SUMMARY OF RESULTS. 
1433 
struo-gle for food not being less severe in the deeper regions of the ocean basins than Economy of 
oo IT o Deep Sea IjIfp’ 
elsewhere. The great majority of deep-sea species live by eating the surface layers of 
the mud, clay or ooze, and by catching or picking up the small organisms and minute 
particles of organic matter which fall from the surface. Many of these species are of 
gigantic size when compared with their shallow-water allies, and in turn they become 
the prey of numerous rapacious animals armed with peculiar tactile, prehensile and 
alluring organs. Phosphorescent light plays an important role in the economy of deep- 
sea life, and is correlated with the prevailing red and brown tints in the majority of 
deep-sea organisms. Some species, in addition to large eyes, are provided with a sort of 
bull’s-eye lantern, from which streams of light are thrown out at the will of the animal. 
Phosphorescent organs act also as a lure, or they may indicate the presence of prey or 
the passage of an enemy. 
In the shallower reaches of the ocean the materials on the bottom are assorted and 
distributed by currents in a way that produces a great variety of conditions. In some 
places there are siliceous or calcareous sands, in other places dead shells and pebbles ; on 
submarine banks, rocks and boulders prevail ; in depressions, fine muds and clays. On The Mud-line. 
each of these bottoms there is usually a very different assemblage of animals. So that, 
although the trawl may not in shallow water bring up such a variety of organisms in any 
single locality as from deeper water, still the total number of genera and species inhabiting 
the whole area of depths less than 50 fathoms is recognised as much greater than in 
deeper water. With increasing depth, not only the nature of the deposits, but the other 
physical conditions, become more and more uniform, till a depth is reached along the 
continental shores facing the great oceans immediately below which the conditions 
become nearly uniform in all parts of the world, and where the fauna likewise presents a 
great uniformity. This depth is usually not far above nor far below the 100-fathom 
line, and is marked out by what I have elsewhere designated as the Mud-line} 
In all modern seas the depth at which minute particles of organic and detrital matters Position of the 
in the form of mud begin to settle on the bottom of the ocean is important both from the 
physical and biological points of view. This depth is determined by the distribution of 
land and water. It is dependent on the depth and extent of the ocean or basin, and varies 
temporarily with seasons of strong winds and calms. In small enclosed arms of the sea, 
like those of the west of Scotland, the mud-line is situated at depths of from 5 to 20 
fathoms, but where currents rush through narrow passages or over submarine barriers 
it is much deeper. In the North Sea, the oceanic mud line to the north occurs at a 
depth of about 80 fathoms, but on the coasts of Scotland facing the great stormy 
Atlantic it is often found at a depth of over 100 fathoms. In the Faroe Channel the 
currents rush over the Wyville-Thomson Eidge with sufficient force to prevent mud 
^ See Deep-Sea Deposits Cliall. Exp., pp. 185 and 252. 
