304 THREE CRUISES OF THE “ BLAKE." 
The oceans, through convection, are the great moderators of 
climate, carrying north the heat of the tropics, while the cold 
water of the arctics gradually finds its way to the tropics to be 
heated, and start on its northward voyage again at a high tem- 
perature. By this action in past geological periods, we may 
seek to explain the remarkable climatic conditions which have 
characterized the cretaceous and the tertiary. Atmospheric tem- 
perature rapidly decreases as we rise to high altitudes, into 
regions where the pressure is reduced, or pass from the tropics 
to the north, where the conditions of pressure are nearly the 
same. In the ocean, where the action of the sun is limited to a 
comparatively shallow belt, the water having the greatest den- 
sity is by no means that having the highest temperature. Oce- 
anic circulation is very slow; and, as in the case of the atmos- 
phere, there are no enormous changes of temperature affecting 
the whole fluid envelope. 
The pressure to which animals living in deep water are sub- 
ject is enormous. At a thousand fathoms, it can be roughly 
stated tobe a ton to the square inch, — more than a hundred 
and twenty times the pressure borne by terrestrial animals at 
the level of the sea. Deep-sea animals are probably no more 
conscious of the pressure acting upon them than animals living 
on dry land ; and they undoubtedly bear, within certain limits, 
much greater extremes of pressure. A difference of pressure 
due to a height of twenty thousand feet is perhaps the utmost 
terrestrial animals can stand; while some invertebrates with a 
wide bathymetrical range may be subject either to the ordinary 
pressure of shallow waters, or, if living near the three-thousand- 
fathom line, to nearly four hundred atmospheres. 
Marine animals readily adapt themselves to these immense 
pressures. "Their tissues are permeated by fluids, and perfect 
equilibrium is established between their circulation and the me- 
dium in which they live. Hence pressure has scarcely any effect 
upon them, as is proved by the wide bathymetrical range of 
a number of invertebrates, and even of fishes belonging to 
families in which the bony skeleton remains more or less carti- 
laginous. Fishes and mollusks are apparently the only animals 
which show very markedly the effect of a diminished pressure. 
