continuity of the modern with the ancient chalk has nothing to do with
the proposition that we can, in any sense whatever, be said to be still
living in the Cretaceous epoch. When the _Challenger's_ trawl brings up
an _Ichthyosaurus_, along with a few living specimens of _Belemnites_ and
_Turrilites_, it may be admitted that she has come upon a cretaceous
"outlier." A geological period is characterized not only by the presence
of those creatures which lived in it, but by the absence of those which
have only come into existence later; and, however large a proportion of
true cretaceous forms may be discovered in the deep sea, the modern types
associated with them must be abolished before the Fauna, as a whole,
could, with any propriety, be termed Cretaceous.
I have now indicated some of the chief lines of Biological inquiry, in
which the _Challenger_ has special opportunities for doing good service,
and in following which she will be carrying out the work already
commenced by the _Lightning_ and _Porcupine_ in their cruises of 1868 and
subsequent years.
But biology, in the long run, rests upon physics, and the first condition
for arriving at a sound theory of distribution in the deep sea, is the
precise ascertainment of the conditions of life; or, in other words, a
full knowledge of all those phenomena which are embraced under the head
of the Physical Geography of the Ocean.
Excellent work has already been done in this direction, chiefly under the
superintendence of Dr. Carpenter, by the _Lightning_ and the
_Porcupine_,[10] and some data of fundamental importance to the physical
geography of the sea have been fixed beyond a doubt.
[Footnote 10: _Proceedings of the Royal Society_, 1870 and 1872]
Thus, though it is true that sea-water steadily contracts as it cools
down to its freezing point, instead of expanding before it reaches its
freezing point as fresh water does, the truth has been steadily ignored
by even the highest authorities in physical geography, and the erroneous
conclusions deduced from their erroneous premises have been widely
accepted as if they were ascertained facts. Of course, if sea-water, like
fresh water, were heaviest at a temperature of 39° F. and got lighter as
it approached 32° F., the water of the bottom of the deep sea could not
be colder than 39°. But one of the first results of the careful
ascertainment of the temperature at different depths, by means of
thermometers specially contrived for the avoidance of the errors produced
by pressure, was the proof that, below 1000 fathoms in the Atlantic, down
to the greatest depths yet sounded, the water has a temperature always
lower than 38° Fahr., whatever be the temperature of the water at the
surface. And that this low temperature of the deepest water is probably
the universal rule for the depths of the open ocean is shown, among
others, by Captain Chimmo's recent observations in the Indian ocean,
between Ceylon and Sumatra, where, the surface water ranging from 85°-81°
Fahr., the temperature at the bottom, at a depth of 2270 to 2656 fathoms,
was only from 34° to 32° Fahr.
As the mean temperature of the superficial layer of the crust of the
earth may be taken at about 50° Fahr., it follows that the bottom layer
of the deep sea in temperate and hot latitudes, is, on the average, much
colder than either of the bodies with which it is in contact; for the
temperature of the earth is constant, while that of the air rarely falls
so low as that of the bottom water in the latitudes in question; and even
when it does, has time to affect only a comparatively thin stratum of the
surface water before the return of warm weather.
How does this apparently anomalous state of things come about? If we
suppose the globe to be covered with a universal ocean, it can hardly be
doubted that the cold of the regions towards the poles must tend to cause
the superficial water of those regions to contract and become
specifically heavier. Under these circumstances, it would have no
alternative but to descend and spread over the sea bottom, while its
place would be taken by warmer water drawn from the adjacent regions.
Thus, deep, cold, polar-equatorial currents, and superficial, warmer,
equatorial-polar currents, would be set up; and as the former would have
a less velocity of rotation from west to east than the regions towards
which they travel, they would not be due southerly or northerly currents,
but south-westerly in the northern hemisphere, and north-westerly in the
southern; while, by a parity of reasoning, the equatorial-polar warm
currents would be north-easterly in the northern hemisphere, and south-
easterly in the southern. Hence, as a north-easterly current has the same
direction as a south-westerly wind, the direction of the northern
equatorial-polar current in the extra-tropical part of its course would
pretty nearly coincide with that of the anti-trade winds. The freezing of
the surface of the polar sea would not interfere with the movement thus
set up. For, however bad a conductor of heat ice may be, the unfrozen
sea-water immediately in contact with the undersurface of the ice must
needs be colder than that further off; and hence will constantly tend to
descend through the subjacent warmer water.
In this way, it would seem inevitable that the surface waters of the
northern and southern frigid zones must, sooner or later, find their way
to the bottom of the rest of the ocean; and there accumulate to a
thickness dependent on the rate at which they absorb heat from the crust
of the earth below, and from the surface water above.
If this hypothesis be correct, it follows that, if any part of the ocean
in warm latitudes is shut off from the influence of the cold polar
underflow, the temperature of its deeps should be less cold than the
temperature of corresponding depths in the open sea. Now, in the
Mediterranean, Nature offers a remarkable experimental proof of just the
kind needed. It is a landlocked sea which runs nearly east and west,
between the twenty-ninth and forty-fifth parallels of north latitude.
Roughly speaking, the average temperature of the air over it is 75° Fahr.
in July and 48° in January.
This great expanse of water is divided by the peninsula of Italy
(including Sicily), continuous with which is a submarine elevation
carrying less than 1,200 feet of water, which extends from Sicily to Cape
Bon in Africa, into two great pools--an eastern and a western. The
eastern pool rapidly deepens to more than 12,000 feet, and sends off to
the north its comparatively shallow branches, the Adriatic and the Aegean
Seas. The western pool is less deep, though it reaches some 10,000 feet.
And, just as the western end of the eastern pool communicates by a
shallow passage, not a sixth of its greatest depth, with the western
pool, so the western pool is separated from the Atlantic by a ridge which
runs between Capes Trafalgar and Spartel, on which there is hardly 1,000
feet of water. All the water of the Mediterranean which lies deeper than
about 150 fathoms, therefore, is shut off from that of the Atlantic, and
there is no communication between the cold layer of the Atlantic (below
1,000 fathoms) and the Mediterranean. Under these circumstances, what is
the temperature of the Mediterranean? Everywhere below 600 feet it is
about 55° Fahr.; and consequently, at its greatest depths, it is some 20°
warmer than the corresponding depths of the Atlantic.
It seems extremely difficult to account for this difference in any other
way, than by adopting the views so strongly and ably advocated by Dr.
Carpenter, that, in the existing distribution of land and water, such a
circulation of the water of the ocean does actually occur, as
theoretically must occur, in the universal ocean, with which we started.
It is quite another question, however, whether this theoretic
circulation, true cause as it may be, is competent to give rise to such
movements of sea-water, in mass, as those currents, which have commonly
been regarded as northern extensions of the Gulf-stream. I shall not
venture to touch upon this complicated problem; but I may take occasion
to remark that the cause of a much simpler phenomenon--the stream of
Atlantic water which sets through the Straits of Gibraltar, eastward, at
the rate of two or three miles an hour or more, does not seem to be so
clearly made out as is desirable.
The facts appear to be that the water of the Mediterranean is very
slightly denser than that of the Atlantic (1.0278 to 1.0265), and that
the deep water of the Mediterranean is slightly denser than that of the
surface; while the deep water of the Atlantic is, if anything, lighter
than that of the surface. Moreover, while a rapid superficial current is
setting in (always, save in exceptionally violent easterly winds) through
the Straits of Gibraltar, from the Atlantic to the Mediterranean, a deep
undercurrent (together with variable side currents) is setting out
through the Straits, from the Mediterranean to the Atlantic.
Dr. Carpenter adopts, without hesitation, the view that the cause of this
indraught of Atlantic water is to be sought in the much more rapid
evaporation which takes place from the surface of the Mediterranean than
from that of the Atlantic; and thus, by lowering the level of the former,
gives rise to an indraught from the latter.
But is there any sound foundation for the three assumptions involved
here? Firstly, that the evaporation from the Mediterranean, as a whole,
is much greater than that from the Atlantic under corresponding
parallels; secondly, that the rainfall over the Mediterranean makes up
for evaporation less than it does over the Atlantic; and thirdly,
supposing these two questions answered affirmatively: Are not these
sources of loss in the Mediterranean fully covered by the prodigious
quantity of fresh water which is poured into it by great rivers and
submarine springs? Consider that the water of the Ebro, the Rhine, the
Po, the Danube, the Don, the Dnieper, and the Nile, all flow directly or
indirectly into the Mediterranean; that the volume of fresh water which
they pour into it is so enormous that fresh water may sometimes be baled
up from the surface of the sea off the Delta of the Nile, while the land
is not yet in sight; that the water of the Black Sea is half fresh, and
that a current of three or four miles an hour constantly streams from it
Mediterraneanwards through the Bosphorus;--consider, in addition, that no
fewer than ten submarine springs of fresh water are known to burst up in
the Mediterranean, some of them so large that Admiral Smyth calls them
"subterranean rivers of amazing volume and force"; and it would seem, on
the face of the matter, that the sun must have enough to do to keep the
level of the Mediterranean down; and that, possibly, we may have to seek
for the cause of the small superiority in saline contents of the
Mediterranean water in some condition other than solar evaporation.
Again, if the Gibraltar indraught is the effect of evaporation, why does
it go on in winter as well as in summer?
All these are questions more easily asked than answered; but they must be
answered before we can accept the Gibraltar stream as an example of a
current produced by indraught with any comfort.
The Mediterranean is not included in the _Challenger's_ route, but she
will visit one of the most promising and little explored of
hydrographical regions--the North Pacific, between Polynesia and the
Asiatic and American shores; and doubtless the store of observations upon
the currents of this region, which she will accumulate, when compared
with what we know of the North Atlantic, will throw a powerful light upon
the present obscurity of the Gulf-stream problem.
III
ON SOME OF THE RESULTS OF THE EXPEDITION OF H.M.S. _CHALLLENGER_
[1875]
In May, 1873, I drew attention[1] to the important problems connected
with the physics and natural history of the sea, to the solution of which
there was every reason to hope the cruise of H.M.S. _Challenger_ would
furnish important contributions. The expectation then expressed has not
been disappointed. Reports to the Admiralty, papers communicated to the
Royal Society, and large collections which have already been sent home,
have shown that the _Challenger's_ staff have made admirable use of their
great opportunities; and that, on the return of the expedition in 1874,
their performance will be fully up to the level of their promise. Indeed,
I am disposed to go so far as to say, that if nothing more came of the
_Challengers_ expedition than has hitherto been yielded by her
exploration of the nature of the sea bottom at great depths, a full
scientific equivalent of the trouble and expense of her equipment would
have been obtained.
[Footnote 1: See the preceding Essay.]
In order to justify this assertion, and yet, at the same time, not to
claim more for Professor Wyville Thomson and his colleagues than is their
due, I must give a brief history of the observations which have preceded
their exploration of this recondite field of research, and endeavour to
make clear what was the state of knowledge in December, 1872, and what
new facts have been added by the scientific staff of the _Challenger_. So
far as I have been able to discover, the first successful attempt to
bring up from great depths more of the sea bottom than would adhere to a
sounding-lead, was made by Sir John Ross, in the voyage to the Arctic
regions which he undertook in 1818. In the Appendix to the narrative of
that voyage, there will be found an account of a very ingenious apparatus
called "clams"--a sort of double scoop--of his own contrivance, which Sir
John Ross had made by the ship's armourer; and by which, being in
Baffin's Bay, in 72° 30' N. and 77° 15' W., he succeeded in bringing up
from 1,050 fathoms (or 6,300 feet), "several pounds" of a "fine green
mud," which formed the bottom of the sea in this region. Captain (now Sir
Edward) Sabine, who accompanied Sir John Ross on this cruise, says of
this mud that it was "soft and greenish, and that the lead sunk several
feet into it." A similar "fine green mud" was found to compose the sea
bottom in Davis Straits by Goodsir in 1845. Nothing is certainly known of
the exact nature of the mud thus obtained, but we shall see that the mud
of the bottom of the Antarctic seas is described in curiously similar
terms by Dr. Hooker, and there is no doubt as to the composition of this
deposit.
In 1850, Captain Penny collected in Assistance Bay, in Kingston Bay, and
in Melville Bay, which lie between 73° 45' and 74° 40' N., specimens of
the residuum left by melted surface ice, and of the sea bottom in these
localities. Dr. Dickie, of Aberdeen, sent these materials to Ehrenberg,
who made out[2] that the residuum of the melted ice consisted for the
most part of the silicious cases of diatomaceous plants, and of the
silicious spicula of sponges; while, mixed with these, were a certain
number of the equally silicious skeletons of those low animal organisms,
which were termed _Polycistineoe_ by Ehrenberg, but are now known as
_Radiolaria_.
[Footnote 2: _Ueber neue Anschauungen des kleinsten nördlichen
Polarlebens_.--Monatsberichte d. K. Akad. Berlin, 1853.]
In 1856, a very remarkable addition to our knowledge of the nature of the
sea bottom in high northern latitudes was made by Professor Bailey of
West Point. Lieutenant Brooke, of the United States Navy, who was
employed in surveying the Sea of Kamschatka, had succeeded in obtaining
specimens of the sea bottom from greater depths than any hitherto
reached, namely from 2,700 fathoms (16,200 feet) in 56° 46' N., and 168°
18' E.; and from 1,700 fathoms (10,200 feet) in 60° 15' N. and 170° 53'
E. On examining these microscopically, Professor Bailey found, as
Ehrenberg had done in the case of mud obtained on the opposite side of
the Arctic region, that the fine mud was made up of shells of
_Diatomacoe_, of spicula of sponges, and of _Radiolaria_, with a small
admixture of mineral matters, but without a trace of any calcareous
organisms.
Still more complete information has been obtained concerning the nature
of the sea bottom in the cold zone around the south pole. Between the
years 1839 and 1843, Sir James Clark Ross executed his famous Antarctic
expedition, in the course of which he penetrated, at two widely distant
points of the Antarctic zone, into the high latitudes of the shores of
Victoria Land and of Graham's Land, and reached the parallel of 80° S.
Sir James Ross was himself a naturalist of no mean acquirements, and Dr.
Hooker,[3] the present President of the Royal Society, accompanied him as
naturalist to the expedition, so that the observations upon the fauna and
flora of the Antarctic regions made during this cruise were sure to have
a peculiar value and importance, even had not the attention of the
voyagers been particularly directed to the importance of noting the
occurrence of the minutest forms of animal and vegetable life in the
ocean.
[Footnote 3: Now Sir Joseph Hooker. 1894.]
Among the scientific instructions for the voyage drawn up by a committee
of the Royal Society, however, there is a remarkable letter from Von
Humboldt to Lord Minto, then First Lord of the Admiralty, in which, among
other things, he dwells upon the significance of the researches into the
microscopic composition of rocks, and the discovery of the great share
which microscopic organisms take in the formation of the crust of the
earth at the present day, made by Ehrenberg in the years 1836-39.
Ehrenberg, in fact, had shown that the extensive beds of "rotten-stone"
or "Tripoli" which occur in various parts of the world, and notably at
Bilin in Bohemia, consisted of accumulations of the silicious cases and
skeletons of _Diatomaceoe_, sponges, and _Radiolaria_; he had proved that
similar deposits were being formed by _Diatomaceoe_, in the pools of the
Thiergarten in Berlin and elsewhere, and had pointed out that, if it were
commercially worth while, rotten-stone might be manufactured by a process
of diatom-culture. Observations conducted at Cuxhaven in 1839, had
revealed the existence, at the surface of the waters of the Baltic, of
living Diatoms and _Radiolaria_ of the same species as those which, in a
fossil state, constitute extensive rocks of tertiary age at Caltanisetta,
Zante, and Oran, on the shores of the Mediterranean.
Moreover, in the fresh-water rotten-stone beds of Bilin, Ehrenberg had
traced out the metamorphosis, effected apparently by the action of
percolating water, of the primitively loose and friable deposit of
organized particles, in which the silex exists in the hydrated or soluble
condition. The silex, in fact, undergoes solution and slow redeposition,
until, in ultimate result, the excessively fine-grained sand, each
particle of which is a skeleton, becomes converted into a dense opaline
stone, with only here and there an indication of an organism.
From the consideration of these facts, Ehrenberg, as early as the year
1839, had arrived at the conclusion that rocks, altogether similar to
those which constitute a large part of the crust of the earth, must be
forming, at the present day, at the bottom of the sea; and he threw out
the suggestion that even where no trace of organic structure is to be
found in the older rocks, it may have been lost by metamorphosis.[4]
[Footnote 4: _Ueber die noch jetzt zahlreich lebende Thierarten der
Kreidebildung und den Organismus der Polythalamien. Abhandlungen der Kön.
Akad. der Wissenchaften._ 1839. _Berlin_. 1841. I am afraid that this
remarkable paper has been somewhat overlooked in the recent discussions
of the relation of ancient rocks to modern deposits.]
The results of the Antarctic exploration, as stated by Dr. Hooker in the
"Botany of the Antarctic Voyage," and in a paper which he read before
the British Association in 1847, are of the greatest importance in
connection with these views, and they are so clearly stated in the former
work, which is somewhat inaccessible, that I make no apology for quoting
them at length--
"The waters and the ice of the South Polar Ocean were alike found to
abound with microscopic vegetables belonging to the order _Diatomaceoe_.
Though much too small to be discernible by the naked eye, they occurred
in such countless myriads as to stain the berg and the pack ice wherever
they were washed by the swell of the sea; and, when enclosed in the
congealing surface of the water, they imparted to the brash and pancake
ice a pale ochreous colour. In the open ocean, northward of the frozen
zone, this order, though no doubt almost universally present, generally
eludes the search of the naturalist; except when its species are
congregated amongst that mucous scum which is sometimes seen floating on
the waves, and of whose real nature we are ignorant; or when the coloured
contents of the marine animals who feed on these Algae are examined. To
the south, however, of the belt of ice which encircles the globe, between
the parallels of 50° and 70° S., and in the waters comprised between that
belt and the highest latitude ever attained by man, this vegetation is
very conspicuous, from the contrast between its colour and the white snow
and ice in which it is imbedded. Insomuch, that in the eightieth degree,
all the surface ice carried along by the currents, the sides of every
berg and the base of the great Victoria Barrier itself, within reach of
the swell, were tinged brown, as if the polar waters were charged with
oxide of iron.
"As the majority of these plants consist of very simple vegetable cells,
enclosed in indestructible silex (as other Algae are in carbonate of
lime), it is obvious that the death and decomposition of such multitudes
must form sedimentary deposits, proportionate in their extent to the
length and exposure of the coast against which they are washed, in
thickness to the power of such agents as the winds, currents, and sea,
which sweep them more energetically to certain positions, and in purity,
to the depth of the water and nature of the bottom. Hence we detected
their remains along every icebound shore, in the depths of the adjacent
ocean, between 80 and 400 fathoms. Off Victoria Barrier (a perpendicular
wall of ice between one and two hundred feet above the level of the sea)
the bottom of the ocean was covered with a stratum of pure white or green
mud, composed principally of the silicious shells of the _Diatomaceoe_.
These, on being put into water, rendered it cloudy like milk, and took
many hours to subside. In the very deep water off Victoria and Graham's
Land, this mud was particularly pure and fine; but towards the shallow
shores there existed a greater or less admixture of disintegrated rock
and sand; so that the organic compounds of the bottom frequently bore but
a small proportion to the inorganic." ...
"The universal existence of such an invisible vegetation as that of the
Antarctic Ocean, is a truly wonderful fact, and the more from its not
being accompanied by plants of a high order. During the years we spent
there, I had been accustomed to regard the phenomena of life as differing
totally from what obtains throughout all other latitudes, for everything
living appeared to be of animal origin. The ocean swarmed with
_Mollusca_, and particularly entomostracous _Crustacea_, small whales,
and porpoises; the sea abounded with penguins and seals, and the air with
birds; the animal kingdom was ever present, the larger creatures preying
on the smaller, and these again on smaller still; all seemed carnivorous.
The herbivorous were not recognised, because feeding on a microscopic
herbage, of whose true nature I had formed an erroneous impression. It
is, therefore, with no little satisfaction that I now class the
_Diatomaceoe_ with plants, probably maintaining in the South Polar Ocean
that balance between the vegetable and the animal kingdoms which prevails
over the surface of our globe. Nor is the sustenance and nutrition of the
animal kingdom the only function these minute productions may perform;
they may also be the purifiers of the vitiated atmosphere, and thus
execute in the Antarctic latitudes the office of our trees and grass turf
in the temperate regions, and the broad leaves of the palm, &c., in the
tropics." ...
With respect to the distribution of the _Diatomaceoe_, Dr. Hooker
remarks:--