304 Prof. Dr. G. PfefFcr on the Mutual Relatiom 



second, a cool- water fauna, wliicli extends over the -whole 

 subsurface-water of the tropics and temperate zone, as well 

 as over the surface-water of the latter ; third, a cold-water 

 fauna, spreading over the whole tloor of the ocean, and 

 embracing also the surface- and subsurface-water of the polar 

 regions. 



The subsurface fauna is certainly not identical with the 

 surface fauna of higher latitudes, nor the deep-water fauna 

 with that of the polar regions; but there is, in the first place, 

 a marked " habit resemblance " between them ; and, in the 

 second place, there is really a gradual transition, in the higher 

 and highest latitudes, between the vertically distributed and 

 the horizontally distributed faunas; and, thirdly, a number of 

 northern and southern species do succeed in spreading far 

 in the direction of the equator through the subsurface-water, 

 just as many species of polar animals are found on the ocean- 

 floor at a great distance from their surface-region. The 

 historical aspect of this point will be dealt with farther on. 



Herewith we conclude the first and descriptive portion of 

 our study. 



Problem of Historical Development. 



The fundamental idea of present-day science, that what- 

 ever exists is intelligible only in the light of its histjry, its 

 evolution, leads us at once to the second part of our subject 

 — the problem, namely, of the historical development of the 

 present conditions o£ our ocean-fauna. 



The fauna of the present day may be described as the 

 impoverished fauna of the Tertiary period. Though a few 

 genera of the present day reach considerably farther back, 

 yet faunistic pictures from before the Tertiary period wear 

 so unfamiliar an aspect that, for the study before us, which 

 is intended only to interpret present conditions, it seems 

 unwise to follow the roots of our fauna farther back than 

 the Early Tertiary or the Later Cretaceous period. 



Tropical Conditions in North Temperate Latitudes. 



In the Early Tertiary period there was in our regions a 

 fauna of tropical character reaching at least to the latitude of 

 Copenhagen, and we must thereture assume that, at that 

 period, these latitudes enjoyed a climate of tropical warmth. 



The legitimacy of this inference has been doubted by some 

 palaeontologists : firstly, because it might be assumed that 

 genera, which now occur only in the tropics, had at that time 



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