6i 



them are very rarely found. The indication, then, is against any 

 extensive development of estuaries in South Carolina during Pleisto- 

 cene times. 



Concerning Geographical Range. — The eastward and westward 

 migration of species along shore is in general more easily done than 

 is the migration northward and southward. In the former case, 

 the temperature, which is the most potent factor by far in influen- 

 cing faunal distribution, remains practically constant, except for 

 changes due to currents in the sea and to fresh-water streams from 

 the land ; while, in the latter case, the northward and southward ex- 

 tent of shore line, temperature continually changes with latitude, 

 and also the currents and rivers play an important role in modifying 

 temperature. It is not so difficult for a northern shallow-bottom 

 species that is not littoral to migrate southward to a Considerable 

 distance, for it can always seek cooler waters by sinking into deeper 

 depths as the southward migration is continued. But it is almost im- 

 possible for a southern shallow-bottom species to migrate very far 

 northward, because it cannot find water of accustomed temperature 

 by rising to more shallow depths, being already in shallow depths. 

 The force of these remarks is plainly evident when it is recalled that 

 the Pleistocene forms were essentially shallow-water forms. They 

 were, therefore, as much limited in their north and south range then 

 by reason of depth conditions as they are today. Furthermore, such 

 species as live between tides and such as are strictly shore forms, 

 getting their food under such conditions, are very much restricted in 

 north and south range, for in this case the privilege of seeking cooler 

 depths as it moves southward is denied the more northern form. 



Also, there is more likelihood of change in bottom conditions close 

 in shore than farther out at deeper depths ; or, putting it the other 

 way, conditions of bottom are more constant at some little distance 

 out than near shore line, where every little inlet has its influences. 

 These things combined point to the fact that the Pleistocene species, 

 no more free to migrate than their living representatives today, can 

 be taken as a safe means for interpreting what must have been the 

 temperature in the Pleistocene Period in the particular latitudes in 

 which they are found fossil. 



Before turning again to the tables, there is another factor affect- 

 ing distribution of marine fauna that must be spoken of, and that is 

 the influence of currents. In this particular case, the influence of the 

 warm Gulf Stream and of the cold Arctic Current has to be consid- 

 ered. In the neighborhood of Cape Hatteras,. according to Dall, close 



