202 THE ANIMALS OF LAKES AND RIVERS 



regard to them. Such discontinuity is common with 

 old-fashioned animals, which owe their local persis- 

 tence to their adaptation to special environmental con- 

 ditions. The tapir in the swampy forests of the Malay 

 region and in South America, but nowhere else, is 

 another example. 



Another very peculiar fish-type in Tanganyika is 

 Polypterus congicus, a member of a ' ganoid ' genus, 

 which is restricted to tropical Africa. Like Protopterus 

 this fish has an accessory respiratory organ, for the 

 organ which forms the air-bladder in many other fish 

 seems here to be used in respiration, perhaps because 

 the fish inhabits the same regions as Protopterus, i. e. 

 rivers whose waters become periodically foul. In very 

 many points of structure, however, the species of 

 Polypterus differ markedly from those of Protopterus. 

 Into these points we cannot go here, but it may be 

 sufiicient to say that many zoologists are of opinion 

 that they show that terrestrial vertebrates arose from 

 a stock common to them and to Polypterus. Though 

 the lung of the dipnoi is much more like the lung of 

 a terrestrial vertebrate than is the air-bladder of 

 Protopterus, it is not believed that dipnoi are near the 

 line of descent of terrestrial vertebrates, largely because 

 of the structure of their Hmbs. It seems as if the limb 

 of Polypterus could more easily give rise to that of 

 a land vertebrate than that of Protopterus. We have 

 already considered several cases of land animals re- 

 acquiring modifications of structure which fit them for 

 life in the water, but Polypterus is interesting as sug- 

 gesting one way in which land animals arose from 

 aquatic ones. Its ancestors were certainly marine, 

 but were driven into fresh water by the competition 

 of better-organized forms which appeared in the sea. 



