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



exists even now a continuous connexion tlirougli the sub- 

 surface-water of tlie^ropics between the identical genera and 

 s]iccios of the higlier hititudes. Curiously enough, this is 

 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 J//c^-Tertiaiy 

 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 tlie 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-mud, which had already 

 passed through the food-canal of fishes as pieces of lime, and 

 of cchinoderms as coral-sand. 



Kow 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 Atrica is practically unknown ; though Von Maltzan men- 

 tions the stunted growth of the Senegambian forms of Plearo- 

 tovia 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 

 Expedition (' Aus den Tiefeu des NN'eltmeers/ Jena, lUOO, p. 76), says 

 concerning the nature of the "West-i^frican 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 



