304 GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION. 



is found exclusively in Australia, would seem to imply a different 

 distribution of land and water masses than now exists. But there 

 appears to be no doubt, from their association with a marine fauna, 

 that the early members of the genus were of an oceanic character, 

 and that the fresh-water habit was obtained as the result of later 

 modifications. The peculiar distribution would then be readily 

 accounted for. It appears more than probable that from the order 

 of fishes represented by this genus have been descended at least 

 some of the earlier amphibians ; if this be true, it would seem that 

 the lungs of the dipnoans had been developed when the animals 

 were still more or less strictly marine. The genetic relationship of 

 the two remaining genera, the South American Lepidosiren and 

 the African Protopterus, to Ceratodus still remains to be deter- 

 mined ; but the development of an additional lung in these forms, 

 coupled with the . circumstance of their broad geographical isola- 

 tion, would indicate an ancient differentiation of the two groups of 

 the Dipneumones and Monopneumones. 



In tracing the phylogeny of the class of fishes as a whole we 

 are presented with certain difficulties which in the present state of 

 our knowledge prove an insuperable obstacle to the solution of the 

 problem. Paleontology thus far offers no positive clue as to what 

 might have been the direct ancestors of the animals in question, 

 and until it does so inferences drawn from purely zoological char- 

 acters will be largely in the nature of pure hypotheses. The earli- 

 est fishes that appear, although obviously of a much less perfect 

 structural type, exhibit very nearly the amount of specialisation 

 seen in the modern forms, and are evidently far removed from the 

 period of their first origination. Whether, therefore, the tunicates, 

 which unquestionably possess many points of structural relation- 

 ship with the fishes, are their true ancestors or not, or whether, 

 instead of representing primitive vertebrate types, they are merely 

 the degenerated remains of a more highly constituted ichthyic 

 stock, must still be considered an open question. Nor would it be 

 safe to affirm that the most ancient representative of the class was 

 a form either closely or remotely allied to amphioxus, the lancelet, 

 or that the latter is an ancient type at all. If the views recently set 

 forth by Professor Cope 114 as to the tunicate affinities of Pterichthys 

 and its allies be proved to be correct, then, indeed, the presumptive 

 evidence would be very great for concluding that what have hither- 



