320 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



From a physiological or functional standpoint it has the same 

 significance, since it is far less effective both for support and for 

 movement than is the segmented osseous skeleton. That this is 

 a prevalent condition of the lower Vertebrata is well known. 

 The bony fishes and Batrachia have but little of the primitive 

 cartilage remaining, and the quantity is still more reduced in the 

 higher classes. Systematically, then, the vertebrate series is in 

 this respect an ascending one. The Leptocardii are membra- 

 nous ; the Marsipobranchii and most of the Elasmobranchii 

 cartihiginous ; the other Pisces and the Batrachia have tlie 

 basicranial axis cartilaginous, so that it is not until the Eeptilia 

 are reached that we have osseous sphenoid and presphenoid bones, 

 such as characterize the birds and mammals. The vertebral 

 column follows more or less inexactly the history of the base of 

 the skull, but its characters do not define the classes. 



As regards the suspensor of the lower jaw, the scale is in the 

 main ascending. We witness a gradual change in the segmenta- 

 tion of the mandibular visceral arch of the skull, which clearly 

 has for its object such a concentration of the parts as will produce 

 the greatest effectiveness of the biting function. This is accom- 

 plished by reducing the number of the segments, so as to bring 

 the resistance of the teeth nearer and nearer to the power, that is, 

 the masse ter and related muscles, and their base of attachment, 

 the brain-case. This is seen in bony Vertebrates in the reduction 

 of the segments between the lower jaw proper and the skull, from 

 four to none. In the fishes we have the hyomandibular, the sym- 

 plectic, the inferior quadrate, and the articular. In the Ba- 

 trachia, reptiles and birds, we have the quadrate and articular 

 only, while in the Mammalia these elements also are wanting. 



The examination of the pectoral and pelvic arches reveals a 

 successive modification of the adaptation of the parts to the me- 

 chanical needs of the limbs. In this regard the air-breathing 

 types display wide diversity from the gill-bearing types or fishes. 

 In the latter, the lateral elements unite below without the inter- 

 vention of a median element or sternum, while in the former the 

 sternum, or parts of it, is generally present. Either arrange- 

 ment is susceptible of much mechanical strength, as witness the 

 Siluroid fishes on the one hand, and the mole on the other. The 

 numerous segments of the fishes' pectoral arch must, however, be 

 an element of weakness, so that from a mechanical standpoint it 

 must take the lowest place. The presence of sternal elements, 



