1880.] ARRANGEMENT OF THE MAMMALIA. 6G1 



animal should be called "vertebrate." This stage is at present 

 represented only by a singularly modified form, the living Jm- 

 2)hio.rus. 

 _ Thus, in the order of Evolution all the Vertebrata hitherto con- 

 sidered may be arranged in nine stages : — I, 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 Eiitheria. All these stages, except that of the 

 Hypotheria, are represented by existing groups of vertebrated ani- 

 mals, which in most cases are composed of greatly modified forms 

 of the 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 I have omitted to mention the Ganoid and 

 the Teleostean Fishes and the Sauropsida. I have done so because 

 they appear to me 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 Tcleostei I 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 Herpetichthyan^ 



Similarly, the suppression of the branchiae, the development of an 

 amnion, and of a respiratory extra-abdominal allantois, and that en- 

 largement of the basioccipital relatively to the exoccipitals which 

 gives rise to a single skuU-coudyle, is all the change required to con- 

 vert a Urodele Amphibian into a Lizard. It is needless to recapi- 

 tulate 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 naturally 

 flows from the considerations now brought forward will stand 

 thus : — 



' That the heart o( Btifinnus affords a complete ti'ansition between the char. 

 Bctenstically Ganoid and the characteristically Teleostean lieart, has recently 

 been proved by Boas (Morphol. Jahrbuch). Thus the last remnant ot the sup- 

 posed hiatus between the Ganoids and the Teleosteans vanishes. 



