302 THREE CRUISES OF THE “ BLAKE." 
between that of 38° and one near the freezing point. Of course, 
the depth at which the belt reaches the surface will change ae- 
cording to the latitude of the intermediate zones of cold and of 
the coldest water. The belt of variable heat is deepest in the 
tropics, while the belt of maximum cold reaches the surface in 
high latitudes. 
In these belts of temperature, we can most satisfactorily trace 
the so-called faunal districts into which the shores of our conti- 
nents have been subdivided. The recognized geographical pro- 
vinces, circumscribed, as first pointed out by Dana, by the lowest 
degree of cold to which they are subject, comprise, as a general 
rule, animals living along the shores stretching north and south 
within certain limits. The bathymetrieal range of these lit- 
toral species is small, and with few exceptions they do not live 
within the limits of either the continental or the abyssal areas, 
where flourish the faunz of the other two belts. The tempera- 
ture of the continental belt is sufficiently low to crop out in 
high latitudes, and hence the large number of so-called arctic 
species which have been found to be quite common in moderate 
depths upon this so-called continental area ; while the strictly 
deep-sea or abyssal forms rarely occur within the range of the 
continental, and still more rarely within that of the littoral belt. 
In making a comparison between the atmospheric and watery 
envelopes of the earth, it is interesting to compare the high 
velocity of the atmospheric currents and their cataclysmic dis- 
turbances with the slow movements of the strongest oceanic 
currents, — the direction of the atmospheric currents being more 
nearly what those of the ocean would be were it not for the 
disturbing effects of continental masses; to the action of these 
currents we undoubtedly owe the general drift of the surface 
flow of the oceanic basins. This, in connection with the ther- 
mal differences and variations in specific gravity, would produce 
surface and under currents, modified in their turn by the rotation 
of the earth. 
The general conditions of temperature below five hundred 
fathoms are very simple. From there an immense body of 
cold water with a temperature of less than 40° (as low as 33.36°, 
or near the freezing point), stretches to the bottom of the 
