Thus for all practical purposes birds may be said to be characterized by wings, which almost acquire the dignity of class characters from their influence on the respiratory function. But in some birds it has been thought that no bone of the fore-limb was ever developed[V]; and the difference between such a phenomenon and the wing of a Swift, for example, is one almost of infinity, as compared with any other aspect that the anterior limb might have assumed. Therefore, since a bird may part with its fore-limbs and yet remain a bird, I infer that it might apply its fore-limbs to the ground, become a quadruped, and be a bird still. And if in this process the other structures remained unchanged, no one would regard the modification as more than an ordinal one. But should the vertebræ change also, or the pelvis, or the covering of the integument, or the jaws become toothed, then, although the heart and lungs and brain of the imaginary animal retained their class characters, the functional differences being more than those of an order would constitute it a sub-class. [Footnote V: According to Prof. Owen, in Dinornis.] In the same way it is conceivable that serpents may have existed with well-developed limbs, and if they retained their other characters the limbed forms would constitute a sub-order of serpents; but if to these characters they added a closed palate united to the cranium, they would constitute a new order of reptiles. A chelonian might be entirely deprived of its bony covering, and it would still be a chelonian, differing only as a separate family. So that structures which to the eye appear fundamental may be lost without affecting an animal's systematic position, just as animals while resembling each other in form may possess dissimilar organization. Even with the living or typical Reptilia, naturalists are divided as to the number of ordinal groups into which they naturally fall. It is however generally agreed that the Amphibia or Dipnoa of Fitzinger, have no near affinity with the true reptiles. Milne-Edwards, Van der Hoeven and Agassiz make the remainder into three orders, as did Cuvier: Chelonia, Sauria, Ophidia. Stannius, Gray, Owen and Huxley, on the other hand, by dividing the Saurians make four orders, to which Dr Günther by his description of Sphenodon has given evidence of a fifth: Crocodilia, Chelonia, Sauria, Ophidia, (Rhynchocephalia.) De Blainville in a remarkable classification (1816), made three orders, Chelonians, Emydosaurians [crocodiles], and Saurophidians; the latter group being subdivided into Saurians and Ophidians. In his "Handbuch der Anatomie der Wirbelthiere" Stannius unites the Crocodilia and Chelonia into a group called Monimostylica; while of the Sauria and Ophidia he makes another group called Streptostylica. Similar groups were made by Dr Gray, and named Cataphracta and Squamata. They are identical with the "cuirassed" and "scaly" reptiles of Dumeril and Bibron.