ANATOMY OF VEETEBEATES. 



183 



mutation of a natatory to a saltatory type of hind-limb, irre- 

 spective of efforts and exercises through successive generations 

 producing and accumulating small changes, and independently of 

 any selection by nature of such generations as were enabled, 

 through the accidental variety of a slightly lengthened hind-limb, 

 to conquer in the battle of life, and to transmit the tendency 

 towards such disproportion to their posterity. 



If the law by which so much of the change of structure adapted 

 to terrestrial life takes place in the active independent aquatic 

 animal be a mystery, and seeming exception, it does not the less 

 impress the believer in the derivative origin of species with the idea 

 of unseen and undiscovered powers, that may operate in produc- 

 ing such result, ' according to a natural Law or Secondary Cause.' ' 



The hind-limb of the Frog (Rand) closely accords at first with 

 that of the Menopome ; a rib-like continuation, fig. 42, 62, of the 

 pleurapophysis, pi, of the last abdominal vertebra, gives attachment 

 to a short femur, 65 ; ossification of the shorter tibia, 56, and fibula, 57, 

 speedily unites them proximally ; five subequal digits bud out of 

 the primitive fin-like projection from the integument; and a simple 

 cartilaginous tarsus, 68, at first intervenes between the toes and 

 the leg. The due length and power of the hind-limb is produced 

 by elongation of all its elements, including the iliac parts of the 

 sustaining arch. In the Toad (Bufo) the sacral process, or anchy- 

 losed riblets, transmitting thereby the weight 

 of the trunk upon the legs, are depressed and 

 expanded at their extremities ; in JPipa, 

 fig. 44, B, s, remarkably so, and resting upon 

 the anterior halves of the ilia. In the Toad 

 the femur is shorter than the ilium, and the 

 tibia is shorter than the femur. In the Frog, 

 fig. 44, contrary proportions prevail. The im- 

 pulse of the hind-limbs is applied, in all tail- 

 less Batrachia, to the hindmost part of the 

 body, beyond the lengthened coccygeal style, 

 fig. 114, a, g, by the remarkable backward 

 production of the ilia, ib. A, 62, which expand 

 and unite, forming a symphysis, above the 

 acetabula ; thence they transmit the impulse 

 of the limbs to the short and strong trans- 

 verse processes of the sacrum, a. A pair 

 of commonly anchylosed semicircular bony 

 plates, ' ischia,' 63, unite with the iliac sym- 

 physis to form the lower half of the joints for the femora, 65. 



1 cxli. p. 86. 



114 



Front and side views of tlie 

 pelvis, Frog 



