August 28, 1884] 



NA TURE 



427 



be employed, to try the effect on a fish of a very gradual appli- 

 cation of pressure, extending over many hours. It is probable 

 that the results would be greatly modified if plenty of time could 

 be given for the fish to accommodate itself to the change of 

 pressure, and the conditions in which it moves in nature slowly 

 from one depth to another be imitated. The results of M. 

 Regnard's further experiments will be looked forward to with 

 great interest. 



A question of the utmost moment, and one that has received a 

 good deal of attention, is that as to the source of food of the 

 deep-sea animals. Certainly a large proportion of this food is 

 derived from the life on the ocean-surface. The debris of pelagic 

 animals sinks slowly downwards, forming on its passage a sparsely 

 scattered supply of food for any animals possibly living at inter- 

 mediate depths, but becoming concentrated as it were on the 

 bottom. The pelagic animals depend for their ultimate source 

 of food, no doubt, largely on the various pelagic plants, the range 

 of which in depth is limited by the penetration of sea-water by 

 the sunlight, and probably to an important extent is dependent 

 on the symbiotic combinations of radiolarians with zooxanthella. 

 But a large part of their food-supply is also constituted by animal 

 and vegetable debris derived from the coasts, either directly from 

 the littoral zone or by rivers and the action of the tides from 

 terrestrial life. Immense quantities of shore-debris have been 

 dredged from deep water near coasts, and deep-sea life appears 

 to diminish in abundance as coasts are receded from. Unfor- 

 tunately our knowledge of pelagic vegetable life is very imperfect, 

 and it is to be hoped that botanists may be led to take up the 

 subject and bring together what is known with regard to the 

 geological ranges and abundance of the various larger sea-weeds, 

 trichodesmium, diatoms, and other Alga; by which the sea-surface 

 is inhabited. It will, then, be possible to form a nearer estimate 

 of the extent to which these plants are capable of forming a 

 sufficient ultimate food-source for the greater part of the pelagic 

 fauna, and through it of deep-sea life. The question is of im- 

 portance, because if the deep sea, having no ultimate source of 

 food in itself, derived its main supply from the coasts and land- 

 surfaces in the early history of the habitation of the globe by 

 animals, there can have existed scarcely any deep-sea fauna until 

 the littoral and terrestrial faunas and floras had become well 

 established. 



Whether the littoral and terrestrial plants or the pelagic be 

 proved to have the larger share in composing the ultimate food- 

 source of the deep sea, it seems certain that the food as it reaches 

 the deep sea is mostly in the form of dead matter, and I imagine 

 that the long but slender backwardly-directed teeth of many 

 deep-sea fish, resembling those of snakes, are used rather as aids 

 for swallowing whole other fishes which have fallen from above 

 dead, and thus making the best of an occasional opportunity of a 

 meal, than for catching and killing living prey. In a lecture on 

 " Life in the Deep Sea," delivered in 1880, 1 I suggested that 

 putrefaction of organic matter, such as ordinarily occurs else- 

 where, may possibly be entirely absent in the deep sea, the 

 Bacteria and other microphytes which cause it being possibly 

 absent. Some interesting experiments with regard to this 

 question have lately been made by M. A. Certes.- He added 

 to sterilised solutions of hay-extract, milk, broth, and other 

 organic nutrient fluids mixed with sea-water, with the usual 

 necessary precautions, small quantities of deep-sea mud, or deep- 

 sea water, procured by the Travailleur and Talisman. In some 

 experiments air was present ; others were made in vacuo. In 

 nearly all the former putrefaction occurred after some time, 

 especially after application of warmth, and micro-organisms were 

 developed, whilst the latter remained without exception sterile, 

 apparently indicating that the microbes which live where air is 

 absent are not present in the deep sea. The others, which 

 developed in the presence of oxygen, may possibly have sunk from 

 the surface to the bottom, and have retained their vitality, al- 

 though it is not improbable that they may be incapable of active 

 existence and multiplication under the physical conditions there 

 existing. M. Certes is to make further experiments on this 

 question under conditions of pressure and temperature as nearly 

 resembling those of the deep sea as possible. In the deep sea 

 the ordinary cycle of chemical changes of matter produced by 

 life is incomplete, there being no plants to work up the decom- 



1 Lecture delivered at the Royal 

 \A. jcxi. p. 592. 



- " Sur la Culture, a l'abri des Gen 

 Sediments rapportes par les Expeditic 

 18S2, 1883," Comptes Rendus, No. 11 



Institution, March 5, 1880, Natur 



les atmospheriaues, des Eaux et di 

 is du Travailleur et du 'I'alisma, 

 11 mars 1884, p. 690. 



position-products. These, therefore, in the absence of any rapid 

 change of the deep-sea waters, must accumulate there, and can 

 only be turned to account when they reach the surface-waters on 

 the littoral regions. 



Many interesting results may be expected to be obtained when 

 the histology of animals from great depths comes to be worked 

 out, and especially that of the special sense-organs. At present 

 very little has been attempted in this direction, principally, no 

 doubt, because deep-sea specimens are too precious to be used 

 for the purpose. In a remarkable scopelid fish dredged by the 

 Challenger from deep water, Ipnops murrayi of Dr. Giinther, 

 the eyes are curiously flattened out and occupy the whole upper 

 surface of the mouth. They are devoid of any trace of lens or 

 iris, and, as appears from observations by Mr. John Murray and 

 my own examination of his preparations, the retina consists of a 

 layer of long rods, with a very thin layer indeed of nerve-fibres 

 in front of it, and apparently no intervening granular ganglionic 

 or other layers. The rods are disposed in hexagonal bundles, 

 the free ends of which rest on corresponding well-defined hex- 

 agonal areas, into which the choroid is divided. It is probable 

 that aberrant structures may be found in the retinas of deep-sea 

 fish, which may conceivably help towards physiological conclu- 

 sions as to the functions of the various components. With 

 regard to the all-important question of the nature of the light 

 undoubtedly present in the deep sea, it is hardly possible to 

 accept Prof. Verrill's recent startling suggestion that sun- 

 light penetrates to the greatest depths with perhaps an intensity 

 at from 2000 to 3000 fathoms equal to that of some of our 

 partially moonlight nights. Such a conjecture is entirely at 

 variance with the results of all experiments on the penetration 

 of sea-water by sunlight as yet made by physicists, results which 

 have prevented other naturalists from adopting this solution of 

 the problem. 



The progress of research by experts on the deep-sea fauna 

 confirms the conclusions early formed that it is impossible to 

 determine any successive zones of depth in the deep-sea regions 

 characterised by the presence of special groups of animals. 

 Within the deep-sea region the contents of a trawl brought up 

 from the bottom give no evidence which can be relied on as to 

 the depth at which the bottom lies within a range of at least 

 2500 fathoms. Some groups of animals appear to be charac- 

 teristic of water of considerable depth, but representatives of 

 them struggle up into much shallower regions. Thus of the 

 remarkable order of Holothurian Elasipoda nearly all the re- 

 presentatives occur at very considerable depths, and their 

 numbers diminish shorewards, but one has been found in only 

 100 fathoms. Again, the Pourtalesidae range upwards into 

 about 300 fathoms, and the Phormosomas, which Loven con- 

 siders as eminently deep-sea forms, range up to a little over ico 

 fathoms depth, and are nearly represented in shallow water at a 

 depth of only five fathoms by Asthenosoma. As has often been 

 pointed out before, there are numerous genera, and even species, 

 which range even from the shore-region to great depths. 



The fact that zones of depth cannot thus be determined adds 

 seriously to the difficulties encountered in the attempt to deter- 

 mine approximately the depths at which geological deposits have 

 been found. Dr. Theodore Fuchs, 1 in an elaborate essay on all 

 questions bearing on the subject, has attempted to determine 

 what geological strata should be considered as of deep-sea 

 formation, but, as he defines the deep-sea fauna as commencing 

 at 100 fathoms and extending downwards to all depths, his 

 results may be considered as merely determining whether certain 

 deposits have been found in as great a depth as 100 fathoms or 

 less, a result of little value as indicating the depths of ancient 

 seas or the extent of upheaval or depression of their bottoms. 

 Mr. John Murray has shown that the depths at which modern 

 deep-sea deposits have 1 een formed can be approximately ascer- 

 tained by the examination of their microscopical composition 

 and the condition of preservation of the contained pelagic and 

 other shells and spicules. 



The most important question with regard to life in the ocean, 

 at present insufficiently answered, is that as to the conditions 

 with regard to life of the intermediate waters between the surface 

 and the bottom. It is most necessary that further investigations 

 should be made in extension of those carried out by Mr. Alex- 

 ander Agassiz with similar apparatus — a net, or vessel, which 

 can be let down to a certain depth whilst completely closed, 



' "Welche Ablagerungen haben wir als Tiefscebildungen zu betrachtenV " 

 A'eues lalirbuch far Mineralogie, Geologic, und Palaoniolegie, 11 Beilage, 



1 ■ 1 1 ■ 



