172 THE DISTRIBUTION OF 
statements made above in regard to the temperature 
of littoral waters hold good here also. An important 
point, however, is that nowhere—not even under the 
Equator—does the warmth of the surface layer extend 
far downwards. In practically all parts of the ocean 
the temperature at depths exceeding 1,000 fathoms is 
permanently low, and varies but little throughout 
the year. 
The conditions as to light demand detailed considera- 
tion. Very careful experiments show that all rays of 
light are present at depths of 50 fathoms. At 300 
fathoms the red and green rays have disappeared, but 
blue and violet rays remain. Some of the violet rays 
are still present at 500 fathoms, for photographic plates 
are slightly affected here. At 900 fathoms they remain 
absolutely unaffected after long exposure, and here we 
must suppose that absolute darkness reigns. 
These statements are true for middle latitudes, but 
another point has also to be considered. Obviously, 
the direction of the rays is not the same in all latitudes, 
and therefore they will penetrate to different depths 
in different latitudes. Dr. Hjort gives the following 
figures in regard to the depths at which the same in- 
tensity of light is found in different latitudes : 
In 33° N. lat. at about 430 fathoms depth. 
In 50° N. lat. at about 270 fathoms depth. 
In 67° N. lat. at about 100 fathoms depth. 
This is important, because, as we shall see directly, 
the intensity of light greatly influences the bathy- 
metrical (i.e. depth) distribution of marine animals. 
The basal food-supply of marine animals—in the 
open as off the shore—consists of algae, so that we 
must add to the above consideration of the conditions 
of life in the open some account of the pelagic algae. 
