114 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



Their origin in the northern land mass or dispersal by this indirect 

 route may explain some of these distributions. Difficulties in the way 

 of this explanation exist in the fact that land bridges may not have 

 their streams favorably directed for the spread of aquatic animals. 

 In Africa and Syria, the long series of depressions from the Jordan 

 Valley to the Central African lakes may once have been a highway 

 for fish migration. The Central American connection between North 

 and South America, however, does not appear to afford opportunity 

 for the exchange of aquatic forms. The conditions governing migration 

 of these forms are, to be sure, quite different from those of the ter- 

 restrial forms. The primary fresh-water animals, such as the fishes, 

 crustaceans, and non-pulmonate snails, all come originally from the 

 sea. Fresh-water groups now restricted to the southern continents may 

 be derived from marine forms which had become restricted to the 

 southern oceans, instead of from ancestral fresh-water forms now 

 extinct in the northern hemisphere. 



The three surviving genera of lungfishes are Lepidosiren in South 

 America, Protopterus in Africa, and Neoceratodus in Australia. This 

 is plainly a relict distribution, as the fossil genus Ceratodus had a 

 wide distribution, throughout the Mesozoic, in Eurasia, Africa, and 

 North America. The temporary and stagnant waters of the tropics and 

 subtropics presented conditions in which a few forms were able to 

 compete with more modern fishes by reason of their ability to breathe 

 air. Protopterus and Lepidosiren burrow into the ground and aestivate 

 for seven to nine months during the dry season. For the same reason 

 Neoceratodus is able to live in the stagnant waters of pools left by 

 the general drying up of the streams in which it lives. The cichlid and 

 characinid fishes of tropical America and Africa (the cichlids in 

 southern India as well) may have been marine and confined to tropical 

 and subtropical seas, whence they entered the fresh waters. Their 

 disappearance in the sea may be laid to the rise of the spiny-rayed 

 fishes, which were at first unable to follow into the fresh waters. Such 

 an explanation of the distribution of Galaxias, confined to the extreme 

 south of South America, Africa, Australia, and New Zealand (Fig. 9) , 

 is highly probable, since this genus is not entirely restricted to fresh 

 water. Members of the Gobiidae, Cottidae, Syngnathidae, Blenniidae, 

 and Elopidae have entered fresh waters independently in different 

 parts of the world. Thus the five species of the elopid genus Megalops 

 are found on the coasts of India and Africa and on the east coast of 

 South America; M. thrissoides occurs in the Magdalena system and 

 in the rivers of West Africa. 51 The gasteropod genus Potamides has 

 probably been independently developed from separate and distinct 



