FOSSILS AND PEDIGREES 359 



but no remains have yet come to light which give any clue to 

 the Hues of descent of these more specialised types. 



The cartilaginous nature of a large part of the skull, coupled 

 with the continuous median fins and the apparently symmetrical 

 tail, led many of the older authorities to suppose that the 

 existing Lung-fishes were the most primitive members of the 

 group, and more nearly related to the ancestral stock than the 

 Palaeozoic forms, Hke Dip- 

 terus, which were regarded 

 as highly specialised offshoots. 

 More recently, however. Pro- 

 fessor Dollo has shown fairly 

 conclusively that evolution 

 has taken place in exactly 

 the opposite direction. The 

 existing Lung-fishes, accord- 

 ing to his view, have been 

 derived from the archaic 

 forms by a process of gradual 



degeneration, accompanied 



also by some speciahsation, 



and they appear to stand 



in much the same relation 



to the Dipterus-like forms as 



do the Sturgeons to their 



Palaeoniscid ancestors. Corn- 



mencing with Dipterus, it is 



possible to select a series of 



genera, living and fossil, 



which not only fall into place 



in the scale of geological time, 



but at the same time illus- ^ig- 130. 



f*^of^ fV.^ T^rnh;ihlp line of Jaws and teeth of the Austrahan Lung- 



trate the probable line ou f^^^^^Epiceratodusforsterilxh- 

 descent of the living forms. 



In addition to the reduction in the number of dermal bones 

 of the skull, other features of degeneration (or specialisation) 

 are exhibited by the gradual assumption of a more eel-like 

 shape the extension of the median fins and their union to torm 

 a continuous fold round the hinder end of the body, the change 

 in the tail from a heterocercal to an outwardly symmetrical 

 form, and the replacement of ganoid plates by thin cycloid 

 scales. In the existing African {Protopterus) and South American 

 {Lepidosiren) Lung-fishes the story has been carried a step or 



