84 



LIFE : OUTLINES OF GENERAL BIOLOGY 



representatives of the five extant orders of Echinoderms, abundant 

 Crustaceans, representatives of most of the Mollusc types, and 

 peculiarly modified Fishes, some with small eyes, others with large 

 eyes, which probably catch the fitful gleams of phosphorescence. 



As to the physical conditions, the deep-sea world is in darkness, 

 for a photographic plate is not influenced below 250-500 fathoms; 

 it is extremely cold, about the freezing-point of fresh water, for the 

 sun's heat is virtually lost at about 150 fathoms; the pressure is 

 enormous — thus at 2,500 fathoms it is about 2§ tons per square 

 inch; the cold water in sinking from the polar regions brings down 

 much oxygen; it is quite calm, for even the greatest storms are 

 relatively shallow in their influence; there are no plants (except 

 {xrhaps the resting phases of some Algae), for typical vegetable life 

 dejx'nds upon light, and not even bacteria, otherwise almost omni- 

 present, are known to flourish in the great depths. A strange, silent, 



Fig. 20. 

 A Dcop-sca Fish, Ga.strostomus, with an enormous gape, the lower jaw 

 Ix'in^ liingcd very far back, an adaptation to the reduced opjX)rtunities 

 for food-capture in abyssal conditions. After Murray and Hjort. 



cold, dark, plantless world! The animals feed upon one another and 

 upon the debris which sinks from above. 



\Vc do not clearly know when the colonising of the depths began, 

 but there is much to be said for the view that an abyssal fauna was, 

 at most, scanty before Cretaceous ages. But whensoever the peopling 

 of the abys.scs occurred, it must have been gradual. It is likely that 

 most of the pioneers migrated outwards and downwards from the 

 shore region (in a wide sense), following the drift of food; it is 

 possible that others, e.g. some Crustaceans, sank from the surface 

 of the open sea. The boreal character of many deep-sea animals has 

 been often remarked, and it is plausible to suppose that there was 

 a particularly abundant colonisation in the Polar regions, and a 

 gradual spreading towards the Equator as the Poles became colder. 

 Perhaps the richness of the fauna at the Equator may be thought of 

 as in part due to the meeting of two great waves of life from the 

 Poles. 



The abyssal conditions of life tend to uniformity over vast areas, 

 just as in the open sea. But, on the whole, Hfe must always have 

 l->een harder in the depths than on the surface. The absence of plants, 

 for instance, involves a keener struggle for existence among animals. 

 Thus, although many abyssal forms, e.g. sea-anemones, live a 

 passive srdentary life, waiting for food to drop into their mouths, 



