PRINCIPLES OF PALEONTOLOGIC CORRELATION 675 



the two provinces, and most of the genera. During Cretaceous 

 time there must have been easy intercommunication between 

 Europe and America by a submerged continental shelf, keeping 

 well within the temperate conditions. This state of things per- 

 sisted through the Eocene, for the same similarity of faunas has 

 been noted on the two continents in strata of that period. 



On a still larger scale the same sort of correlation has been 

 carried out between the western American and the Alpine Upper 

 Trias, where many of the species and nearly all the genera are 

 common to the two localities, although they are not in the same 

 province, nor even in the same faunal region, and separated by 

 six thousand miles in a direct line, and by at least twelve thou- 

 sand miles by the nearest direction in which migration could 

 have taken place. Yet there must have been easy intercommu- 

 nication by continental margins from the American region, 

 through the Oriental, to the Mediterranean region, along the 

 borders of the ancient Mesozoic "Tethys" or central Mediter- 

 ranean sea, that stretched eastward from the Alpine province 

 through Asia Minor, India, and at least to the borders of China 

 and Japan. 



Direct correlation is possible even where there is no com- 

 munity of species, if a number of characteristic short-lived genera 

 be common to the two regions. Thus the student of stratigraphic 

 paleontology has no difficulty in correlating the Meekoceras beds 

 of the Lower Trias, whether they occur in the Himalayas, 

 Siberia, California, or Idaho ; the fauna is essentially similar in 

 all these regions, although species common to them are not yet 

 identified. These faunas must have had a common origin either 

 in one of these regions or in some unknown outside region, and 

 reached the American and Asiatic provinces by migration. The 

 place of origin may have been distant enough for the migrant 

 faunas to have become specifically differentiated by the time 

 they had reached their distant goals. In fact this is probably 

 by far the more common case. Absolute specific identity 

 between regions as distant as Asia and America must be rare ; 

 in reality there are usually in common to such regions only what 



