Wdliher — Origin and Peopling of the Deep Sea. 57 



Only imperceptibly slow diffusion streams constantly mix the 

 waters. 



Just as the sun's heat can only warm the upper water strata, 

 so the sunlight, even in clear water, penetrates only to a 

 depth of about 400 meters. Photographic plates which were 

 exposed at such a depth at Nice show no influence of light. 

 Only the delicate shimmer of phosphorescent animals lights 

 the dark abyss. The consumption of carbon dioxide, so 

 important for the life of plants, is only possible in sunlight ; 

 therefore we need not wonder that the deep sea harbors no 

 single plant and that with their absence fail also the plant- 

 eating animals. 



To sum up the characteristics of the abyssal region so far 

 noted, — a uniformly low temperature, quiet water of normal 

 salinity disturbed by no noticeable movement, no light and no 

 plant life : these are the bionomically important characteristics 

 of the deep sea. 



These conditions of existence are, moreover, very constant 

 over vast areas and cause the world-wide distribution of most 

 of the deep-s"ea dwellers. The fauna of the deep sea is 

 undoubtedly poorer than that of the shallower parts of the sea, 

 but if we consider that all light-hungry and plant-eating 

 animals are lacking there as well as all inhabitants of the 

 moving and warm sea water, we are still astonished at the 

 animal world of the abyss. For each dragnet brings up deep 

 sea animals and even the small bottom samples of the sounding 

 apparatus have afforded traces of organic life at the maximum 

 depth of more than 8000 meters. 



Ten years ago, Sir John Murray gathered the results of the 

 earlier deep sea expeditions and thereby showed that 



dpwn to 200 meters about 4200 kinds of bottom-dwelling animals exist 



down to 2000 " " 600 " " 



at 4000 " " 400 " " 



below 5000 " " 150 " " " " " " 



To this must be added the great number of animals floating 

 and swimming in the deeper water strata. Hut, while in 

 shallow water a great number of individuals of each species 

 are captured in each cast of the net, the deeper strata are rich 

 in species, but poor in individuals. A cast of the dragnet in 

 1000 meters depth still gave 100 examples of the same ani- 

 mal, but in greater depths there were often in a net only 2 

 examples each of 10 different species. 



It is difficult to point out a peculiar characteristic of the 

 known deep-sea animals without giving a specific description 

 of individual forms. JBut one can safely state that they 

 mostly possess very weak structures. Their hard parts are 



