567.3 245 



III. 



Sharks as Ancestral Fishes. 



PRIMAEVAL sharks are certainly to be looked upon with great 

 interest by the student of fishes ; for recent discoveries have 

 seemed to warrant the belief that by a better knowledge of these 

 ancient forms the clearest light is to be thrown on the problem of 

 ancestral fishes. The study of living sharks has been in many ways 

 an unsatisfactory one ; for, while many of their adult structures are 

 usually conceded to be generalised and therefore primitive, other and 

 important structural characters must be regarded as much modified. 

 Indeed, on this account, Kupffer, Beard, Pollard, Dollo, Retzius, and 

 Klaatsch, among others, appear to have come to regard the sharks as 

 perhaps a more divergent group than some of the archaic ganoids. 

 Thus, sharks appear to have lost entirely their ancient mode of repro- 

 duction — the eggs are no longer fertilised externally, " claspers " 

 being present in the male of every recent shark 1 ; the eggs are now 

 few in number, enormous in size, and elaborate in their range of 

 protective capsules ; their developmental characters, especially those 

 of the earliest and of larval stages, are remarkable in their adaptations. 

 Whether the recent sharks, in spite of their many specialised 

 characters, are yet to be regarded as representing most nearly the 

 ancestral stem-form of the fishes, becomes, accordingly, a doubtful, 

 if an interesting, question. Considered on broader grounds, they have 

 certainly a strong claim to be looked upon as primitive ; the 

 characters of their integument and skeleton, of their circulatory, renal, 

 and nervous systems, and end organs, together with their more 

 numerous gills and body-segments, and the simpler condition of tail 

 and fins, stand as weighty evidence in favour of their generalised 

 position. 



On the other hand, it becomes equally clear that this problem 

 can be settled definitely only when evidence shall be forthcoming 

 to demonstrate that the ancestors of the living sharks were, 

 not only clearly shark-like (i.e., not approaching in structural 

 characters the stems of other groups), but were also clearly lacking 

 in whatever might be interpreted as the specialised features of their 

 descendants. The solution of the entire problem, accordingly, must 

 naturally fall to the palaeontologist, and we must await the discovery 



1 Professor Liitken, in a recent conversation with the writer, stated that he had 

 determined functional claspers in L^margus. 



