300 THE EVOLUTION OF MAN. 



main gTOup consists of the two classes of true Fishes, and of 

 the Dipneusta ; in these, two pairs of lateral limbs, in the ' 

 shape of many-fingered swimming-fins — one pair of pectoral 

 fins (the fore legs) and one pair of abdominal fins (hind legs) — 

 are originally always present (Figs. 191, 192, Plate XII.). 

 Finally, the third main group embraces the four higher 

 vertebrate classes: Amphibia, Reptiles, Birds, and Mammals; 

 in these the same two pairs of legs exist originally, but in 

 the form of five-fingered feet. The digits or fingers are 

 often fewer than five ; sometimes, also, the feet are quite 

 aborted. But the original parent-form of the entire group 

 had anteriorly and posteriorly five digits (Pentadactylism, 

 p. 123). 



As regards the Phylogeny of the limbs, from their 

 Comparative Anatomy it appears, therefore, that the extre- 

 mities originated in the Fishes, in the Primitive Fishes 

 (Selachii), and were transmitted from these to all higher 

 Vertebrates (all the Amphirhina), first in the form of 

 many-fingered fins, and afterwards as five-fingei'ed feet 

 (Figs. 267-272). The anterior extremity- — the pectoral fin 

 (or the fore leg) — is originally shaped precisely like the 

 posterior extremity — the ventral fin (or the hind leg). In 

 the one, as in the other, the true limb, externally promi- 

 nent, is distinguishable from the internal, concealed girdle, 

 by which the limb is attached to the spinal column — the 

 shoulder-girdle above, the pelvic girdle below. 



The genuine primitive form of the paired limbs, as it 

 existed in the most ancient of the Primitive Fishes durinar 

 the Silurian Period, occurs to this day in perfect preserva- 

 tion in the ancient Ceratodus, and very curious Mud-fish of 

 Australia (p. 119, Plate XII.). In this, both the pectoral and 



