Time-table for North America. 7 



nental uplift which marks the close of periods. During the 

 middle age of the periods, when the oceanic transgressions are 

 greatest, the faunas throughout a continent are most alike in 

 composition and have the greatest number of species in com- 

 mon ; they have therefore been called by Chamberlin " cos- 

 mopolitan faunas." Again, faunas of the same age are most 

 dissimilar in the early times of the periods, when the oceanic 

 realms are most localized and the transgressions upon the con- 

 tinents are smallest. Similar restriction also takes place dur- 

 ing the closing age of the periods, though at these times there 

 are many more hold-over species from the earlier, widely dis- 

 persed faunas ; in other words, there is no marked introduction 

 of new organic types during 'the recession of the continental 

 seas. However, when the oceans again spread over the con- 

 tinents, a long time has elapsed, many of the old familiar forms 

 have disappeared under the stress of restricted habitat, and 

 new forms have been developed, the prophets of a new period 

 and indicative of the next trend in evolution. 



The appearance of identical fossil genera of land animals, and 

 more especially of mammals, in two continents that are now 

 widely separated, is often taken as proof of the former con- 

 nection of the two areas by land bridges that have since vanished 

 beneath the sea, or of migration by routes in high latitudes 

 which are no longer available because of the present frigid 

 climates. This method of correlation is undoubtedly correct 

 in the main, but as genera apparently alike have been developed 

 under similar stimuli from unrelated stocks (" parallel develop- 

 ment " and " homoeomorphy "), single appearances on two con- 

 tinents cannot be accepted as migrant individuals from a 

 common center of evolution and dispersal until the ancestral 

 relationship (phylogeny) has been established in each case. In 

 the same way, single marine invertebrate genera appearing in 

 two or more oceanic realms at the same time may be parallel 

 developments or independent evolutions from different species 

 of the same genus. 



When the lands are least overlapped by the oceans, the fresh 

 waters more often record themselves, and especially is this true 

 in the areas where mountains have just been born. Unfor- 

 tunately, however, these, the " continental deposits," are 

 frequently devoid of fossils, because here the great bulk of 

 organisms live, not in the sediments, under the protective 

 covering of water, but on the dry land, where after death, 

 instead of receiving natural burial, they are exposed to the 

 atmosphere and thus either eaten by other animals or attacked 

 by bacteria and so reduced through further oxidation into the 

 elements from which they came. In the same way, plants fail 

 to fossilize. As for animals living in the fresh waters, they 



