;-i02 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 

 niodevaie 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, circunipolarity, 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 tlnoughout the whole tropical zone, yet, conditioned 

 by tlie 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 tlie 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 horizontal decrease in warmth there is a corre- 

 sponding vertical decrease, inasmuch as — speaking quite 

 generally — 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. For 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 ; aiid accordingly tiie ocean, and the fauna which it 



