98 INTRODUCTION TO EVOLUTION 



bony fishes lived in fresh water, where a salt concentration less than that 

 of sea water (in which, as we have seen, life began) was acquired. When 

 some of the descendants of these ancestral fresh-water fishes returned to 

 the ocean they were faced with the problem of living in a medium hav- 

 ing greater salt concentration than did their own body fluids. They were 

 in the position of bags of difl'erentially permeable membrane enclosing a 

 dilute salt solution and immersed in a more concentrated salt solution 

 (Fig. 5.3). The contents of such a bag would decrease as water passed out- 



Salts 



Water 



Salts 



Water 



Copious hypotonic 

 urine 



water 



Scant and slightly 

 hypotonic urine 



B 



FIG. 5.4. Diagram of osmotic regulation in bony (teleost) fishes. A, 

 fresh-water species. B, marine species. (After Baldwin; from Florkin and 

 Morgulis, Biochemical Evolution, Academic Press, Inc., p. 84.) 



ward through the membrane. In other words, strange as it may seem, ma- 

 rine fishes have to fight desiccation or drying out, owing to loss of water 

 through exposed membranes. 



How could marine descendants of fresh-water fishes meet the problem? 

 From their fresh-water ancestors they inherited kidneys which excreted 

 quantities of water. But marine fishes need to conserve water! Such kid- 

 neys would prove a liability. So we find that in marine bony fishes the 

 kidneys are greatly reduced in structure and excrete but small amounts of 

 urine. But since even that amount of water loss must be compensated for, 



