294 



THE RISE OF ANIMAL LIFE 



■H kAi^tUOiti/kiilAMlMS i **<■ 



Fig. 13-18. Ancient lobe-finned fish (lofimeria) found off the coast of Africa in 1939, supposedly extinct for 



millions of years. 



water, and that was some means of utilizing 

 the oxygen of the air. The "lobe-finned" fish 

 of the past, as well as the lungfish of today, 

 accomplished this rather satisfactorily. Ap- 

 parently these fish lived in regions where 

 nearly all the water dried up for extended 

 periods during the year, and in order to 

 survive such arid periods, they found it 

 necessary to come to the surface of drying 

 pools and take in air, since there was little 

 oxygen in the water. These animals devel- 

 oped a pair of sac-like lungs from the ven- 

 tral side of the pharynx which allowed 

 them to gulp air during periods when their 

 gills were useless. This, it must be remem- 

 bered, is a primitive condition. Its counter- 

 part is found in present-day fishes in the 

 form of a swim bladder which functions as 

 a hydrostatic organ rather than a lung, since 

 these fish have no need for a lung-like struc- 

 ture at any time during their lives. It is 

 therefore easy to see that once this lung- 

 like structure developed among the "lobe- 

 finned" fish, it was utilized on land and 

 there eventually became the complex organ 

 that is found in such animals as the birds 

 and mammals of today (Fig. 13-19). 



Thus two features, the bony appendage 

 and the lung, made it possible, for animals 

 to attempt the greatest of all transitions — 

 from the water onto land. 



INVASION OF THE LAND: 

 THE AMPHIBIANS 



Of all the changes that have occurred in 

 animals during their long evolution to pres- 

 ent-day forms, one of the most intriguing 

 is the fishes' forsaking of their aquatic life 

 for life on land. According to Romer this 

 was the "result of a happy accident." They 

 would hardly have left the water in search 

 of food, since during these times most ani- 

 mals were aquatic except a few insects, and 

 fish would hardly leave a food-laden world 

 for one almost devoid of food. They had 

 already supplied themselves with a means 

 of breathing air, so this could not have been 

 the cause. Romer reasons that, if drought 

 periods were too extensive, those fish which 

 could breathe air and walk about on the 

 land were able to move to other ponds, and 

 survive. Thus the appendages and lungs 

 aided them in finding water rather than 



