112 FROM FISH TO PHILOSOPHER 



Up and down). It is required of them only that they 

 swim about in their crepuscular and static world waving 

 their phosphorescent lures to guide unwary victims into 

 their gaping maws; they epitomize life living itself out 

 in the nearest approach to absolute quietude, and they 

 are correspondingly delicate and fragile things. Though 

 geologically recent in origin, they are, in respect to spe- 

 cialization, farthest removed from the primitive fresh- 

 water forms. 



None of the elasmobranchs, in spite of their long resi- 

 dence in the sea, is aglomerular; having always had ade- 

 quate water available for filtration, they have had no 

 need to abandon the glomeruli. 



How long the marine teleosts have lived in the sea is, 

 as we have noted, an unsettled question, but we believe 

 that the structure of the kidney a£Fords some information 

 on this point. The primitive 'ganoid' fishes— the gar pike 

 (Lepidosteus) y the bowfin (Amia), the bichir of the 

 Nile (Polypterus) , the sturgeon (Acipenser), the pad- 

 dlefish (Polyodon) of the Mississippi— are either fresh- 

 water forms or migrate into fresh water at some period 

 in their life cycle, a statement that is equally true of the 

 'centrally placed' teleosts. With rare and obvious excep- 

 tions, all permanently fresh- water fishes are phyletically 

 primitive. And all primitive, fresh- water fishes have 

 large and well-vascularized glomeruli. The supposition 

 that these fishes have recently invaded fresh water from 

 the sea is incompatible with the structure of their 

 glomeruli: had they had a long marine history one 

 would expect them to show the reduction in glomerular 

 development characteristic of the more specialized ma- 

 rine forms, and we believe that, on the evidence afforded 

 by the kidney, we may infer that the 'ganoids' and 

 primitive fresh-water teleosts have had a continuous 

 fresh-water (or migratory) history since their origin in 

 the Devonian. 



Conversely, the marine fishes are under continuous 



