2C8 THE DOCTRINE OF DESCENT. 



powers of adaptation, but have not proved capable of 

 any further development. Comparative anatomy has 

 vainly spent much labour in attempting to trace the 

 condition of the higher animals from the spec'al organ- 

 ization 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 indicated 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 in 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, however, 

 almost resembles that of an amphibian, and they also 

 provisionally 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 

 African Polypterus ; and the discovery of a very re- 

 markable Australian fish, the Ceratodus, confirms this 

 affinity. 



Through forms thus resembling the Dipnoi, the 

 advance from the fishes to the amphibians was probably 

 accomplished. But, as a scientific friend, profoundly 

 versed in the history of development, has pointed out 

 to me, — supporting his remark on the comparison of the 



