no 



CHAPTERS ON EVOLUTION. 



and prove to us the essential similarity of the one-fingered hand of the 

 horse with the five-fingered hands of its higher and lower neighbours. 

 If we investigated the limbs of reptiles and those of the frogs 

 (Fig. 48) and their kind, we should detect a like agreement in 

 fundamental structure with the limbs of man and his nearest allies 

 the upper arm (h\ fore-arm (r), wrist (wr), palm (mc\ and fingers 

 being duly represented. The fishes, as the lowest members of the 



vertebrate group, would, however, 

 present us with grave difficulties in 

 the way of reconciling the structure 

 of their limbs with that of higher 

 animals. Fishes (Fig. 38) possess 

 two sets of fins. These consist 

 of the first set, or paired fins the 

 "pectorals" or "breast" fins (/) 

 and "ventrals"^). The second set, 

 forming the unpaired fins is placed 

 in the middle line of the body, that 

 is, on the back (d l , d*) and on 

 the belly (a) of the fish, whilst the 

 tail-fin (c) also belongs to the un- 

 paired series. It is evident that 

 the "paired" fins must represent 

 the limbs of other vertebrates, such 

 limbs being invariably developed 

 in pairs. The breast-fins (Fig. 38, /) 

 of the fish are in reality its arms, 

 whilst the ventral fins (v) represent 

 its lower or hinder limbs. Com- 

 parative anatomists are not agreed 

 as to the exact or detailed cor- 

 respondence of fish-limbs with 

 those of other vertebrates, but 

 that such a correspondence exists 

 no one may doubt, since, were any 

 other proof wanting, the naturalist might point to the fact that the 

 representatives of the limb-girdles (shoulder-bones and haunch-bones) 

 of higher animals are developed in fishes for the support of their 

 paired fins. The " limbs " of fishes, in short, belong to a very primi- 

 tive type of limb, which, from its early origin and special development 

 in relation to the surroundings of fishes, exhibits but little corre- 

 spondence with limbs of later forms. But we may thus discover the 

 important fact that the limbs of vertebrate animals are modelled on 

 a common plan, and the task of discovering how such identity may 

 be explained, forms a legitimate subject of further inquiry. 



FIG. 48. SKELETON OF FROG. 



