366 E. O. ULRICH — REVISION OF THE PALEOZOIC SYSTEMS 



existence of these is said to be largely owing to such transgressions. The 

 term cosmopolitan faunas sounds well, but when fairly analyzed it is 

 but seldom found to have any basis in fact. Except in a very broa;d 

 generic way, I question if there ever was such a thing as a cosmopolitan 

 littoral or shallow-water bottom fauna. Some of these faunas doubtless 

 attained great distribution in certain geologic times, but, aside from a 

 few highly adaptable species and genera, they have always been limited 

 by barriers of some kind to smaller or larger areas beyond which they 

 could not spread. We have then comparatively few perhaps truly 

 "cosmopolitan" shallow-water species and genera living with varying 

 hosts of species whose geographic range is limited; but as to faunas to 

 which the designation cosmopolitan may justly be applied, only those 

 adapted to pelagic modes of existence could ever become so. Of such 

 the Paleozoic graptolite faunas are notable examples. 



There are some wide ranging bottom faunas and small groups of 

 species that are of great service in correlating between eastern- America 

 and western Europe and others between western America and Eurasia, 

 but I know of no fauna that lived at the same time, or at different times, 

 in all of the oceanic basins of the northern hemisphere. When any con- 

 siderable number of either land or marine species is common to two 

 continents the fact argues strongly for land connection between them 

 at such times; and given the continuous shoreline thus afforded the 

 more vigorous elements of the bottom faunas migrated more rapidly 

 than we can express it in any correlation table yet attempted. But only 

 in rare instances did such faunas pass directly from the Atlantic into 

 the Pacific or from the Pacific into the Atlantic. As to the occasional 

 small associations of species that are so frequently cited as common to 

 these great faunal realms, they are in every instance, especially the corals, 

 of vigorous, long-lived types whose evidence in correlation paleontologists 

 have learned from sad experience to view with suspicion. When critically 

 studied the distant occurrences are usually found to be not only distin- 

 guishable from the species with which they have been identified, but 

 they have often been shown to belong to very different geological ages. 

 How often have authors thought they recognized middle Silurian corals 

 in specimens since found to belong to middle Ordovician, Richmond, late 

 Silurian, or even early Devonian horizons! Therefore, instead of being 

 contemporaneous phases the local appearances of so-called cosmopolitan 

 faunas are more probably either later or earlier stages of slowly modi- 

 fying species, and consequently, if the mutations are not accurately 

 discriminated, of little value in exact correlation. Quite certainly, too. 



