32 Routes from the Sea 



water which is much saltier than the ocean (Beattie, 1932; Pearse, 

 1931, 1932e; Vogel, 1927) . Bony fishes appear to have originated in 

 fresh water and to have spread to all parts of the ocean. 



An estuary has been called "the doorway by which marine forms 

 have populated fresh water." This statement is perhaps in part 

 true, but an estuarine doorway is not wide open and easily passed. 

 There are many difficulties to be surmounted. Many animals 

 struggle long ages to get through and fail. Only a few attain fresh 

 water by this route. 



PooLS^ Swamps^ and Marshes 



Marshes and swamps resemble estuaries and are often connected 

 with them. They are always shallow; the amount of water in them 

 varies more or less at different seasons; they contain considerable 

 submerged and emergent vegetation, such as eel grasses, potamoge- 

 tons, rushes, and grasses. A salty or brackish marsh near the sea 

 may be subject to tidal influences; a fresh-water inland swamp may 

 be overflowed during heavy rains or become nearly dry at other 

 times. Pools are like marshes and swamps in many respects, but 

 are largely without emergent aquatic vegetation. 



Stagnant, shallow, vegetation-filled bodies of water near the ocean 

 often tend to become fresher. In a Long Island, N. Y., salt marsh, 

 Conrad 6C Galligar (1929) found that fresh-water vegetation ad- 

 vanced seaward and that salt-water plants retreated before them. 

 Such migrations were in part due to the filling in of the land. On 

 the coast of Tunis in areas which are periodically submerged by 

 the ocean, dried, and subjected to rainfall, only ephemeral halo- 

 phytes grow; but in similar areas which are not submerged, marshes 

 develop in which Salicornia is the dominant plant (BuroUet, 1926) . 

 Salt in itself is apparently not the chief factor in making the dif- 

 ference between an ocean beach and a marsh, but several factors are 

 concerned. Animals which live in marshes, swamps, and pools are 



