Estuaries 23 



animals which ascended from the mouths of great rivers and from 

 estuaries" (Murray, 1895). Annandale (1922) maintains that a 

 large proportion of the animals which leave the ocean to live in 

 fresh and brackish water are primitive types which have been unable 

 to compete with more progressive marine animals. He looks upon 

 estuaries as "a refuge for spent races." In the Ganges Delta he 

 found that most of the animals in fresh and brackish tidal waters 

 were marine in their affinities, but few such animals were to be 

 found in the river above. Annandale was impressed by the continual 

 pressure to enter new habitats which animals displayed, and sur- 

 prised that so few marine species were able to establish themselves 

 in the Ganges River. He affirmed that the attempts to invade fresh 

 water were not anadromous or seasonal migrations but manifesta- 

 tions of a general tendency to spread. Pelseneer (1906) long ago 

 expressed the view that river animals not only arose where tempera- 

 tures were uniformly high, throughout the tropics, but also where 

 the ocean was diluted by heavy rains. He cited Indo-China and the 

 Bay of Bengal as a region which was particularly favorable for 

 the transformation of marine into fresh-water animals. 



Any stream is subject to change in volume and level. The Ama- 

 zon River may rise 12 to 50 feet during the rainy season. The Nile 

 periodically overflows the bottom lands along its banks, and the 

 Egyptians have long correlated their agricultural activities with its 

 rise and fall. Some small streams are intermittent. Their beds are 

 actually dry at certain seasons. The instability which is associated 

 with variations in volume, speed, and level of stream waters is prob- 

 ably not of particular significance in relation to the migrations of 

 marine animals into estuaries, for there is always a stratification of 

 waters in the mouth of a river which empties into the sea. The 

 fresh, usually warmer, water flows over the denser saline water from 

 the ocean. Marine animals may therefore live in the deeper parts 

 of a river without being in fresh water. There is also more or less 



