38 THE DEPTHS OF THE SEJ. [CHAP. i. 



In moderate depths sometimes the whole mass of 

 water from the surface to the bottom is abnormally 

 warm, owing to the movement in a certain direction 

 of a great body of warm water, as in the ' warm area ' 

 to the north-west of the Hebrides ; and sometimes the 

 whole body of water is abnormally cold, as in the 

 ' cold area ' between Scotland and Fseroe, and in the 

 northern part of the German Ocean. In deep water 

 however, after the first few hundred fathoms, the 

 thermometer usually sinks gradually and very slowly 

 till it reaches its minimum at the bottom, a little 

 above or below the zero of the centigrade scale. 



The temperature of the sea apparently never sinks 

 at any depth below 3*5 C., a degree of cold which, 

 singularly enough, is not inconsistent with abundant 

 and vigorous animal life, so that in the ocean, except 

 perhaps within the eternal ice-barrier of the antarctic 

 pole, life seems nowhere to be limited by cold. But 

 although certain sea-animals many of them, such as 

 the siphonophora, the salpa3, and the ctenophorous 

 medusae, of the most delicate and complicated organiza- 

 tion are tolerant of such severe cold, it would appear 

 to be temperature almost entirely which regulates the 

 distribution of species. The nature of the ground 

 can have little to say to it, for on every line of coast 

 of any extent almost every condition and every kind 

 of sediment is usually represented. Prom their inha- 

 biting a medium which differs but little in weight 

 from the substance of their bodies, and from the great 

 majority of them producing free-moving larvae or fry 

 in vast numbers which are floated along from, place 

 to place by currents, marine animals would seem to 

 have every possible chance of extending their area, 



