INTRODUCTION 453 



consequence in the composition of the fauna, they represent invasions 

 from different oceanic basins. 



Though experience shows it to be an unlikely condition, it is yet con- 

 ceivable and physically possible that such diversely originating invasions 

 might have occurred at the same time and their heads approached each 

 other closely. So long as they remained separate, their respective faunas 

 would have maintained their characteristics; but so soon as confluence 

 was established, either one or the other fauna must have dominated the 

 life in the joined embayinents, or a mingling of the two must have ensued. 

 Either result is to be included among the means by which the faunas of 

 different oceanic basins acquired or interchanged specific and varietal 

 units that are of the highest importance in correlating their respective 

 deposits. 



The reason why many correlation criteria fail when they are employed 

 in other provinces than the one in which their value has been unimpeach- 

 ably established is that the circumstances on whose unvarying mainte- 

 nance they depend change from place to place. Indeed, all nature attests 

 the impossibility of conditions continuing unchanged everywhere. Each 

 case, therefore, demands a prior determination of the probable changes in 

 circumstances which have made it different in some corresponding degree 

 from the proved cases that are accepted as the standard by which the 

 others are to be measured. 



These changes commonly are grounded in physical phenomena, which 

 bring about corresponding modifications in the locally prevailing organic 

 facies. The lands that contribute material to build up the floors of the 

 continental and oceanic basins may be higher or lower than the average; 

 one area may be elevated, another depressed; the climate may be moist or 

 uncommonly dry; the run-off of the streams may be rapid and then- 

 waters loaded with sediment, or it may be slow and the waters clear. All 

 these variations would have some effect on the existence and character of 

 the land animals and plants, and on the character of the deposits that 

 were laid down in the water-filled basins or on favored parts of the land; 

 and they would also tend to change the character of the faunas that live 

 in the seas. Conditions favoring the existence of certain kinds of life 

 would be introduced in one set of places, while other places would have 

 become inhospitable to the same organisms, though perhaps exceptionally 

 attractive to other kinds. 



Differential movements of the surface of the lithosphere also may 

 result in great and sometimes complete changes in the local composition 

 of marine faunas. For instance, a continental basin that had previously 

 been connected with the Gulf of Mexico mi^ht be so tilted that the south- 



