318 Prof. Dr. G. Pfeffer on the Mutual Relations 
exists even now a continuous connexion through the sub- 
surface-water of the tropics between the identical genera and 
species of the higher latitudes. Curiously enough, this 1s 
corroborated by actual observations only in part as regards 
genera, and not at all as regards species. And so it seems 
certain, just as in the case of the deep sea, that the species 
occurring alike in the surface-water of the higher northern 
and southern latitudes have in general in the tropics an 
interrupted discontinuous distribution, notwithstanding the 
fact that it must have been continuous up to M/id-Tertiary 
times. 
The remarkably poor development of the fauna of the 
tropical subsurface-water, as revealed by deep-sea investi- 
gations, gives us a hint as to the cause of this phenomenon. 
‘The reason of this retrogression may lie in the extraordinary 
development in the tropics of reef-facies, which, absorbing 
almost all the supplies of the surface-water, may have over- 
whelmed the other members of the old fauna, or crowded 
them into the deeper water: the forms adapted to the region 
of light perished, the mud-eaters went down to the deep sea. 
Thus the subsurface-water fauna by no means corresponds to 
the surface-fauna of higher latitudes, but only to the mud- 
eating portion of it. ‘The change in the internal economic 
conditions of this community, the gradually enforced economic 
dependence on an altered surface-water fauna, and the change 
of the mud-bottom to one of coral-mud, must assuredly have 
worked towards the impoverishment of the fauna; but a still 
stronger influence must have been exerted by the probable 
scantiness of nutrition in the coral-emud, which had already 
passed through the food-canal of fishes as pieces of lime, and 
of echinoderms as coral-sand. 
Now the coral-reefs are not developed on the west coasts 
of Africa and America, so that we might expect to find there 
the continuity in distribution of at least some bipolar species, 
which is rendered difficult, if not altogether suppressed, within 
the coral-region. But the state of the subsurface- water fauna 
of Africa is practically unknown ; though Von Maltzan men- 
tions the stunted growth of the Senegambian forms of Pleuro- 
toma as compared with the same species from the Mediter- 
ranean *, We have gained some knowledge of tropical West 
* Professor Chun, in his admirable work on the German Deep-sea 
Ixpedition (‘ Aus den Tiefen des Weltmeers,’ Jena, 1900, p. 75), says 
concerning the nature of the West-African coast:—‘* We were less satis- 
fied with the results of the trawling operations, which we made to depths 
of 4900 m. The bottom of the deep sea in these regions is covered with 
a disagreeable, viscous, blackish ooze, apparently mixed with the mud 
