AIATTJJJJW, CLIMATE AXD LJ\ OLUTJOX 189 



desert barrier to the south less developed in the early Tertiary than they 

 are to-day, we should expect that the autoehthonic element would be 

 dominant and that Tertiary Egypt belonged to the Ethiopian zoological 

 region, although modern Egypt does not. 



These may serve as instances of the caution with which the geological 

 record must be used in attempting to estimate the position and source of 

 regional fauna?. 



The regions here adopted are based primarily upon the present and 

 past distribution of mammals. Birds, reptiles, amphibinns, fresh-water 

 fishes and the various groups of terrestrial invertebrates are not wholly 

 in accord with this arrangement so far as their present distribution is 

 concerned. This is partly because the means and limitations of their 

 dispersal differ, chiefly, as I shall attempt to show, because so little is 

 known of their former distribution. 



Former Barriers axd Bridges 



The general principle of dispersal on the lines of the present continents 

 is open to an obvious objection. The outlines and connections of the 

 continents were different in former times. The relations of land and 

 water were not the same. In fact, if one depends upon a text-book knowl- 

 edge of geology he may find authority for an assured belief that they 

 were fundamentally and altogether different in different geologic periods. 

 It is necessary therefore to jjoint out that the stratigraphic no less than 

 the life record is a defective one, and that the really proven changes in 

 the .distribution of land and water are limited to those summarized on 

 page 175. The geotectonic hypotheses so ably and })rilliantly elaborated 

 by Suess,^* Haug^^ and other writers, are not facts but theories, and I 

 must confess to a decidedly skeptical attitude towards some of their con- 

 clusions. There are too many gaps in the chain of their arguments ; too 

 many knoA^Ti facts with which their conclusions appear to be inconsistent. 



The permanency of the continental platforms is indicated by the ab- 

 sence of abyssal deposits in their sedimentary succession wherever this 

 has been adequately studied. The platforms have been extensi\ely over- 

 flowed by shallow seas, but such submergences were temporary, and inter- 

 vening periods of uplift are indicated by gaps in the marine succession. 

 Where the geologic records are fragmentary, widely scattered and imper- 

 fectly correlated, there often is a tendency to exaggerate the extent and 

 permanency of such overflows, as also to assume extensi\e unknown con- 

 tinents to account for the existence of clastic sediments which were more 



"E. SuESs : Antlitz tier Erflo. lSSS-1901. 

 '5 E. Haug : Traits de Geolosrie. 1012. 



