258 THE DOCTRINE OF DESCENT. 



external shape and integuments, have exhibited great 

 powers of adaptation, but have not proved capable o£ 

 any further development. Comparative anatomy has 

 vainly spent much labour in attempting to trace the con- 

 dition of the higher animals from the special organiza- 

 tion of the Teleostei, or to explain the peculiarities of 

 the Teleostei from above downwards. It was labour lost, 

 for the solution is to be reached only by the method in- 

 dicated in the derivation of the Teleostei, through the 

 Ganoids, from the shark-like fishes. 



Hence, at the present period, a development is con- 

 cluded with the Teleostei, and we must look to another 

 grade for the transition from the fishes to the amphibians. 

 We find one in the order of the mud-fishes (Dipnoi), 

 scantily represented by only few species (Lepidosiren 

 Protopterus). These fish-like animals, living iri American 

 and African rivers which dry up in the hot season of 

 the year, are fish by right of their skeleton and scales, 

 and some other characteristics; the skull, howfever, al- 

 most resembles that of an amphibian, and they also pro- 

 visionally use their swim-bladders as lungs; and by 

 thus breathing alternately water and air, they set before 

 us the transition of the gill-breathing larvae of the 

 amphibians to the phase of air-breathing. Of the true 

 fishes at the present time, they most nearly approach the 

 family of the Crossopterygii, represented by the Af- 

 rican Polypterus; and the discovery of a very re- 

 markable Australian fish, the Ceratodus, confirms this 

 affinity. 



Through forms thus resembling the Dipnoi, the ad- 

 vance from the fishes to the amphibians was probably 

 accomplished. But, as a scientific friend, profoundly 



