8 Pratt, Bipolar Theory. 



"almost entirely, shut off from the deep water of the 

 "temperate zone." On p. 317 he says :" It would seem 

 " that the time which has elapsed since the present surface- 

 " water species of the higher north and south descended 

 " to the depths has not sufficed for a migration beyond 

 " the equator to the opposite hemisphere." 



For the littoral fauna he maintains that, although the 

 connection of northern and southern faunas was con- 

 tinuous through the tropics in mid-tertiary times, actual 

 observations at the present time show that " littoral " 

 species occurring in north and south higher latitudes have, 

 in general, in the tropics, an interrupted discontinuous 

 distribution. 



The fauna of the West Coast of America, from the 

 temperate southern to the temperate northern zone, he 

 states to be " of a nearly homogeneous character, inter- 

 " rupted only in the narrow province of Panama, where 

 " the littoral fauna is of a tropical character. In general 

 " character it may be described as a cool water fauna, but 

 " it has undergone quite remarkable local differentiation. 

 "... This fauna springs apparently from the southern 

 " hemisphere . . . and has crowded out, more or less, 

 " the members of the universal fauna." The narrowness 

 of the continental slope and the presence of the mouths of 

 subterranean rivers would readily afford an opportunity 

 for an interruption of faunal continuity. 



In conclusion he says (p. 322) : " P'aunas of higher 

 " latitudes represent the coeval relics of the almost uniformly 

 " developed and almost universally distributed early- 

 " tertiary faunas, as they have been evolved under the 

 " influence of the cooling of the climate by a process of 

 "separating out and selection. The similarity of the 

 " operating causes secured that the same components of the 

 " old fauna remained behind in both north and south, and 



