120 EVOLUTION OF THE EARTH 



portance if we would find a cause. In search of this, the mind 

 turns at once to the land-encircling strand or tidal zone, stretch- 

 ing as it does over the many thousands of miles of the coast, 

 the omnipresent frontier between land and sea. Here the 

 conditions are varied, here there is food and light, and here 

 the stimulus to double-breathing is enforced by the twice daily 

 baring of the zone and its consequent transition from aquatic 

 to terrestrial condition and back again with monotonous regu- 

 larity. But in spite of its convenience, few indeed have been 

 the stragglers of the vast army of aquatic life which have ever 

 essayed its passage land crabs and other crustaceans, certain 

 molluscs, a few of the higher fishes such as the mud-skippers 

 Periophthalmus and Boleophthalmus, all of which are impelled 

 to a temporary or permanent migration shoreward by the need 

 of food. And while they have developed accessory respiratory 

 devices, the latter are merely spongy or other extensions or 

 modifications of the water-breathing gills, and in no instance is 

 there developed anything comparable in structure or ultimate 

 efficiency to the terrestrial vertebrate's lung. Such structures 

 as the latter have been developed only in fresh waters, all 

 fishes which show it or its homologue, the swim-bladder, being 

 either fresh-water or the descendants of fresh-water forms. 

 The sharks forsook their ancestral waters before or when the 

 need of such a structure was felt and have never developed it; 

 on the other hand, all other fishes except the cyclostomes either 

 have it or have lost it through specialization. In the modern 

 fish, the swim-bladder has a hydrostatic function in that it 

 serves to alter the specific gravity of the creature and thus 

 enables it to maintain any given level in the water without 

 further effort. As such, it is analogous to the water ballast 

 tanks in a submersible ship, although the mechanism whereby 

 the effect is produced naturally varies. 



The lungs of higher vertebrates are therefore identical with 

 the swim-bladder of the fish and have retained and vastly im- 



