40 Mr. H. G. Seelcy on the Origin 



treatise on limbs, and by otlicrs. Each limb consists of a 

 sequence of bones, of which the number of parts in each seg- 

 ment in most animals increases from above downward, and is 

 usually the same, part for part, in the fore limb and in the 

 hind limb. Thus in the first segment there is one bone, the 

 humerus or femur ; in the second segment two bones, the ulna 

 and radius or the tibia and fibula ; in the third segment three 

 bones, in the proximal row of the carpals or tarsals ; in the 

 fourth segment four bones, in the distal row of the carpus or 

 tarsus ; and in the fifth segment the five digits. Variations 

 occur in great number, but chiefly by suppression of parts; 

 and so true is the correspondence in general, that Professor 

 Humphrey offered an interpretation of the structure by sup- 

 posing that there were originally in each limb five rays, 

 which in the humerus are blended into one, while in the pha- 

 langes they remain more frequently distinct. 



It will be necessary to ask, what are these limbs, and in 

 obedience to what mechanical law are they where found, and 

 why do the fore and hind limbs correspond in their parts ? 



But, besides the limbs, the skeleton possesses the arches 

 with which they articulate : — for the hind limb a pelvis, made 

 up of an ilium, ischium, and a pubis ; and for the fore limb a 

 scapular arch consisting of a scapula and coracoid, and some- 

 times having associated with it a clavicle and interclavicle. 



If we turn to comparative anatomy for an explanation of 

 the phenomena, in sharks and rays the pectoral and pelvic 

 regions will be found to be well developed, and long limbs are 

 attached to them Avhich are already Avell segmented and limited 

 at the sides to fore limbs and hind limbs. In osseous fishes, 

 however, the fins represent, as a rule, more than two pairs, 

 and are often strongly developed down the back. So the first 

 difficulty is, why should there be but two pairs of limbs ? To 

 that question, perhaps, an examination of a skeleton Avill fur- 

 nish an answer ; for the two arches will be seen to be at the 

 two ends of the primitive soft animal enclosed by the skeleton, 

 and at the two chief points of flexure of the skeleton — one 

 where the neck bends with the body, the other where the tail 

 bends with the body ; and in those animals in which there is 

 little or no special flexure in one part more than another, 

 limbs arc wanting, the potential tendency to the development 

 of limbs nevertheless notwithstanding. Now if Ave can dis- 

 cover Avhy they are wanting, we obtain a clue to their law of 

 development. 



In serpents the power expended in motion is distributed 

 equally along the whole body, and there is scarcely greater 

 pressure in one part than in another; so that its intlueuce 



