[Enclosure]

The Persistence of forms of life in the Depths of the Sea—

Among the many points of interest presented by the deep sea dredging which of late years has added so much to our knowledge of the quantity and kind of life existing in the depths of the ocean, there is one which seems as yet to have attracted but little attention viz— the bearing of those researches on the origin of species

For effecting the production of new species the mutual relation of organic beings is considered of far more importance by M^r Darwin than change in the inorganic conditions of existence.³ Now although such change unaided might be able to do no more than produce local and short lived varieties still the entire absence of such change would preclude the possibility of new species arising, for the small individual differences on which Natural Selection depends and without which it is of no avail must be due either directly or remotely to change in the inorganic conditions of existence.

If then it is necessary that such change should take place or should have taken place in order that individual differences may exist and on being subject to the action of Natural Selection give rise to new Species it follows that if there were a large portion of the earths surface throughout which the conditions of life were the same and had remained so for a long period of time we might expect to find less variation in the forms of life inhabiting such an area than on other parts of the earths surface where the conditions of existence were more complex and more subject to change.

Now the conditions of existence would be less liable to vary on the bed of a deep ocean than on that of a shallow sea. If the bed of an ocean 12000 feet deep were elevated or depressed 1000 feet usually far less change would be required in the forms of life proper to it in order that they might adapt themselves to the change of condition than would be the case if a shallow sea or if any portion of the dry land were elevated or depressed to a like amount.

An alteration of level great enough to convert a a shallow sea into dry land or low lying land into sea would in either case necessitate a total change of fauna throughout the areas so affected. Some local forms would possibly become extinct; others would migrate though in most cases they would be unable to find a habitat adjoining and precisely similar to that from which they had been displaced, so that the new adaptations required and still more the competition with the species already occupying the new area would give rise to new species. Variations of level of less amount in shallow seas would by altering the flow of tidal and other currents change the nature of the sediment brought by them and deposited on the sea bottom. At the same time the changes in temperature would probably be much greater than on the bed of a deep ocean subjected to a similar alteration of level, and species inhabiting shallow seas would thus be more likely to become extinct or to be forced to migrate; and new species would arise as the change in circumstances and increased competition necessitated new adaptations of structure and habits. In the same way changes in the level of dry land would generally require corresponding changes in the fauna as the temperature rainfall and drainage would be all liable to great variation.

An alteration in the level of the bed of a deep ocean to the extent suggested above would produce some change in the pressure, and possibly, some change in the nature of the sediment deposited there and in the temperature.

If the present state of our knowledge of deep sea temperatures allowed us to conclude that the temperature is independant of the depth and that areas adjoining one another and having the same depth of water may have climates differing to so great an extent as to render them incapable of supporting similar forms of life then we should have a disturbing cause which would make it very difficult to draw any deductions as to the persistence of forms of life in deep seas. The remarkable discovery of the warm & cold areas near the Faroe channel by the Lightning expedition in 1868 has excited so much interest that they are likely to be considered of more than due importance in their relation to the distribution of deep sea life and their influence on it.⁴ There is no doubt that a warm area would occasionally be converted by a change of level into a cold area and a change in fauna might in such cases be required. Professor Wyville Thompson has shewn that this would be the case over a part of the bed of the North Atlantic if it were depressed 1500 feet.⁵ Such a change however would only occur when the ocean bed reached the surface of contact of the warm & cold currents; a considerable elevation of the bed of a deep sea would be required to effect it and the area whose climate was altered would in most cases remain continuous with an area not affected by the change, possessing an identical climate and containing a similar fauna. As yet no warm area has been discovered at so great a depth as 1000 fathoms

The Gulf Stream has a depth which does not reach 400 fathoms from the point where it leaves the Florida Strait till its course is obstructed by the cold currents and shallows of the old world. Warm currents are normally surface currents. It is only in the neighborhood of land and shallow water that the warmer surface water is “banked down” so as to produce the phenomena of warm and cold areas side by side in deep water. The proximity of land and shallow water in addition to its tendency to produce these contrasts of climate by interfering with the free flow of currents would at all times be a great disturbing cause to deep sea life in its neighborhood as its subsidence or elevation even to a small amount might effect great changes in the local currents and deposit of sediment on the bed of the sea. Warm areas would therefore only produce disturbance in those parts of the deep sea where disturbance is most likely to arise from other causes, and if we confine our attention to life in depths greater than say 1000 fathoms this cause of disturbance disappears. Referring to a chart of the Atlantic it will be seen that with the exception of a narrow strip along the coast lines that ocean has everywhere a depth so far as it is known exceeding 1000 fathoms thus any exceptional conditions which may be shewn to prevail in these greater depths will affect the life over a large proportion of the surface of the globe which is covered by the sea. The range of bottom temperature over this area will be found to be very small. A consideration of the bottom temperatures obtained in the dredging expeditions since 1869 render it probable that the temperature will not rise higher than 4°C and rarely fall as low as 0°.C. {for Porcupine expedition 1869 & 1870 cf. pg^s. 142, 202   Depths of the sea for Challenger expedition cf. notes on the Challenger by Prof. Wyville Thompson in Nature Vol. 8 pg^s. 28, 51, 109, 246, 266, 347 & 400.}⁶ In the Porcupine expedition of 1869 the highest temperature found at a depth greater than 1000 fathoms was 3.2°C at Station 17, in 1230 fath^s. and the lowest temperature was 2.4°C at Station 38 in 2090 fath^s. In the Porcupine expedition of 1870 the highest temperature in like depths was 4.5°C at Station 18 in 1065 f^s. and the lowest 4.3°C at Station 17 in 1095 fath^s. Both these soundings were very near the coast of Portugal which may account for the high temperature. But few deep soundings were taken in the Atlantic during this expedition. In the cruise of the Challenger up to the present time {Oct 73.} the highest temperature given in the Notes published in Nature is 3°C (Mar 14) in 1420 faths^s. not far from the Island of Sombrero and the lowest temperature is 1.5.C in 2575 fath^s. (June. 16) and again in 2025 fath^s. (June. 11). On the line of soundings taken between the Cape de Verde Islands & Sombrero the temperature varies from 2°.C. to 1.6°C. The lower temperatures were found on the line between Bermuda and the Azores but only two temperatures are given in the Notes from the Challenger and both are near land. Should equally low temperatures be met with throughout this line of soundings we must look to the Arctic regions as the source in part at least of this bottom water and not to the Antarctic as has been suggested by Prof. Wyville Thompson.⁷ The soundings taken by Commander Chimmo RN & Lieut. Johnson RN in the North Atlantic when the errors arising from the use of unprotected thermometers have been corrected give a temperature of 3°.9.C for depths of 1000 fathoms and of 1°.7C for depths of 2270 fathoms. {Proceedings of the Royal Society Vol XVIII pg. 472.}⁸ Lower temperatures were found in the Faroe channel in depths less than 1000 fathoms: the lowest was -1.3°C at Station 59 in 580 fathoms. This channel is on the boundary of the Atlantic ocean near the source of the cold water and in the neighborhood of land. In the deep parts of the ocean which are away from the disturbing influence of land the temperature diminishes very gradually as the depth increases so that temperatures lower than 1.5°C which is the lowest found as yet will probably be met with at greater depths and also in northern an southern parts of the ocean which are near to the sources of the cold water

So far then we may expect to find the bottom temperature over the greater part of the ocean bed subject to little variation in depths greater than 1000 fath^s.

The deposit of sediment in greater or lesser quantity on any part of the ocean bed would affect the life inhabiting that part and might destroy it if the quantity deposited became excessive. Finely divided matter suspended in water settles so slowly that there is probably no part of the ocean bed to which it might not be transported. The locality to which sediment from any given source is carried depends during the earlier part of its transportation on surface and local currents and as the velocity and direction of such currents would vary with small alterations in the coast line or level of the land so also would the area of the sea bed vary on which the sediment was ultimately deposited. An immense area over which red mud is now being deposited has been explored by the Challenger. In dredging on the line from the Cape de Verde’s to Sombrero and thence to Bermuda where this clay was found there was a great scarcity of life   this may in some measure be due to the turbid state of the water which forms the deposit. When the bad hauls made with the dredge from the Challenger over this ground are compared with some made during the Porcupine expedition in somewhat similar depths and with like temperatures (at station 37 for instance. Porcupine 1869) some other cause than temperature or pressure must be found to account for the difference in the quantity of life. {cf. Jour^l. of Geol^l Soc^y. Vol. XXVII pg. LXXI Presidents address}   The absence of life in the depths of the Ægean sea and also in the Atlantic at the entrance to the States of Gibraltar has been attributed the abundance of fine tenacious mud which is being deposited there.⁹

Hence the deposit of sediment considered as a disturbing cause would be liable to much variation and the conditions of life might be altered on any part of the ocean bed independantly of large movements of subsidence or elevation.

Of the varying conditions which affect animal life in the depths of the Sea pressure is probably the only one which varies directly with the depth. If extreme depths were always to be found unfavorable to animal life it would be a reasonable conclusion that the scarcity of life was due to the great pressure rather than to other disturbing causes which are more or less independant of the depth.

In his Notes from the Challenger {Nature Vol 8 pg. 266} Prof^r. Wyville Thompson says that great depths appear to be unfavorable to animal life   this he found to be the case in dredging between the Cape de Verdes and Sombrero and between Sombrero & Bermuda when the scarcity of life in great depths was very striking.

As pressure is constant for a given depth if it is detrimental to life it should be constant in its effect at similar depths. If an abundance of life were found in one place and a scarcity in another the depth in the two places being the same the scarcity of life in the latter could not be attributable to pressure and again if pressure were hostile to life we should expect to find the higher forms of life dissappear as the depth increased. They do appear to diminish in numbers as the depth increases but an abundance of life was found at Station 37 (of the Porcupine 1869) in a depth of 2435 fath^s. with a temperature 2.5.° C. In one haul of the dredge at this station all the invertebrate subkingdoms were represented. {Depth of the Sea pg. 95 & 96.} The tube building annelid found on Mar. 11 at a depth of 2975. by the Challenger is said by Prof. Wyville Thompson to be closely allied to the Clymenidæ “a well known shallow water group of high organisation”. {Nature vol 8 pg. 52.}¹⁰

So far we have no conclusive evidence that the extreme pressure in great depths is inimical to life or that considerable alterations in the level of the bed of the ocean would necessitate great changes in the forms of life living on it to adapt them to the change in pressure.

The quantity of life at great depths will depend in some measure on the velocity of the current if that term can be applied to movements so slow as those which suffice to convey the surface water from the poles to the ocean bed at the equator. The scarcity of life in the deeper parts of the Mediterranean is due in some measure to the tranquillity prevailing in the bottom water. The Protozoa are dependant on movement of the bottom water for the supply of organic matter in solution on which as has been shewn by Prof. Wyville Thompson they feed by absorption through the surface. When once the Protozoa are provided for “there is no difficulty in accounting for the alimentation of the higher animal types”. {Proceedings of the Royal Society. Vol XVIII pg. 477.}¹¹ Some organic matter might penetrate to the lower strata of ocean water by diffusion downwards without the aid of currents or lateral movements of the water but the supply of food would not in that case be so great as when a current provided a more rapid change of bottom water   The rate at which the bottom water is flowing along the bed of the ocean must vary in different localities   In deep basins there will be a near approach to absolute tranquillity and in such places the supply of food will be small. The quantity of movement taking place in the bottom water over a given area might vary with an alteration in the disposition of land & water at a great distance from the area in question but it would not be affected by small alterations so that regarded as a condition influencing life at great depths it would not be liable to vary much during long periods.

The above remarks will apply to the supply of gases contained in the bottom water and requisite for the respiration of the abyssal fauna   Evidence of the effect of movement of the water in accelerating the diffusion of the combined gases was obtained during the cruise of the Porcupine when there was found a reduction in the quantity of carbonic acid and an increase in the quantity of oxygen after the agitation of the surface water of the sea by wind. Of the composition of the bottom water we have as yet but little accurate knowledge. {see Prof. Wyville Thompsons remarks on this subject in Depths of the Sea pg. 499.} M^r Lant Carpenters observations on the quantity of gases contained in sea water seem to indicate that the quantity of oxygen diminishes and the quantity of carbonic acid increases with the depth but at the bottom the quantities of oxygen & carbonic acid do not conform to this law {Depths of the Sea Appendix A. Summary of the Results of the Examination of Samples of Sea Water &ct. by W^m Lant Carpenter pg 508}.¹² He observes that a large percentage of carbonic acid in the bottom water was frequently accompanied by an abundance of life. Further than this it does not appear that the quantity of life depends on any observed law of the distribution of these gases. There are doubtless many & far more intricate conditions than those given above which affect life at great depths with which we are so far unacquainted.

Considering the large proportion of the globe which is covered with water and the great depth of the ocean as compared with the height of the land above the sea level it seems not only possible but highly probable that the continuity of oceans of the present day with those of former times has been rarely broken. It is difficult to conceive the slow disappearance of one ocean and equally slow reappearance of another without some connection having existed between the two throughout the time. The law of the succession of types or the wonderful relation in the same continent between the dead and the living so long ago insisted on by M^r Darwin,¹³ is strong evidence in favour of the antiquity of the existing continents, Evidence that they have existed in somewhat the same position as that which they now occupy for a vast period throughout which their continuity has remained unbroken, or been subjected only to temporary interruption. If this be the case then although it would not necessarily follow there would be a strong probability in favour of the antiquity of the existing oceans.

When the probability that Deep sea life would be found persistant in form from geological periods to the present time first occurred to the writer some years since the exploration of the Deep sea was only beginning   It then appeared to him that the inorganic conditions of existence would be almost without change in in very deep oceans. The published results of the Deep Sea dredging expeditions have shewn that such cannot be the case and that changes in the conditions are taking place there as elsewhere. Still from the foregoing remarks it it would appear that on the bed of a deep ocean we have a combination of circumstances more unfavorable than obtain elsewhere for the production of new species; that is to say less change in the inorganic conditions of existence as far as we know them over large areas and for long periods. Slow and gradual changes in the level of the ocean bed would most likely preserve the continuity of former oceans with those of the present day and this owing to the small changes which would generally be produced in the conditions of existence at the sea bottom by even considerable alterations in the level of the ocean bed would enable some proportion of those forms of life which can exist at great depths to migrate slowly and live for long periods all but unchanged. From these considerations we might expect to find surviving in the deeper parts of the ocean some forms of life which have hitherto been known only as fossils embedded in strata built up in the depths of bygone oceans.

In his Presidential Address to the Geological Society in 1871 M^r Prestwich gave a very complete summary of the results of the Deep Sea exploration so far as it had been carried out up to that time, with tables shewing the relationship of the Atlantic Deep sea fauna to extinct and living forms. {Journal of the Geological Society of London Vol. XXVII}¹⁴ He sums up this part of the question by saying “while the Foraminifera obtained in these deep sea explorations shew a strong community of genera and a partial identity of species with chalk Foraminifera, the sponges markedly representative forms, the Echinoderms allied genera, and the Brachiopoda a similarity of groups, on the other hand as we ascend the scale so far from any resemblance being maintained we find actual identities with the upper Miocene, Pliocene and Quaternary fauna of Western Europe.” {ibid. pg. LVI}¹⁵ The species enumerated in the tables given by M^r Prestwich from which these deductions are drawn were taken in comparatively moderate depths. Out of the 170 dredging stations of the Lightning and Porcupine expeditions only 22 were in depths over 800 fathoms, and only 16 in depths over 1000 fathoms and I have endeavoured to shew that in the latter depth we are most likely to find forms of life which have been persistent through long periods as it is there that the fewest changes in the conditions of life can have taken place.

With the same data as M^r Prestwich much the same conclusions are arrived at by Prof Wyville Thompson in the Chapter in the Depths of the Sea on the Continuity of the Chalk.¹⁶

The Challenger expedition judging from the Notes published in Nature has not so far brought forward any very striking facts bearing on the persistence of life   Species having much resemblance to fossil forms have in a few cases been found but on the whole life has been very scarce in the more extreme depths explored during this expedition.

As the deepest parts of the ocean are more fully explored and more especially those parts where even at great depths life is, as we have seen that it is in some places abundant we shall doubtless find that species have been more slow to change where “all things always seemed the same” than elsewhere. I cannot better conclude than by quoting the words of Prof. Wyville Thompson who says “our dredgings only shew that the abysses of the ocean—abysses which Sir Charles Lyell admits to have outlived a succession of geological epochs¹⁷—are inhabited by a special fauna possibly as persistent in its general features as the abysses themselves”. {Depths of the Sea pg. 495.}