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 with 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 the 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 offer no explanation of the exist- 
ing plankton conditions of higher latitudes, but are to be 
regarded either as likewise relics of the Karly Tertiary fauna 
or as local and relatively transitory pushings forward of the 
fauna of higher latitudes. 
OBJECTIONS TO THE THEORY OF BIPOLARITY. 
The position we have reached is thus as follows :—There 
