PHYLOGENY. 97 



bony vertebrates in the reduction of the segments be- 

 tween the lower jaw proper and the skull, from four to 

 none. In the fishes we have the hyomandibular, the 

 symplectic, the inferior quadrate, and the articular. 

 In the Batrachia, reptiles and birds, we have the quad- 

 rate 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 mechanical needs of the limbs. In 

 this regard the air-breathing types display wide di- 

 versity from the gill-bearing types or fishes. In the 

 latter, the lateral elements unite below without the in- 

 tervention of a median element or sternum, while in 

 the former the sternum, or parts of it, is generally pres- 

 ent. Either arrangement is susceptible of much me- 

 chanical 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 pres- 

 ence of sternal elements, with both clavicle, epicora- 

 coid, and coracoid bones on each side, gives the Rep- 

 tilia the highest place for mechanical strength. The 

 loss of the bony coracoid seen in the tailed Batrachia, 

 and loss of coracoid and epicoracoid in the Mammalia, 

 constitute an element of weakness. The line is not 

 then one of uniform ascent in this respect. 



The absence of pelvis, or its extremely rudimental 

 condition, in fishes, places them at the foot of the line 

 in this respect. The forward extension of the ilium in 

 some Batrachia and in the Mammalia, is to be com- 

 pared with its backward direction in Reptilia, and its 

 extension both ways in the birds. These conditions 



