88 THE HISTORY OF THE NORTH ATLANTIC 



an inhabitant of shallow water, and very limitedly locomotive 

 when young, has survived all the changes since the Carbon- 

 iferous age, and has spread itself over the whole northern 

 hemisphere, 1 though a warm water rather than Arctic type. 



I have collected fossil oysters in the Cretaceous clays of the 

 coulees of Western Canada, in the Lias shales of England, in 

 the Eocene and the Cretaceous beds of the Alps, of Egypt, of 

 the Red Sea coast, of Judea, and the heights of Lebanon. 

 Everywhere and in all formations they present forms which are 

 so variable and yet so similar that one might suppose all the 

 so-called species to be mere varieties. Did the oyster originate 

 separately on the two sides of the Atlantic, or did it cross over 

 so promptly that its appearance seems to be identical on the 

 two sides ? Are all the oysters of a common ancestry, or did 

 the causes, whatever they were, which introduced the oyster in 

 the Carboniferous act over again in later periods ? Who can 

 tell ? This is one of the cases where causation and develop- 

 ment the two scientific factors which constitute the basis of 

 what is called evolution cannot easily be isolated. I would 

 recommend to those biologists who discuss these questions to 

 devote themselves to the oyster. This familiar mollusk has 

 successfully pursued its course, and has overcome all its enemies, 

 from the flat-toothed selachians of the Carboniferous to the 

 oyster dredges of the present day, has varied almost indefinitely, 

 and yet has continued to be an oyster, unless, indeed, it may at 

 certain portions of its career have temporarily assumed the 

 guise of a Gryphaea or an Exogyra. The history of such an 

 animal deserves to be traced with care, and much curious in- 

 formation respecting it will be found in the report which I have 

 cited in the note. 



But in these respects the oyster is merely an example of 

 many forms. Similar considerations apply to all those Pliocene 

 and Pleistocene mollusks which are found in the raised sea 

 1 White, Report U. S. Geol. Survey, 1882-83. 



