172 THE DISTRIBUTION OP 



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 Mttle 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 stiU 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 wiU 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 hght 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. 



