ANATOMY OF VERTEBRATES. 



183 



imitation of a natatory to a saltatory type of hlnd-limlj, irre- 

 spective of efforts and exercises throuo-h successive o-cnerations 

 producing and accumulating small changes, and independently of 

 any selection liy nature of such generations as were enal)led, 

 through the accidental A'ariety of a slightly lengthened hind-limb, 

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

 towards such dispro]iortion to their posterity. 



It the law l)y 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 lielicver in the derivative origin of species with the idea 

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

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

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

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

 pleurapophysis, ^/, of the last abdominal vertebra, gives attachment 

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

 S]icodily unites them proximally ; five subeqnal digits bud out of 

 the primitive fin-like i)rojection from the integument; and a simple 

 cartllaginouR tarsus, 08, at first intervenes between the toes and 

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

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

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

 loscd riblets, transmitting thereby the weight 

 of the trunk upon the legs, are depressed and 

 expanded at their extremities ; in Pipo, 

 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 

 tiliia is shorter than the femur. In tlie 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 lengtJiencd coccygeal style, 

 fig. 114, A, f/, \)y the remarkable backward 

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

 and unite, forming a symphysis, above the 

 acctabula ; 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,' C3, unite with the iliac sym- 

 physis to form the lower half of the j(3ints for the femora, 65. 



' CXLI. p. 86. 



IM 



