200 



THE GENESIS OF SPECIES. 



[Chap. 



and those sustaining the leg, ankle, and toes, \vith the 

 exception that the toes exceed the fingers in number 



by one. 



Yet these animals are far from being the root-forms 

 from which all the Vertcbrata have diverged, as is evi- 

 denced from the degree of specialization which their 

 structure presents. If they have descended from such 

 primitive forms as Professor Gegenbaur imagines, tlien 



A. SKELETON OF ANTERIOR EXTREMITY OF AN EFT. 



B. SKELETON OF POSTERIOR EXTKEIIITY OF THE SAME. 



they have built up a secondary serial homology — a repe- 

 tition of similar modifications — fully as remarkable as if 

 it were primary. The Plesiosauria — those extinct marine 

 reptiles of the Secondary period, with long necks, small 

 heads, and paddle-like limbs — are of yet higher organi- 

 zation than are the efts and other Amphibia : nevertheless 

 they present us with a similarity of structure l)etween 

 the fore and hind limb, which is so great as almost to be 



