Geographical Distribution. 181 



task, have already greatly increased our understanding 

 of the " metabolism of the ocean", as Hensen expressed 

 his ultimate aim. 



One must not forget the pioneer work of Wallich, 

 Carpenter, and others, but our knowledge 

 of the abyssal fauna practically begins with 

 the Challenger expedition. 



The researches of the Challenger and analogous ex- 

 peditions have made it certain that there is no depth- 

 limit to the distribution of animal life, that there are in 

 the great abysses representatives of most of the classes 

 from Protozoa to fishes, and that the distribution of 

 some types tends to be cosmopolitan in correspondence 

 with the uniformity of the physical conditions. 



As to these physical conditions, the deep-sea world is 

 in darkness, apart from occasional "phosphorescence", 

 for a sensitive photographic plate is not influenced 

 below 250-500 fathoms ; the temperature is about freez- 

 ing point, the heat of the sun being practically lost at 

 about 150 fathoms; the pressure is enormous, about 2^ 

 tons per square, inch at 200 fathoms ; the cold water in 

 sinking brings down a relatively large proportion of 

 oxygen ; it is quite calm, for the effects of the greatest 

 storms are only felt near the surface. 



There are no plants, apart from the resting stages of 

 a few doubtful algoid forms, for typical vegetable life is 

 dependent upon light, and not even bacteria, otherwise 

 so omnipresent, are known to occur in the great depths. 

 The animals feed on one another and on the organic 

 debris which sinks down from above. 



Modern research has yielded no result more stimula- 

 ting to the imagination than the tidings of this strange, 

 silent, cold, dark, plantless world and its numerous 

 inhabitants. 



The Challenger and subsequent expeditions yielded 

 results which have been worked up in many of the 

 leading biological laboratories of Europe and America, 

 and there is now an abundance of reliable data; not 

 enough, however, to settle some of the most interesting 

 questions which the facts raise. 



What of the metabolism of deep-sea animals, the 



