Discourses - Thomas H. Huxley
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"All along the edge of the ice-pack--everywhere, in fact, to the south of
the two stations--on the 11th of February on our southward voyage, and on
the 3rd of March on our return, we brought up fine sand and grayish mud,
with small pebbles of quartz and felspar, and small fragments of mica-
slate, chlorite-slate, clay-slate, gneiss, and granite. This deposit, I
have no doubt, was derived from the surface like the others, but in this
case by the melting of icebergs and the precipitation of foreign matter
contained in the ice.
"We never saw any trace of gravel or sand, or any material necessarily
derived from land, on an iceberg. Several showed vertical or irregular
fissures filled with discoloured ice or snow; but, when looked at
closely, the discoloration proved usually to be very slight, and the
effect at a distance was usually due to the foreign material filling the
fissure reflecting light less perfectly than the general surface of the
berg. I conceive that the upper surface of one of these great tabular
southern icebergs, including by far the greater part of its bulk, and
culminating in the portion exposed above the surface of the sea, was
formed by the piling up of successive layers of snow during the period,
amounting perhaps to several centuries, during which the ice-cap was
slowly forcing itself over the low land and out to sea over a long extent
of gentle slope, until it reached a depth considerably above 200 fathoms,
when the lower specific weight of the ice caused an upward strain which
at length overcame the cohesion of the mass, and portions were rent off
and floated away. If this be the true history of the formation of these
icebergs, the absence of all land _debris_ in the portion exposed above
the surface of the sea is readily understood. If any such exist, it must
be confined to the lower part of the berg, to that part which has at one
time or other moved on the floor of the ice-cap.
"The icebergs, when they are first dispersed, float in from 200 to 250
fathoms. When, therefore, they have been drifted to latitudes of 65 deg. or
64 deg. S., the bottom of the berg just reaches the layer at which the
temperature of the water is distinctly rising, and it is rapidly melted,
and the mud and pebbles with which it is more or less charged are
precipitated. That this precipitation takes place all over the area where
the icebergs are breaking up, constantly, and to a considerable extent,
is evident from the fact of the soundings being entirely composed of such
deposits; for the Diatoms, _Globigerinoe_, and radiolarians are present
on the surface in large numbers; and unless the deposit from the ice were
abundant it would soon be covered and masked by a layer of the exuvia of
surface organisms."
The observations which have been detailed leave no doubt that the
Antarctic sea bottom, from a little to the south of the fiftieth
parallel, as far as 80 deg. S., is being covered by a fine deposit of
silicious mud, more or less mixed, in some parts, with the ice-borne
_debris_ of polar lands and with the ejections of volcanoes. The
silicious particles which constitute this mud, are derived, in part, from
the diatomaceous plants and radiolarian animals which throng the surface,
and, in part, from the spicula of sponges which live at the bottom. The
evidence respecting the corresponding Arctic area is less complete, but
it is sufficient to justify the conclusion that an essentially similar
silicious cap is being formed around the northern pole.
There is no doubt that the constituent particles of this mud may
agglomerate into a dense rock, such as that formed at Oran on the shores
of the Mediterranean, which is made up of similar materials. Moreover, in
the case of freshwater deposits of this kind it is certain that the
action of percolating water may convert the originally soft and friable,
fine-grained sandstone into a dense, semi-transparent opaline stone, the
silicious organized skeletons being dissolved, and the silex re-deposited
in an amorphous state. Whether such a metamorphosis as this occurs in
submarine deposits, as well as in those formed in fresh water, does not
appear; but there seems no reason to doubt that it may. And hence it may
not be hazardous to conclude that very ordinary metamorphic agencies may
convert these polar caps into a form of quartzite.
In the great intermediate zone, occupying some 110 deg. of latitude, which
separates the circumpolar Arctic and Antarctic areas of silicious
deposit, the Diatoms and _Radiolaria_ of the surface water and the
sponges of the bottom do not die out, and, so far as some forms are
concerned, do not even appear to diminish in total number; though, on a
rough estimate, it would appear that the proportion of _Radiolaria_ to
Diatoms is much greater than in the colder seas. Nevertheless the
composition of the deep-sea mud of this intermediate zone is entirely
different from that of the circumpolar regions.
The first exact information respecting the nature of this mud at depths
greater than 1,000 fathoms was given by Ehrenberg, in the account which
he published in the "Monatsberichte" of the Berlin Academy for the year
1853, of the soundings obtained by Lieut. Berryman, of the United States
Navy, in the North Atlantic, between Newfoundland and the Azores.
Observations which confirm those of Ehrenberg in all essential respects
have been made by Professor Bailey, myself, Dr. Wallich, Dr. Carpenter,
and Professor Wyville Thomson, in their earlier cruises; and the
continuation of the _Globigerina_ ooze over the South Pacific has been
proved by the recent work of the _Challenger_, by which it is also shown,
for the first time, that, in passing from the equator to high southern
latitudes, the number and variety of the _Foraminifera_ diminishes, and
even the _Globigerinoe_ become dwarfed. And this result, it will be
observed, is in entire accordance with the fact already mentioned that,
in the sea of Kamschatka, the deep-sea mud was found by Bailey to contain
no calcareous organisms.
Thus, in the whole of the "intermediate zone," the silicious deposit
which is being formed there, as elsewhere, by the accumulation of sponge-
spicula, _Radiolaria_, and Diatoms, is obscured and overpowered by the
immensely greater amount of calcareous sediment, which arises from the
aggregation of the skeletons of dead _Foraminifera_. The similarity of
the deposit, thus composed of a large percentage of carbonate of lime,
and a small percentage of silex, to chalk, regarded merely as a kind of
rock, which was first pointed out by Ehrenberg,[5] is now admitted on all
hands; nor can it be reasonably doubted, that ordinary metamorphic
agencies are competent to convert the "modern chalk" into hard limestone
or even into crystalline marble.
[Footnote 5: The following passages in Ehrenberg's memoir on _The
Organisms in the Chalk which are still living_ (1839), are conclusive:--
"7. The dawning period of the existing living organic creation, if such a
period is distinguishable (which is doubtful), can only be supposed to
have existed on the other side of, and below, the chalk formation; and
thus, either the chalk, with its widespread and thick beds, must enter
into the series of newer formations; or some of the accepted four great
geological periods, the quaternary, tertiary, and secondary formations,
contain organisms which still live. It is more probable, in the
proportion of 3 to 1, that the transition or primary period is not
different, but that it is only more difficult to examine and understand,
by reason of the gradual and prolonged chemical decomposition and
metamorphosis of many of its organic constituents."
"10. By the mass-forming _Infasoria_ and _Polythalamia_, secondary are
not distinguishable from tertiary formations; and, from what has been
said, it is possible that, at this very day, rock masses are forming in
the sea, and being raised by volcanic agencies, the constitution of
which, on the whole, is altogether similar to that of the chalk. The
chalk remains distinguishable by its organic remains as a formation, but
not as a kind of rock."]
Ehrenberg appears to have taken it for granted that the _Globigerinoe_
and other _Foraminifera_ which are found in the deep-sea mud, live at the
great depths in which their remains are found; and he supports this
opinion by producing evidence that the soft parts of these organisms are
preserved, and may be demonstrated by removing the calcareous matter with
dilute acids. In 1857, the evidence for and against this conclusion
appeared to me to be insufficient to warrant a positive conclusion one
way or the other, and I expressed myself in my report to the Admiralty on
Captain Dayman's soundings in the following terms:--
"When we consider the immense area over which this deposit is spread, the
depth at which its formation is going on, and its similarity to chalk,
and still more to such rocks as the marls of Caltanisetta, the question,
whence are all these organisms derived? becomes one of high scientific
interest.
"Three answers have suggested themselves:--
"In accordance with the prevalent view of the limitation of life to
comparatively small depths, it is imagined either: 1, that these
organisms have drifted into their present position from shallower waters;
or 2, that they habitually live at the surface of the ocean, and only
fall down into their present position.
"1. I conceive that the first supposition is negatived by the extremely
marked zoological peculiarity of the deep-sea fauna.
"Had the _Globigerinoe_ been drifted into their present position from
shallow water, we should find a very large proportion of the
characteristic inhabitants of shallow waters mixed with them, and this
would the more certainly be the case, as the large _Globigerinoe_, so
abundant in the deep-sea soundings, are, in proportion to their size,
more solid and massive than almost any other _Foraminifera_. But the fact
is that the proportion of other _Foraminifera_ is exceedingly small, nor
have I found as yet, in the deep-sea deposits, any such matters as
fragments of molluscous shells, of _Echini_, &c., which abound in shallow
waters, and are quite as likely to be drifted as the heavy
_Globigerinoe_. Again, the relative proportions of young and fully formed
_Globigerinoe_ seem inconsistent with the notion that they have travelled
far. And it seems difficult to imagine why, had the deposit been
accumulated in this way, _Coscinodisci_ should so almost entirely
represent the _Diatomaceoe_.
"2. The second hypothesis is far more feasible, and is strongly supported
by the fact that many _Polycistineoe [Radiolaria]_ and _Coscinodisci_ are
well known to live at the surface of the ocean. Mr. Macdonald, Assistant-
Surgeon of H.M.S. _Herald_, now in the South-Western Pacific, has lately
sent home some very valuable observations on living forms of this kind,
met with in the stomachs of oceanic mollusks, and therefore certainly
inhabitants of the superficial layer of the ocean. But it is a singular
circumstance that only one of the forms figured by Mr. Macdonald is at
all like a _Globigerina_, and there are some peculiarities about even
this which make me greatly doubt its affinity with that genus. The form,
indeed, is not unlike that of a _Globigerina_, but it is provided with
long radiating processes, of which I have never seen any trace in
_Globigerina_. Did they exist, they might explain what otherwise is a
great objection to this view, viz., how is it conceivable that the heavy
_Globigerina_ should maintain itself at the surface of the water?
"If the organic bodies in the deep-sea soundings have neither been
drifted, nor have fallen from above, there remains but one alternative--
they must have lived and died where they are.
"Important objections, however, at once suggest themselves to this view.
How can animal life be conceived to exist under such conditions of light,
temperature, pressure, and aeration as must obtain at these vast depths?
"To this one can only reply that we know for a certainty that even very
highly-organized animals do continue to live at a depth of 300 and 400
fathoms, inasmuch as they have been dredged up thence; and that the
difference in the amount of light and heat at 400 and at 2,000 fathoms is
probably, so to speak, very far less than the difference in complexity of
organisation between these animals and the humbler _Protozoa_ and
_Protophyta_ of the deep-sea soundings.
"I confess, though as yet far from regarding it proved that the
_Globigerinoe_ live at these depths, the balance of probabilities seems
to me to incline in that direction. And there is one circumstance which
weighs strongly in my mind. It may be taken as a law that any genus of
animals which is found far back in time is capable of living under a
great variety of circumstances as regards light, temperature, and
pressure. Now, the genus _Globigerina_ is abundantly represented in the
cretaceous epoch, and perhaps earlier.
"I abstain, however, at present from drawing any positive conclusions,
preferring rather to await the result of more extended observations."[6]
[Footnote 6: Appendix to Report on Deep-sea Soundings in the Atlantic
Ocean, by Lieut.-Commander Joseph Dayman. 1857.]
Dr. Wallich, Professor Wyville Thomson, and Dr. Carpenter concluded that
the _Globigerinoe_ live at the bottom. Dr. Wallich writes in 1862--"By
sinking very fine gauze nets to considerable depths, I have repeatedly
satisfied myself that _Globigerina_ does not occur in the superficial
strata of the ocean."[7] Moreover, having obtained certain living star-
fish from a depth of 1,260 fathoms, and found their stomachs full of
"fresh-looking _Globigerinoe_" and their _debris_--he adduces this fact
in support of his belief that the _Globigerinoe_ live at the bottom.
[Footnote 7: The _North Atlantic Sea-bed_, p. 137.]
On the other hand, Mueller, Haeckel, Major Owen, Mr. Gwyn Jeffries, and
other observers, found that _Globigerinoe_, with the allied genera
_Orbulina_ and _Pulvinulina_, sometimes occur abundantly at the surface
of the sea, the shells of these pelagic forms being not unfrequently
provided with the long spines noticed by Macdonald; and in 1865 and 1866,
Major Owen more especially insisted on the importance of this fact. The
recent work of the _Challenger_ fully confirms Major Owen's statement. In
the paper recently published in the proceedings of the Royal Society,[8]
from which a quotation has already been made, Professor Wyville Thomson
says:--
"I had formed and expressed a very strong opinion on the matter. It
seemed to me that the evidence was conclusive that the _Foraminifera_
which formed the _Globigerina_ ooze lived on the bottom, and that the
occurrence of individuals on the surface was accidental and exceptional;
but after going into the thing carefully, and considering the mass of
evidence which has been accumulated by Mr. Murray, I now admit that I was
in error; and I agree with him that it may be taken as proved that all
the materials of such deposits, with the exception, of course, of the
remains of animals which we now know to live at the bottom at all depths,
which occur in the deposit as foreign bodies, are derived from the
surface.
[Footnote 8: "Preliminary Notes on the Nature of the Sea-bottom procured
by the soundings of H.M.S. _Challenger_ during her cruise in the Southern
Seas, in the early part of the year 1874."--_Proceedings of the Royal
Society_, Nov. 26, 1874.]
"Mr. Murray has combined with a careful examination of the soundings a
constant use of the tow-net, usually at the surface, but also at depths
of from ten to one hundred fathoms; and he finds the closest relation to
exist between the surface fauna of any particular locality and the
deposit which is taking place at the bottom. In all seas, from the
equator to the polar ice, the tow-net contains _Globigerinoe_. They are
more abundant and of a larger size in warmer seas; several varieties,
attaining a large size and presenting marked varietal characters, are
found in the intertropical area of the Atlantic. In the latitude of
Kerguelen they are less numerous and smaller, while further south they
are still more dwarfed, and only one variety, the typical _Globigerina
bulloides_, is represented. The living _Globigerinoe_ from the tow-net
are singularly different in appearance from the dead shells we find at
the bottom. The shell is clear and transparent, and each of the pores
which penetrate it is surrounded by a raised crest, the crest round
adjacent pores coalescing into a roughly hexagonal network, so that the
pores appear to lie at the bottom of a hexagonal pit. At each angle of
this hexagon the crest gives off a delicate flexible calcareous spine,
which is sometimes four or five times the diameter of the shell in
length. The spines radiate symmetrically from the direction of the centre
of each chamber of the shell, and the sheaves of long transparent needles
crossing one another in different directions have a very beautiful
effect. The smaller inner chambers of the shell are entirely filled with
an orange-yellow granular sarcode; and the large terminal chamber usually
contains only a small irregular mass, or two or three small masses run
together, of the same yellow sarcode stuck against one side, the
remainder of the chamber being empty. No definite arrangement and no
approach to structure was observed in the sarcode, and no
differentiation, with the exception of round bright-yellow oil-globules,
very much like those found in some of the radiolarians, which are
scattered, apparently irregularly, in the sarcode. We never have been
able to detect, in any of the large number of _Globigerinoe_ which we
have examined, the least trace of pseudopodia, or any extension, in any
form, of the sarcode beyond the shell.
* * * * *
"In specimens taken with the tow-net the spines are very usually absent;
but that is probably on account of their extreme tenuity; they are broken
off by the slightest touch. In fresh examples from the surface, the dots
indicating the origin of the lost spines may almost always be made out
with a high power. There are never spines on the _Globigerinoe_ from the
bottom, even in the shallowest water."
There can now be no doubt, therefore, that _Globigerinoe_ live at the top
of the sea; but the question may still be raised whether they do not also
live at the bottom. In favour of this view, it has been urged that the
shells of the _Globigerinoe_ of the surface never possess such thick
walls as those which are fouled at the bottom, but I confess that I doubt
the accuracy of this statement. Again, the occurrence of minute
_Globigerinoe_ in all stages of development, at the greatest depths, is
brought forward as evidence that they live _in situ_. But considering the
extent to which the surface organisms are devoured, without
discrimination of young and old, by _Salpoe_ and the like, it is not
wonderful that shells of all ages should be among the rejectamenta. Nor
can the presence of the soft parts of the body in the shells which form
the _Globigerina_ ooze, and the fact, if it be one, that animals living
at the bottom use them as food, be considered as conclusive evidence that
the _Globigerinoe_ live at the bottom. Such as die at the surface, and
even many of those which are swallowed by other animals, may retain much
of their protoplasmic matter when they reach the depths at which the
temperature sinks to 34 deg. or 32 deg. Fahrenheit, where decomposition must
become exceedingly slow.
Another consideration appears to me to be in favour of the view that the
_Globigerinoe_ and their allies are essentially surface animals. This is
the fact brought out by the _Challenger's_ work, that they have a
southern limit of distribution, which can hardly depend upon anything but
the temperature of the surface water. And it is to be remarked that this
southern limit occurs at a lower latitude in the Antarctic seas than it
does in the North Atlantic. According to Dr. Wallich ("The North Atlantic
Sea Bed," p. 157) _Globigerina_ is the prevailing form in the deposits
between the Faroe Islands and Iceland, and between Iceland and East
Greenland--or, in other words, in a region of the sea-bottom which lies
altogether north of the parallel of 60 deg. N.; while in the southern seas,
the _Globigerinoe_ become dwarfed and almost disappear between 50 deg. and
55 deg. S. On the other hand, in the sea of Kamschatka, the _Globigerinoe_
have vanished in 56 deg. N., so that the persistence of the _Globigerina_
ooze in high latitudes, in the North Atlantic, would seem to depend on
the northward curve of the isothermals peculiar to this region; and it is
difficult to understand how the formation of _Globigerina_ ooze can be
affected by this climatal peculiarity unless it be effected by surface
animals.
Whatever may be the mode of life of the _Foraminifera_, to which the
calcareous element of the deep-sea "chalk" owes its existence, the fact
that it is the chief and most widely spread material of the sea-bottom in
the intermediate zone, throughout both the Atlantic and Pacific Oceans,
and the Indian Ocean, at depths from a few hundred to over two thousand
fathoms, is established. But it is not the only extensive deposit which
is now taking place. In 1853, Count Pourtales, an officer of the United
States Coast Survey, which has done so much for scientific hydrography,
observed, that the mud forming the sea-bottom at depths of one hundred
and fifty fathoms, in 31 deg. 32' N., 79 deg. 35' W., off the Coast of Florida,
was "a mixture, in about equal proportions, of _Globigerinoe_ and black
sand, probably greensand, as it makes a green mark when crushed on
paper." Professor Bailey, examining these grains microscopically, found
that they were casts of the interior cavities of _Foraminifera_,
consisting of a mineral known as _Glauconite_, which is a silicate of
iron and alumina. In these casts the minutest cavities and finest tubes
in the Foraminifer were sornetilnes reproduced in solid counterparts of
the glassy mineral, while the calcareous original had been entirely
dissolved away.
Contemporaneously with these observations, the indefatigable Ehrenberg
had discovered that the "greensands" of the geologist were largely made
up of casts of a similar character, and proved the existence of
_Foraminifera_ at a very ancient geological epoch, by discovering such
casts in a greensand of Lower Silurian age, which occurs near St.
Petersburg.
Subsequently, Messrs. Parker and Jones discovered similar casts in
process of formation, the original shell not having disappeared, in
specimens of the sea-bottom of the Australian seas, brought home by the
late Professor Jukes. And the _Challenger_ has observed a deposit of a
similar character in the course of the Agulhas current, near the Cape of
Good Hope, and in some other localities not yet defined.
It would appear that this infiltration of _Foraminifera_ shells with
_Glauconite_ does not take place at great depths, but rather in what may
be termed a sublittoral region, ranging from a hundred to three hundred
fathoms. It cannot be ascribed to any local cause, for it takes place,
not only over large areas in the Gulf of Mexico and the Coast of Florida,
but in the South Atlantic and in the Pacific. But what are the conditions
which determine its occurrence, and whence the silex, the iron, and the
alumina (with perhaps potash and some other ingredients in small
quantity) of which the _Glauconite_ is composed, proceed, is a point on
which no light has yet been thrown. For the present we must be content
with the fact that, in certain areas of the "intermediate zone,"
greensand is replacing and representing the primitively calcareo-
silicious ooze.
The investigation of the deposits which are now being formed in the basin
of the Mediterranean, by the late Professor Edward Forbes, by Professor
Williamson and more recently by Dr. Carpenter, and a comparison of the
results thus obtained with what is known of the surface fauna, have
brought to light the remarkable fact, that while the surface and the
shallows abound with _Foraminifera_ and other calcareous shelled
organisms, the indications of life become scanty at depths beyond 500 or
600 fathoms, while almost all traces of it disappear at greater depths,
and at 1,000 to 2,000 fathoms the bottom is covered with a fine clay.
Dr. Carpenter has discussed the significance of this remarkable fact, and
he is disposed to attribute the absence of life at great depths, partly
to the absence of any circulation of the water of the Mediterranean at
such depths, and partly to the exhaustion of the oxygen of the water by
the organic matter contained in the fine clay, which he conceives to be
formed by the finest particles of the mud brought down by the rivers
which flow into the Mediterranean.
However this may be, the explanation thus offered of the presence of the
fine mud, and of the absence of organisms which ordinarily live at the
bottom, does not account for the absence of the skeletons of the
organisms which undoubtedly abound at the surface of the Mediterranean;
and it would seem to have no application to the remarkable fact
discovered by the _Challenger_, that in the open Atlantic and Pacific
Oceans, in the midst of the great intermediate zone, and thousands of
miles away from the embouchure of any river, the sea-bottom, at depths
approaching to and beyond 3,000 fathoms, no longer consists of
_Globigerina_ ooze, but of an excessively fine red clay.