302 Prof. Dr. G. Pfeffer on the Mutual Relations 
the year; second, the polar region, with low temperature 
and slight variations; thirdly, the temperate region, with 
moderate temperature and great yearly variations. To these 
natural thermal zones there correspond similar faunistic 
regions; but this statement requires certain qualifications, 
chiefly in regard to what we are here especially con- 
sidering—the animal life of the ocean-floor, the Benthos of 
Haeckel. 
The arctic fauna shows zonal development, or, as it has 
been called, circumpolarity, very perfectly ; while in the ant- 
arctic fauna, with the weak development and the wide 
separation of the coast-area characteristic of that region, 
circumpolarity is much less observable. 
The tropical fauna is relatively uniform in its repre- 
sentation throughout the whole tropical zone, yet, conditioned 
by the formation of continents on the one hand, and by the 
unique horizontal and vertical motion of the water on the 
western tropical coasts on the other, faunas of a peculiar 
kind are differentiated on the west coasts of Africa and 
America. 
In the fauna of the temperate zones circumpolarity dimin- 
ishes considerably, giving place to the development of local 
faunas. ‘This corresponds to the enormous formation of 
continents in the north, and the wide separation of coast- 
regions in the south ; and the local occurrence of extraordinary 
yearly variations of temperature has a similar influence. The 
parts of the temperate zone which border on the tropics show 
likeness in many respects to the tropical zones, and those 
bordering on the polar zones similarly approach these, and 
we speak therefore of two subtropical faunas, and of a boreal 
and a notal fauna. 
Besides the borizontal decrease in warmth there is a corre- 
sponding vertical decrease, inasmuch as—speaking quite 
geucrally—the temperature of the ocean, from the surface to 
the floor, gradually falls, so that all gradations from tropical 
warm to polar cold water are to be found. 
Two regions may be distinguished in the water of the 
open sea: first, a superficial region, through which light 
penetrates, and in which both variations of temperature and 
the movements of the water are felt; and, secondly, a deeper 
region, reaching to the ocean-floor, constant in temperature 
and without either light or water-movements. [or pelagic 
animals this division at once suggests a corresponding 
faunistic division; but, with regard to the dwellers on the 
ocean -floor, other considerations have to be taken into 
account; and accordingly the ocean, and the fauna which it 
