XXXV 



Second. To what other forms is that fish most nearly related ? 



(1.) In response to the first question, no doubt has been expressed, the 

 admission that the Dipnoans (and & fortiori the Lepidosirenids) are most 

 nearly allied to the Batrachians being universal, even among those who 

 place in the " highest" rank the Elasmobranchiates. 



(2.) In response to the second question, the admission {now universal) 

 that the Dipnoans are fishes determines the question that they are to be 

 treated as fishes, and collocated in the series of fishes. 



And now, if it becomes necessary to enumerate the forms of animals in 

 a linear series, there are the alternatives of doing so at the expense of one 

 or the other classes, for (it is scarcely necessary to add) a linear series can- 

 not exhibit all the affinities of living beings. 



But it being admitted that the Dipnoans are Fishes, it would surely be 

 unreasonable to overturn the natural series of the latter only to exhibit 

 representatives thereof in juxtaposition to the Batrachians. The alterna- 

 tive then remains to accommodate ourselves to the facts of the case, to 

 build upon the sure foundations furnished by the concurrent admission of 

 what are the most generalized types, and then successively approximating 

 whatever forms are most nearly related to the preceding, and without ne- 

 cessary consideration of where we may end — for, commencing aright, we 

 cannot wander very far from the right path. 



And if it is admitted that the sequence up to the Dipnoans is not an 

 unnatural one, we have chiefly to inquire what are the forms most nearly 

 related to them. It must be admitted that (among living forms) the 

 Crossopterygians are nearest related on one side, and the Batrachians on the 

 other, but the former in very much closer bonds than the latter. And with 

 this concession, we have next to inquire what are the most nearly related to 

 the Crossopterygians. And, in the direction of the Teleosts, it can scarcely 

 be denied that the Ilyoganoids are such forms. The relations of the last to 

 the Teleosts are so obvious that it is unnecessary to proceed further. 



And if it be demanded, how then can the facts be best expressed ? refer- 

 ence may be made to the genealogist. He has to deal with similar prob- 

 lems so far as linear sequence is concerned, and the methods employed by 

 him may be advantageously adapted in biological taxonomy. 



Let the Dipnoan be considered as the eldest representative of the an- 

 cestral stock equally of the Fishes and of the Batrachians, from which the 

 respective forms have descended, diverging more and more in the course 

 of time. Of course, the Dipnoan will be more nearly related to the Ba- 

 trachians than the Fishes diverging from the same stem — as the grand- 

 parent is more nearly related to the children of two sons than such grand- 

 children by the different sons are to each other. 



But the genealogist takes the eldest branch of the family, and continues 



