258 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 special 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 larve 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 
