304 Prof. Dr. G. Pfeffer on the Mutual Relations 
second, a cool-water fauna, which extends over the whole 
subsurtace-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 floor 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 history, its 
evolution, leads us at once to the second part of our subject 
—the problem, namely, of the historical development of the 
present conditions of 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 
paleontologists : firstly, because it might be assumed that 
genera, Which now occur only in the tropics, had at that time 
