78 CLARK: DISTRIBUTION OF CRINOIDS 



of its fauna from that ocean. Its fauna, therefore, is composed 

 of the same types that occur in the ocean with which it is most 

 intimately connected, with the less plastic and adaptable weeded 

 out and the remainder modified in proportion to the difference 

 between the physics and chemistry of the inland sea and that of 

 the parent ocean. 



All inland seas necessarily differ physically to a greater or 

 lesser degree from the oceans with which they are connected. 

 Their abyssal water cannot form a part of the general abyssal 

 circulation of the oceans, moving slowly anticlockwise about the 

 oceanic basins, and therefore tends to become more or less stag- 

 nant and, under certain conditions, either abnormally cold, as 

 in the x4.rctic, or abnormally warm, as in the Mediterranean. 

 Their surface water, no longer a part of the general superficial 

 oceanic circulation, unless there be an outlet sufficiently large 

 so that a continuous flow is maintained, increases in salinity 

 through an excess of evaporation, as in the Mediterranean and 

 in the Red Sea, or decreases in salinity through an excess of 

 rainfall in the tributary drainage area as in the Baltic. Either 

 of these changes is fatal to a certain percentage of the organisms 

 which enter inland seas, so that necessarily their fauna is com- 

 posed only of the more resistant and adaptable organisms of the 

 parent oceans. 



On account of the physical alteration of the waters of an in- 

 land sea, through which they become less favorable than oceanic 

 waters for the support of marine organisms, inland seas never 

 serve as the cradle for new organic types; their fauna is entirely 

 derived from outside, though the component elements may be 

 forced to undergo a certain amount of modification in order to 

 meet the new conditions imposed. 



An inland sea of the present epoch may be a derivative from a 

 much greater sea of the past, as in the case of the Mediterranean. 



The restriction in size of any large portion of an ocean immedi- 

 ately alters and restricts the circulation of the enclosed water, 

 bringing it more and more under the influence of the local meteor- 

 ological conditions; the effect upon the fauna is therefore exactly 

 the same as though the sea arose through a sinking of the land 



