nature, namely , that the animal was provided 
with enormous lungs, required by no peculiar economy, 
but simply to employ this unusual apparatus as a protec¬ 
tion. Whatever solution may be in reserve for this extra¬ 
ordinary structure of the Plesiosaurus, it must be admitted 
