THE PHYSIOLOGY OF DEEP-SEA LIFE. 301 
perature to which man is known to have been exposed is prob. 
ably 78° F. below zero, while in the Colorado River desert a 
maximum temperature of 120° is not uncommon. The highest 
temperature of the surface of the sea is about 89°, and the 
lowest only a couple of degrees below freezing! 
It is well known that many species of marine invertebrates 
have a range in depth corresponding to the whole scale of this 
difference of temperature, say 60°, while in the case of land 
animals the extremes differ by nearly 200°. But the facts are 
exactly opposite where pressure is concerned. At moderate 
heights, when compared to the whole depth of the atmosphere, 
the conditions of pressure are sufficiently changed to affect most 
painfully all terrestrial animals ; indeed, these animals are prac- 
tically limited in their range to a small proportion of the total 
depth of the atmosphere. With marine animals the conditions 
are reversed. A difference in bathymetrical range, in which a 
marine animal may pass from a pressure of a few pounds to the 
square inch to as тапу hundred is not uncommon; while a 
difference of twenty pounds to the square inch will represent 
the maximum to which terrestrial animals can with safety be 
subjected. 
Enormous as is the range of pressure which marine animals 
can endure, it is not to this, but to the slight differences in the 
temperature of the succeeding belts of the sea, that we must 
look for an explanation of the principal causes of bathymetrical 
distribution. The heat of the sun, judging from the tempera- 
ture sections, does not extend farther than to a depth of one 
hundred and fifty fathoms. This is the range within which at 
different latitudes are limited the widest variations of tempera- 
ture (to about 50° F.), and the range within which the oceanic 
faunze, as formerly understood, are restricted. Below this belt 
— of different depth, according to the latitude — we find a 
second belt of from three to four hundred fathoms, within 
which the temperature falls very rapidly (to about 38°), until 
it reaches the depth at which the remaining mass of water may 
practically be said to have a uniform temperature, varying only 
1 The highest surface temperatures Gulf of Mexico, and along the Guate- 
have been observed in the Red Sea, the mala coast. 
