and the Elements in which they live. 175 



lowest representatives occur even in brackish swamps, and, as 

 soon as attention is called to tliis subject, it cannot fail to be per- 

 ceived 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 Ser- 

 pents with their flattened tail are inferior to the terrestrial ge- 

 nera, and that among these it is a well-known fact there are some 

 with rudimentary posterior extremities which assign them a 

 superior rank ? Some objections might be di-awn from the con- 

 sideration of the Saurians, among which the highest type, the 

 Crocodiles, are chiefly fluviatile ; but it has elsewhere been shown 

 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 geological periods, when their first re- 

 presentatives were marine types provided with fins instead of 

 distinct fingers ; so that, far from being an exception, the Croco- 

 diles of our days, which are either fluviatile or terrestrial, must 

 be considered as the highi:st representatives of that almost extinct 

 type of Reptiles, the earliest forms of which were marine, fol- 

 lowed by freshwater. Finally, among Chelonians the gradation 

 in connection with the natural elements in which they live is 

 most striking, for the inferiority 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 numerous as those 

 of Fishes. Next we place the freshwater Turtles with palmate 

 fingers, and highest, tei'restrial Testudines with their short undi- 

 vided fingers. So that we have in this class, with its various ma- 

 rine and freshwater and terrestrial types, not only a full illus- 

 tration of these laws, but so intimate a connection between gra- 

 dation 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. 



Ever since the class of Birds has been the object of regular 

 investigation, then- 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 ai*e so, 

 not only on account of the more imperfect development of their 

 legs, which preserve throughout their embryonic form, but also 

 in the less extensive development of their wings, in the more 

 scale-like form of their feathers, and the greater number of eggs 

 they lay, and the less care they take of their young, which are 

 liatchcd in a state of development in which they are already pre- 



