The Vertebrata in general possess two pairs of appendages or limbs—pectoral and pelvic—both liable to great modifications in adaptation to particular habits. The earliest known vertebrates (early ostracoderms) possessed no true limbs, and this limbless ness is shared by the cyclostomes. These facts have led many authorities to believe that the vertebrates were originally without limbs. But it must be remembered that with the development of a specially elongated form of body, the limbs tend to disappear (many reptiles such as serpents and certain lizards : and the Apoda amongst Amphibia) and this disappearance may be so complete as to leave no vestige even in the embryo.
The advancement of knowledge entails greater caution in accepting dogmatic conclusions as to the evolutionary history of the Vertebrata than was customary a few years ago. It is clear that the normal jawed vertebrates (Gnathostomata) of to-day fall naturally into two distinct sets: (I) Fish, constructed for swimming and (2) Tetrapods adapted for movement upon a solid substratum.
The former fall into a number of subsidiary groups : Elasmo branchii with Holocephali; Crossopterygii; Actinopterygii, in cluding a few more archaic types (sturgeons, gar-pike, bowfins) together with the vast assemblage of modern bony fishes or Teleostei; and the Dipnoi or lungfish. Each of these groups
represents a terminal twig of the evolutionary tree.
Existing tetrapods also fall into well-marked groups—amphib ians, reptiles, birds and mammals. Here again evolutionary con clusions must be limited to broad general principles. On the whole the amphibians are the most archaic while the birds are the most highly evolved. The mammals hold their dominating position not in virtue of high organization in general but rather in virtue of their special development of brain.
In earlier days it was also customary to debate the claims of various groups of invertebrates to represent the ancestral type from which the vertebrates originated. Here again the advance of knowledge has indicated the need for greater caution.
In the opinion of the writer of this article our present-day knowledge of the facts of vertebrate morphology forbids our going farther than to suggest that amongst the post-coelenterate phases of vertebrate evolution was a stage having "features in common" with annelids. It should be mentioned however that at the present time many zoologists are inclined to regard the echinoderms, and still more Balanoglossus and its allies, as being related to the ancestral stock of the vertebrates. (See FISHES, SELACHIANS, AMPHIBIA, REPTILES, BIRDS, ORNITHOLOGY, MAM