Proceedings of the scientific meetings of the Zoological Society of London, 1880

Page 714

1880.] ARRANGEMENT OF THE MAMMALIA. 661 animal should be called " vertebrate." This stage is at present represented only by a singularly modified form, the living Am­phioxus. Thus, in the order of Evolution all the Vertebrata hitherto con­sidered may be arranged in nine stages:-1, that of the Hypichthyes ; 2, that of the Myzichthyes; 3, that of the Chondrichthyes ; 4, that of the Herpetichthyes; 5, that of the Amphibia; 6, that of the Hypotheria; 7, that of the Prototheria ; 8, that of the Metatheria; and, 9, that of the Eutheria. All these stages, except that of the Hypotheria, are represented by existing groups of vertebrated ani­mals, which in most cases are composed of greatly modified forms of tbe type to which they belong, only the Amphibia and the Eutheria exhibiting near approximations to the unmodified type in some of their existing members. It will be observed that 1 have omitted to mention the Ganoid and the Teleostean Fishes and the Sauropsida. I have done so because they appear to m e to lie off the main line of evolution- to represent, as it were, side tracks starting from certain points of that line. The Ganoidei and the Teleostei 1 conceive to stand in this relation to the stage of the Herpetichthyes, and the Sauropsida to the stage of the Amphibia. There is nothing, so far as I can see, in the organization of the Ganoid and Teleostean fishes which is not readily explicable by the application of the law of Evolution to the Herpetichthyes. They may be interpreted as effects of the excessive development, reduction, or coalescence of the parts of a Herpetichthyan1. Similarly, the suppression of the branchiae, the development of an amnion, and of a respiratory extra-abdominal allantois, and that en­largement of the basioccipital relatively to the exoccipitals which gives rise to a single skull-condyle, is all the change required to con­vert a Urodele Amphibian into a Lizard. It is needless to recapi­tulate the evidence of the transition from the Reptilian to the Bird type which the study of extinct animal remains has brought to light. The scheme of arrangement of the Vertebrata which naturallv flows from the considerations now brought forward will stand thus:- 1 That the heart of Butirinus affords a complete transition between tbe char-acteristically Ganoid and the characteristically Teleostean heart, has recently been proved by Boas (Morphol. Jahrbuch). Thus the last remnant of the sup­posed hiatus between the Ganoids and the Teleosteans vanishes.