of the Arctic and the Anturctic Fuunas. 303 
contains, may be divided vertically into the three following 
regions :— 
First: the surface-water,—the warmest of all the vertical 
zones, with variations in warmth, with movement of the 
water, with the influence of light, and therefore with plant- 
rowth, with a terrigenous floor consisting of rock, gravel, 
and sand. ‘This region reaches, according to the locality, 
from the surface to a depth of from 50-150 fathoms. ‘The 
expressions “ surface-water”’ and ‘“surface-fauna”’ are here 
used as practically equivalent to “ littoral zone ” and “ littoral 
fauna.” 
Second: the subsurface-water,—cool, without variations in 
warmth, without light, without plant-growth; its floor lies 
on the slope of the continents and is covered with terrigenous 
mud. According to locality, this region reaches to a depth 
of 600-1000 fathoms; its fauna consists only of the mud- 
eaters, and is therefore economically dependent on supplies 
from other regions, particularly on the assimilating flora and 
fauna of the adjoining surface-water, ‘The fauna of this region 
resembles the fauna of the surface-water of higher latitudes. 
Third: the deep water, the deep sea,—resembling the sub- 
surface zone in its lack of variations of warmth, of light, of 
plant-growth, and water-movement ; its floor is the bottom 
of the ocean, and is covered with fine slimy ooze or clay of 
pelagic origin; its fauna is economically independent of that 
of the coasts, because of the great distance between them, 
but, on the other hand, it is dependent on the pelagic 
animals, whose dead bodies form its food-supply. Apart 
from archaic or highly specialized forms, the tauna of this 
region has an arctic character; that is, it resembles the 
surface-fauna of the highest latitudes. 
Disregarding for the present the nature of the ocean-floor, 
and considering the water with reference to its temperature 
alone, we have, first, a tropical warm water occurring only 
as the surface-water of the tropical zone; second, a cool 
subsurface-water, which, in the higher temperate zones, 
gradually passes over into the surface-water of a similar 
temperature; third, a cold, deep water, which covers the 
whole ocean-floor, and within the polar zones passes over 
into equally cold surface-water. Thus the cold water has a 
universal spatial distribution over the whole earth, the cool 
water an almost universal distribution over the torrid and 
temperate zones, while the warm water occurs solely as the 
surface-water of the tropics. 
Corresponding to this there is, first, a warm-water fauna, 
which is developed only in the surface-water of the tropics ; 
aie 
