62 FROM FISH TO PHILOSOPHER 



concentrated than the blood— was not to be achieved in 

 any significant degree until the evolution of the mam- 

 mals. For this invention, three hundred-odd million years 

 in the future, the Devonian fishes could not wait. 



So when the elasmobranchs were driven from fresh 

 water into the sea they sought another solution, and 

 found one so simple that it would probably escape the 

 imagination of the most teleologically minded biologist. 

 They simply reduced the renal excretion of urea, letting 

 this substance accimiulate in the blood and tissues until 

 it reached concentrations of 2.0 to 2.5 per cent (figures 

 to be compared with 0.01 to 0.03 per cent in aU other 

 vertebrates). As this urea accumulates it contributes its 

 share to lowering the diffusion pressure of water in the 

 blood, imtil the latter falls below that of sea water and 

 water begins to move by passive osmotic absorption into 

 the body through the giUs. The end result is that the 

 animal, instead of losing water to the sea water, draws 

 water out of sea water at no direct physiological expense. 



For reasons probably not connected with the retention 

 of urea, the skeleton of the cartilaginous fishes, as the 

 name reveals, is not calcified into true bone. The general 

 tendency in evolution is to lose bone rather than to gain 

 it, and in the elasmobranchs this loss has gone to com- 

 pletion: they do not have a calcified bone in the body 

 and are therefore easily dissected with scalpel and small 

 scissors, for which reason the common dogfish is univer- 

 sally used for dissection in the teaching of comparative 

 anatomy. The skin is generally covered with homy scales 

 or dermal denticles (shagreen) having a structure very 

 similar to that of teeth: an inner pulpy core surrounded 

 by a layer of calcareous dentine and, outermost, a film 

 of enamel secreted by the underlying ectoderm. This 

 skin is impermeable to both water and urea, and repre- 

 sents an effective, pliable armor. 



The term Tiabitus,' borrowed from botany, is used by 

 the paleontologist to designate any adaptive feature in 



