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 sufiiciently 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. AYith 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 many 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 jDressure 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 bathymeti-ical 

 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 

 faunae, 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, aud along the Guate- 

 have been observed in the Red Sea, the mala coast. 



