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 
sufficient 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 limbs. 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. 
