BATRACHIANS. 123 



of the yet extant Amphibians have a smooth and slippery 

 skin. The latter, also, are on an average smoother than the 

 former, and must be regarded as their stunted posterity. 



Among the Amphibia of the present time we are, 

 therefore, unable to find any forms that are directly referable 

 to the pedigree of the human race, or that are to be re- 

 garded as ancestors of the three higher Vertebrate classes ; 

 yet, in important points of their internal anatomical struc- 

 ture, and especially in their germ-development, they cor- 

 respond so closely with us, that we are justified in affirming 

 that between the Double-breathers (Dipneusta) on the one 

 hand, and the three higher Vertebrate classes (grouped 

 together as Amniota) on the other, there existed a series of 

 extinct intermediate forms which, if we had them before us, 

 we should class among Amphibia. The whole organization 

 of the extant Amphibia represents a transitional group of 

 this kind. In the important matters of respiration and 

 circulation of the blood, they are still closely allied to the 

 Double-breathers, although in other respects they rise above 

 the latter. This is especially true with respect to the ad- 

 vanced structure of their limbs or extremities. The latter 

 here for the first time appear as feet with five digits. The 

 thorough researches of Gegenbaur have shown that the fins 

 of Fishes, concerning which very erroneous views were pre- 

 viously held, are feet with numerous digits ; that is to say, 

 the several cartilaginous or osseous rays, many of which occur 

 in every Fish-fin, correspond to the fingers or digits on the 

 limbs of higher Vertebrates. The several joints of each ray 

 correspond to the several joints of each digit. In the Double- 

 breathers the fin yet retains the same structure as in Fishes, 

 and it was only gradually that the five-toed form of foot, 



