The Primitives 



the salt in their tissues, bringing their blood plasma to the same 

 consistency as itself. The sharks, however, counteracted this 

 tendency by continually adding soluble urea from their kidneys to 

 their blood and this urea served to stabilise their internal saHnity 

 at a level approximating to that of the sea around them. These 

 tv^o methods of solving the same physiological problem - the one 

 byabsorbingw^ater against the chemical gradient, the other by manu- 

 facturing salts to decrease the gradient - w^ere so dissimilar from 

 one another that they could not be regarded as tw^o stages in the 

 same process. They w^ere too radically different processes and, 

 without any anatomical data whatever, they would have shown 

 that the bony and the cartilaginous fishes did not form part of the 

 same evolutionary sequence but that each was a sequence in itself, 

 though each ran parallel to the other : two different lines of ani- 

 mals each beginning in fresh water and adapting themselves to 

 ever-increasing concentrations of salt in their environment. 



Among biologists it was often thought that the distaste of the 

 public for the dogfish could be explained by their surplus of urea - 

 a substance definitely excretory and therefore not likely to be 

 palatable. But fishermen gave the lie to these theories. It was not 

 only that large areas of the world regarded shark-fin soup as among 

 the most delicate of delicacies, nor that many of the large sharks 

 were highly prized for the taste of their flesh as well as for the 

 richness of their oil : there was another group of cartilaginous fish, 

 closely related to these massive brutes as well as to the little dog- 

 fish, and this group was hunted by the Aberdeen fishermen and 

 eaten by them. In Aberdeen itself, in the ports of the Moray Firth 

 and over a good deal of the farming interior, these fish were in- 

 deed among the favourite foods of the bulk of the population, 

 though their popularity was declining as the southern fashions of 

 England and the industrial belt moved north. This meant, of 

 course, that the price of skates and rays was never very high but 

 it did not prevent large numbers of them from being landed. And 

 they always sold. 



It was again the strangeness of public taste that baffled Jan. In 



125 



