Animals and the Elements in which they Live. 221 



families, and only a few marine and aquatic ones ; but who 

 can fail to perceive that the marine serpents, with their flat- 

 tened tail, are inferior to the terrestrial genera, and that 

 among these it is a well-known fact there are some with 

 rudimentary posterior extremities, which assigns them a 

 superior rank. Some objections might be drawn from the 

 consideration of the Saurian s, among which, the highest 

 type, the Crocodiles, are chiefly fluviatile ; but it has else- 

 where been shewn that Crocodiles are not truly Saurians of 

 the same type with our Lizards, but modern representatives 

 of a large family, which was very numerous in former geo- 

 logical 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 highest representatives of that almost extinct type of 

 Reptiles, the earliest forms of which were marine, followed 

 by fresh- water. Finally, among Chelonians, the gradation, 

 in connection with the natural elements in which they live, 

 is most striking ; for the inferiority of the 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 in- 

 ternal organs, especially of their ovaries, which contain eggs 

 almost as numerous as those of fishes. Next we place the 

 fresh-water Turtles, with palmate fingers, and highest, ter- 

 restrial 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 of 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 guide to ascertain the natural 

 gradation of types, as the study of their internal structure. 



3. Birds. — ^Ever since the class of Birds has been the object 

 of regular investigation, their aquatic types have been con- 

 sidered as inferior to the terrestrial ones ; and among the for- 

 mer, those which live entirely an aquatic life are decidedly the 

 lowest. They are so, not only on account of the more imper- 

 fect development of their legs, which preserve throughout their 

 embryonic form, but also in the less extensive development of 



