50 THE OCEAN RIVER 



by the garpikes and sturgeons; the lung fishes; and their rela- 

 tives, the lobe-fins. The few remaining lung fishes are found in 

 the streams of South America, Africa, and Australia, where 

 their air-breathing lungs permit them to survive the dry seasons 

 when the streams disappear and they are forced to estivate in 

 the drying mud of the river beds. It is from the ancestral lobe- 

 fins, inhabitants of the Devonian streams and lagoons, that 

 there evolved not only the successful groups of bony fishes 

 which populate both sea and river today but also the entire 

 line of four-legged land animals and birds and man himself. 

 Some of the lobe-fins, the coelacanths, returned early to the 

 sea but were generally thought to have died out in Mesozoic 

 time, 100 million years ago. When, in 1939, a fisherman hauled 

 his trawl off the South African coast at East London and 

 found a living coelacanth in his net, he was looking at a sur- 

 vivor of the age of dinosaurs. 



The air-breathing lung of the lobe-fins and lung fishes was 

 an outgrowth of the gullet, modified to permit the breathing 

 of air. When some of the ancient lobe-fins began to develop 

 into the fishes which dominate the seas today, the one-time 

 lung lost its air-breathing function and became a hydrostatic 

 organ or swim bladder, which enables a fish to adjust itself to 

 the changes due to the varying depths of water in which it 

 swims. Other lobe-fins, venturing into strange territory, left 

 the fresh water entirely to live on land. We can imagine that 

 the seasonal drying up of lakes and streams during the Devon- 

 ian period did much to hasten this process. As the descendants 

 of these venturesome fishes began to live upon land, the skele- 

 tons of their pelvic and pectoral fins became modified to 

 permit them to act, first as flippers and later as legs. In some 

 such fashion as this the fishes gave rise to amphibians and they 

 in turn to reptiles. 



The oceans and seas today are inhabited by the many sur- 

 viving forms of life whose long evolutionary paths during 

 periods of hundreds of millions of years were confined solely 



