38 THE DEPTHS OF THE SEA. [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 Froe, 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 salpze, and the ctenophorous 
medusee, 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. From 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 larvee 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, 
