R. T. NEWTON ON FISHES* TAILS. 89 



The homocercal tail is doubtless the most highly developed, that 

 is the most specialized, form to be found among the class of fishes ; 

 and taking the class as a whole, there seems to be a gradual de- 

 velopment traceable from the lowest to the highest. But if we en- 

 deavour to arrange the fishes in a linear series, we find that it 

 cannot be done, and this is more especially the case when the whole 

 organization of each group is considered. 



To take perhaps the most remarkable instance, the Dipnoi. 

 These fishes, although much below the Teleostei, in the form of the 

 tail and structure of the skeleton generally, yet in the possession of 

 lungs and other characters they are much above them. It is 

 obvious, therefore, that the Teleostei are not derived from the 

 Dipnoi, nor vice versa ; the relationship which exists between the 

 different groups of recent fishes is not one of direct descent the one 

 from the other, but that of common parentage, not one line of 

 descent, but many. And this leads us to inquire into the history of 

 fishes in past times, and more especially to see what information can 

 be gained from the study of fossil fishes' tails. 



Part II. 



COMPARISON OF RECENT WITH FOSSIL FISHES* TAILS. 



The possibility of a parallelism between the development of the 

 tail of a high-class fish of the present day and the development of 

 fishes' tails during geological times, has not escaped the notice of 

 those astute observers who have already studied this matter. The 

 history of the different opinions on this subject is fully stated by 

 Profs. Huxley and A. Agassiz (loc. cit.), and it will only be necessary 

 now to mention the more prominent points of this discussion. MM. 

 Agassiz and Vogt were of opinion that such a parallelism did exist, 

 because they were under the impression that the tail of the salmon, 

 which in a young condition is heterocercal, became in the adult truly 

 homocercal (that is, diphycercal as we now understand it) and seeing 

 that among the older fossil fishes the heterocercal tail predominated, 

 and that the homocercal appeared later and superseded it, there 

 seemed to be a kind of parallel development. M. Van Beneden, 

 finding that the earliest condition of the Plagiostomes was diphy- 

 cercal, came to just the opposite conclusion, because, for such a 

 parallelism to exist, the oldest forms ought to have been diphycercal, 

 whereas they were heterocercal. Prof. Huxley was also of opinion 

 that such a parallelism could not be traced, because he had shown 

 that the apparently homocercal fishes were really only heterocercal 



