320 Prof. Dr. G. Pfeffer on the Mutual Relations 



forward in connexion with the discontinuity of the cool-water 

 fauna in the subsurface-water of the tropics — namely, the 

 influx of subterranean rivers. As most subsurface animals 

 may dispense witli pelagic larval stages, submarine river- 

 mouths would possibly form barriers to distribution. And 

 in general I wish to call attention to the fact that all the 

 conditions which may have contributed to the impoverishment 

 of tlie tropical subsurface-fauna need not extend over the 

 whole area to bring about this result. 



Pelagic Fauna. 



Finally, I should like to touch, in a few words, on the 

 bipolarity of the pelagic animals, although this does not 

 really form part of our present theme. The theory has been 

 promulgated, on the strength of isolated results, that the 

 bipolar plankton species only seem to be bipolar, but really 

 have a continuous distribution either through the deeper 

 water (Chun) or in the surface-water (Lohmann) of the 

 tropics. No objection can be offered to either assumption in 

 itself; the Early Tertiary condition would have persisted till 

 the present day, just as has occurred exceptionally among 

 benthos forms. Moreover, all the objections which have 

 been cited above against a general meeting of northern and 

 southern forms in the subsurface-water of the tropics refer to 

 conditions which affect the benthos animals alone. But it is 

 certain that a connexion through the deeper water is scarcely 

 possible for the plankton plants and the animals directly 

 dependent on these. Therefore this theory yields no general 

 principle of explanation applicable to the whole of the con- 

 ditions. But we know enough to be justified in assuming 

 that there was in Early Tertiary times a pelagic fauna of 

 almost universal distribution and composition, and that, 

 therefore, the presence of similar genera and species of 

 plankton animals and plants in the higher latitudes of the 

 earth must date back to the Tertiary period. The pelagic 

 fauna of higher latitudes may therefore be looked upon as a 

 relic of the Early Tertiary fauna, and the connexions now 

 existing through the tropics oflfer no explanation of the exist- 

 ing plankton conditions of higher latitudes, but are to be 

 regarded either as likewise relics of the Early Tertiary fauna 

 or as local and relatively transitory pushings forward of the 

 fauna of higher latitudes. 



Objections to the Theory of Bipolaeity. 

 The position we have reached is thus as follows : — There 



i 



