Discourses - Thomas H. Huxley
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Professor Thomson gives the following account of this capital
discovery:--
"According to our present experience, the deposit of _Globigerina_ ooze
is limited to water of a certain depth, the extreme limit of the pure
characteristic formation being placed at a depth of somewhere about 2,250
fathoms. Crossing from these shallower regions occupied by the ooze into
deeper soundings, we find, universally, that the calcareous formation
gradually passes into, and is finally replaced by, an extremely fine pure
clay, which occupies, speaking generally, all depths below 2,500 fathoms,
and consists almost entirely of a silicate of the red oxide of iron and
alumina. The transition is very slow, and extends over several hundred
fathoms of increasing depth; the shells gradually lose their sharpness of
outline, and assume a kind of 'rotten' look and a brownish colour, and
become more and more mixed with a fine amorphous red-brown powder, which
increases steadily in proportion until the lime has almost entirely
disappeared. This brown matter is in the finest possible state of
subdivision, so fine that when, after sifting it to separate any
organisms it might contain, we put it into jars to settle, it remained
for days in suspension, giving the water very much the appearance and
colour of chocolate.
"In indicating the nature of the bottom on the charts, we came, from
experience and without any theoretical considerations, to use three terms
for soundings in deep water. Two of these, Gl. oz. and r. cl., were very
definite, and indicated strongly-marked formations, with apparently but
few characters in common; but we frequently got soundings which we could
not exactly call '_Globigerina_ ooze' or 'red clay,' and before we were
fully aware of the nature of these, we were in the habit of indicating
them as 'grey ooze' (gr. oz.) We now recognise the 'grey ooze' as an
intermediate stage between the _Globigerina_ ooze and the red clay; we
find that on one side, as it were, of an ideal line, the red clay
contains more and more of the material of the calcareous ooze, while on
the other, the ooze is mixed with an increasing proportion of 'red clay.'
"Although we have met with the same phenomenon so frequently, that we
were at length able to predict the nature of the bottom from the depth of
the soundings with absolute certainty for the Atlantic and the Southern
Sea, we had, perhaps, the best opportunity of observing it in our first
section across the Atlantic, between Teneriffe and St. Thomas. The first
four stations on this section, at depths from 1,525 to 2,220 fathoms,
show _Globigerina_ ooze. From the last of these, which is about 300 miles
from Teneriffe, the depth gradually increases to 2,740 fathoms at 500,
and 2,950 fathoms at 750 miles from Teneriffe. The bottom in these two
soundings might have been called 'grey ooze,' for although its nature has
altered entirely from the _Globigerina_ ooze, the red clay into which it
is rapidly passing still contains a considerable admixture of carbonate
of lime.
"The depth goes on increasing to a distance of 1,150 miles from
Teneriffe, when it reaches 3,150 fathoms; there the clay is pure and
smooth, and contains scarcely a trace of lime. From this great depth the
bottom gradually rises, and, with decreasing depth, the grey colour and
the calcareous composition of the ooze return. Three soundings in 2,050,
1,900, and 1,950 fathoms on the 'Dolphin Rise' gave highly characteristic
examples of the _Globigerina_ formation. Passing from the middle plateau
of the Atlantic into the western trough, with depths a little over 3,000
fathoms, the red clay returned in all its purity; and our last sounding,
in 1,420 fathoms, before reaching Sombrero, restored the _Globigerina_
ooze with its peculiar associated fauna.
"This section shows also the wide extension and the vast geological
importance of the red clay formation. The total distance from Teneriffe
to Sombrero is about 2,700 miles. Proceeding from east to west, we have--
About 80 miles of volcanic mud and sand,
" 350 " _Globigerina_ ooze,
" 1,050 " red clay,
" 330 " _Globigerina_ ooze,
" 850 " red clay,
" 40 " _Globigerina_ ooze;
giving a total of 1,900 miles of red clay to 720 miles of _Globigerina_
ooze.
"The nature and origin of this vast deposit of clay is a question of the
very greatest interest; and although I think there can be no doubt that
it is in the main solved, yet some matters of detail are still involved
in difficulty. My first impression was that it might be the most minutely
divided material, the ultimate sediment produced by the disintegration of
the land, by rivers and by the action of the sea on exposed coasts, and
held in suspension and distributed by ocean currents, and only making
itself manifest in places unoccupied by the _Globigerina_ ooze. Several
circumstances seemed, however, to negative this mode of origin. The
formation seemed too uniform: wherever we met with it, it had the same
character, and it only varied in composition in containing less or more
carbonate of lime.
"Again, the were gradually becoming more and more convinced that all the
important elements of the _Globigerina_ ooze lived on the surface, and it
seemed evident that, so long as the condition on the surface remained the
same, no alteration of contour at the bottom could possibly prevent its
accumulation; and the surface conditions in the Mid-Atlantic were very
uniform, a moderate current of a very equal temperature passing
continuously over elevations and depressions, and everywhere yielding to
the tow-net the ooze-forming _Foraminifera_ in the same proportion. The
Mid-Atlantic swarms with pelagic _Mollusca_, and, in moderate depths, the
shells of these are constantly mixed with the _Globigerina_ ooze,
sometimes in number sufficient to make up a considerable portion of its
bulk. It is clear that these shells must fall in equal numbers upon the
red clay, but scarcely a trace of one of them is ever brought up by the
dredge on the red clay area. It might be possible to explain the absence
of shell-secreting animals living on the bottom, on the supposition that
the nature of the deposit was injurious to them; but then the idea of a
current sufficiently strong to sweep them away is negatived by the
extreme fineness of the sediment which is being laid down; the absence of
surface shells appears to be intelligible only on the supposition that
they are in some way removed.
"We conclude, therefore, that the 'red clay' is not an additional
substance introduced from without, and occupying certain depressed
regions on account of some law regulating its deposition, but that it is
produced by the removal, by some means or other, over these areas, of the
carbonate of lime, which forms probably about 98 per cent. of the
material of the _Globigerina_ ooze. We can trace, indeed, every
successive stage in the removal of the carbonate of lime in descending
the slope of the ridge or plateau where the _Globigerina_ ooze is
forming, to the region of the clay. We find, first, that the shells of
pteropods and other surface _Mollusca_ which are constantly falling on
the bottom, are absent, or, if a few remain, they are brittle and yellow,
and evidently decaying rapidly. These shells of _Mollusca_ decompose more
easily and disappear sooner than the smaller, and apparently more
delicate, shells of rhizopods. The smaller _Foraminifera_ now give way,
and are found in lessening proportion to the larger; the coccoliths first
lose their thin outer border and then disappear; and the clubs of the
rhabdoliths get worn out of shape, and are last seen, under a high power,
as infinitely minute cylinders scattered over the field. The larger
_Foraminifera_ are attacked, and instead of being vividly white and
delicately sculptured, they become brown and worn, and finally they break
up, each according to its fashion; the chamber-walls of _Globigerina_
fall into wedge-shaped pieces, which quickly disappear, and a thick rough
crust breaks away from the surface of _Orbulina_, leaving a thin inner
sphere, at first beautifully transparent, but soon becoming opaque and
crumbling away.
"In the meantime the proportion of the amorphous 'red clay' to the
calcareous elements of all kinds increases, until the latter disappear,
with the exception of a few scattered shells of the larger
_Foraminifera_, which are still found even in the most characteristic
samples of the 'red clay.'
"There seems to be no room left for doubt that the red clay is
essentially the insoluble residue, the _ash_, as it were, of the
calcareous organisms which form the _Globigerina_ ooze, after the
calcareous matter has been by some means removed. An ordinary mixture of
calcareous _Foraminifera_ with the shells of pteropods, forming a fair
sample of _Globigerina_ ooze from near St. Thomas, was carefully washed,
and subjected by Mr. Buchanan to the action of weak acid; and he found
that there remained after the carbonate of lime had been removed, about 1
per cent. of a reddish mud, consisting of silica, alumina, and the red
oxide of iron. This experiment has been frequently repeated with
different samples of _Globigerina_ ooze, and always with the result that
a small proportion of a red sediment remains, which possesses all the
characters of the red clay."
* * * * *
"It seems evident from the observations here recorded, that _clay_, which
we have hitherto looked upon as essentially the product of the
disintegration of older rocks, may be, under certain circumstances, an
organic formation like chalk; that, as a matter of fact, an area on the
surface of the globe, which we have shown to be of vast extent, although
we are still far from having ascertained its limits, is being covered by
such a deposit at the present day.
"It is impossible to avoid associating such a formation with the fine,
smooth, homogeneous clays and schists, poor in fossils, but showing worm-
tubes and tracks, and bunches of doubtful branching things, such as
Oldhamia, silicious sponges, and thin-shelled peculiar shrimps. Such
formations, more or less metamorphosed, are very familiar, especially to
the student of palaeozoic geology, and they often attain a vast thickness.
One is inclined, from the great resemblance between them in composition
and in the general character of the included fauna, to suspect that these
may be organic formations, like the modern red clay of the Atlantic and
Southern Sea, accumulations of the insoluble ashes of shelled creatures.
"The dredging in the red clay on the 13th of March was usually rich. The
bag contained examples, those with calcareous shells rather stunted, of
most of the characteristic deep-water groups of the Southern Sea,
including _Umbellularia, Euplectella, Pterocrinus, Brisinga, Ophioglypha,
Pourtalesia_, and one or two _Mollusca_. This is, however, very rarely
the case. Generally the red clay is barren, or contains only a very small
number of forms."
It must be admitted that it is very difficult, at present, to frame any
satisfactory explanation of the mode of origin of this singular deposit
of red clay.
I cannot say that the theory put forward tentatively, and with much
reservation by Professor Thomson, that the calcareous matter is dissolved
out by the relatively fresh water of the deep currents from the Antarctic
regions, appears satisfactory to me. Nor do I see my way to the
acceptance of the suggestion of Dr. Carpenter, that the red clay is the
result of the decomposition of previously-formed greensand. At present
there is no evidence that greensand casts are ever formed at great
depths; nor has it been proved that _Glauconite_ is decomposable by the
agency of water and carbonic acid.
I think it probable that we shall have to wait some time for a sufficient
explanation of the origin of the abyssal red clay, no less than for that
of the sublittoral greensand in the intermediate zone. But the importance
of the establishment of the fact that these various deposits are being
formed in the ocean, at the present day, remains the same; whether its
_rationale_ be understood or not.
For, suppose the globe to be evenly covered with sea, to a depth say of a
thousand fathoms--then, whatever might be the mineral matter composing
the sea-bottom, little or no deposit would be formed upon it, the
abrading and denuding action of water, at such a depth, being exceedingly
slight.
Next, imagine sponges, _Radiolaria, Foraminifera_, and diatomaceous
plants, such as those which now exist in the deep-sea, to be introduced:
they would be distributed according to the same laws as at present, the
sponges (and possibly some of the _Foraminifera_), covering the bottom,
while other _Foraminifera_, with the _Radiolaria_ and _Diatomacea_, would
increase and multiply in the surface waters. In accordance with the
existing state of things, the _Radiolaria_ and Diatoms would have a
universal distribution, the latter gathering most thickly in the polar
regions, while the _Foraminifera_ would be largely, if not exclusively,
confined to the intermediate zone; and, as a consequence of this
distribution, a bed of "chalk" would begin to form in the intermediate
zone, while caps of silicious rock would accumulate on the circumpolar
regions.
Suppose, further, that a part of the intermediate area were raised to
within two or three hundred fathoms of the surface--for anything that we
know to the contrary, the change of level might determine the
substitution of greensand for the "chalk"; while, on the other hand, if
part of the same area were depressed to three thousand fathoms, that
change might determine the substitution of a different silicate of
alumina and iron--namely, clay--for the "chalk" that would otherwise be
formed.
If the _Challenger_ hypothesis, that the red clay is the residue left by
dissolved _Foraminiferous_ skeletons, is correct, then all these deposits
alike would be directly, or indirectly, the product of living organisms.
But just as a silicious deposit may be metamorphosed into opal or
quartzite, and chalk into marble, so known metamorphic agencies may
metamorphose clay into schist, clay-slate, slate, gneiss, or even
granite. And thus, by the agency of the lowest and simplest of organisms,
our imaginary globe might be covered with strata, of all the chief kinds
of rock of which the known crust of the earth is composed, of indefinite
thickness and extent.
The bearing of the conclusions which are now either established, or
highly probable, respecting the origin of silicious, calcareous, and
clayey rocks, and their metamorphic derivatives, upon the archaeology of
the earth, the elucidation of which is the ultimate object of the
geologist, is of no small importance.
A hundred years ago the singular insight of Linnaeus enabled him to say
that "fossils are not the children but the parents of rocks,"[9] and the
whole effect of the discoveries made since his time has been to compile a
larger and larger commentary upon this text. It is, at present, a
perfectly tenable hypothesis that all siliceous and calcareous rocks are
either directly, or indirectly, derived from material which has, at one
time or other, formed part of the organized framework of living
organisms. Whether the same generalization may be extended to aluminous
rocks, depends upon the conclusion to be drawn from the facts respecting
the red clay areas brought to light by the _Challenger_. If we accept the
view taken by Wyville Thomson and his colleagues--that the red clay is
the residuum left after the calcareous matter of the _Globigerinoe_ ooze
has been dissolved away--then clay is as much a product of life as
limestone, and all known derivatives of clay may have formed part of
animal bodies.
[Footnote 9: "Petrificata montium calcariorum non filii sed parentes
sunt, cum omnis calx oriatur ab animalibus."--_Systema Naturae_, Ed. xii.,
t. iii., p. 154. It must be recollected that Linnaeus included silex, as
well as limestone, under the name of "calx," and that he would probably
have arranged Diatoms among animals, as part of "chaos." Ehrenberg quotes
another even more pithy passage, which I have not been able to find in
any edition of the _Systema_ accessible to me: "Sic lapides ab
animalibus, nec vice versa. Sic runes saxei non primaevi, sed temporis
filiae."]
So long as the _Globigerinoe_;, actually collected at the surface, have
not been demonstrated to contain the elements of clay, the _Challenger_
hypothesis, as I may term it, must be accepted with reserve and
provisionally, but, at present, I cannot but think that it is more
probable than any other suggestion which has been made.
Accepting it provisionally, we arrive at the remarkable result that all
the chief known constituents of the crust of the earth may have formed
part of living bodies; that they may be the "ash" of protoplasm; that the
"_rupes saxei_" are not only _"temporis,"_ but "_vitae filiae_"; and,
consequently, that the time during which life has been active on the
globe may be indefinitely greater than the period, the commencement of
which is marked by the oldest known rocks, whether fossiliferous or
unfossiliferous.
And thus we are led to see where the solution of a great problem and
apparent paradox of geology may lie. Satisfactory evidence now exists
that some animals in the existing world have been derived by a process of
gradual modification from pre-existing forms. It is undeniable, for
example, that the evidence in favour of the derivation of the horse from
the later tertiary _Hipparion_, and that of the _Hipparion_ from
_Anchitherium_, is as complete and cogent as such evidence can reasonably
be expected to be; and the further investigations into the history of the
tertiary mammalia are pushed, the greater is the accumulation of evidence
having the same tendency. So far from palaeontology lending no support to
the doctrine of evolution--as one sees constantly asserted--that
doctrine, if it had no other support, would have been irresistibly forced
upon us by the palaeontological discoveries of the last twenty years.
If, however, the diverse forms of life which now exist have been produced
by the modification of previously-existing less divergent forms, the
recent and extinct species, taken as a whole, must fall into series which
must converge as we go back in time. Hence, if the period represented by
the rocks is greater than, or co-extensive with, that during which life
has existed, we ought, somewhere among the ancient formations, to arrive
at the point to which all these series converge, or from which, in other
words, they have diverged--the primitive undifferentiated protoplasmic
living things, whence the two great series of plants and animals have
taken their departure.
But, as a matter of fact, the amount of convergence of series, in
relation to the time occupied by the deposition of geological formations,
is extraordinarily small. Of all animals the higher _Vertebrata_ are the
most complex; and among these the carnivores and hoofed animals
(_Ungulata_) are highly differentiated. Nevertheless, although the
different lines of modification of the _Carnivora_ and those of the
_Ungulata_, respectively, approach one another, and, although each group
is represented by less differentiated forms in the older tertiary rocks
than at the present day, the oldest tertiary rocks do not bring us near
the primitive form of either. If, in the same way, the convergence of the
varied forms of reptiles is measured against the time during which their
remains are preserved--which is represented by the whole of the tertiary
and mesozoic formations--the amount of that convergence is far smaller
than that of the lines of mammals between the present time and the
beginning of the tertiary epoch. And it is a broad fact that, the lower
we go in the scale of organization, the fewer signs are there of
convergence towards the primitive form from whence all must have
diverged, if evolution be a fact. Nevertheless, that it is a fact in some
cases, is proved, and I, for one, have not the courage to suppose that
the mode in which some species have taken their origin is different from
that in which the rest have originated.
What, then, has become of all the marine animals which, on the hypothesis
of evolution, must have existed in myriads in those seas, wherein the
many thousand feet of Cambrian and Laurentian rocks now devoid, or almost
devoid, of any trace of life were deposited?
Sir Charles Lyell long ago suggested that the azoic character of these
ancient formations might be due to the fact that they had undergone
extensive metamorphosis; and readers of the "Principles of Geology" will
be familiar with the ingenious manner in which he contrasts the theory of
the Gnome, who is acquainted only with the interior of the earth, with
those of ordinary philosophers, who know only its exterior.
The metamorphism contemplated by the great modern champion of rational
geology is, mainly, that brought about by the exposure of rocks to
subterranean heat; and where no such heat could be shown to have
operated, his opponents assumed that no metamorphosis could have taken
place. But the formation of greensand, and still more that of the "red
clay" (if the _Challenger_ hypothesis be correct) affords an insight into
a new kind of metamorphosis--not igneous, but aqueous--by which the
primitive nature of a deposit may be masked as completely as it can be by
the agency of heat. And, as Wyville Thomson suggests, in the passage I
have quoted above (p. 17), it further enables us to assign a new cause
for the occurrence, so puzzling hitherto, of thousands of feet of
unfossiliferous fine-grained schists and slates, in the midst of
formations deposited in seas which certainly abounded in life. If the
great deposit of "red clay" now forming in the eastern valley of the
Atlantic were metamorphosed into slate and then upheaved, it would
constitute an "azoic" rock of enormous extent. And yet that rock is now
forming in the midst of a sea which swarms with living beings, the great
majority of which are provided with calcareous or silicious shells and
skeletons; and, therefore, are such as, up to this time, we should have
termed eminently preservable.
Thus the discoveries made by the _Challenger_ expedition, like all recent
advances in our knowledge of the phenomena of biology, or of the changes
now being effected in the structure of the surface of the earth, are in
accordance with and lend strong support to, that doctrine of
Uniformitarianism, which, fifty years ago, was held only by a small
minority of English geologists--Lyell, Scrope, and De la Beche--but now,
thanks to the long-continued labours of the first two, and mainly to
those of Sir Charles Lyell, has gradually passed from the position of a
heresy to that of catholic doctrine.
Applied within the limits of the time registered by the known fraction of
the crust of the earth, I believe that uniformitarianism is unassailable.
The evidence that, in the enormous lapse of time between the deposition
of the lowest Laurentian strata and the present day, the forces which
have modified the surface of the crust of the earth were different in
kind, or greater in the intensity of their action, than those which are
now occupied in the same work, has yet to be produced. Such evidence as
we possess all tends in the contrary direction, and is in favour of the
same slow and gradual changes occurring then as now.
But this conclusion in nowise conflicts with the deductions of the
physicist from his no less clear and certain data. It may be certain that
this globe has cooled down from a condition in which life could not have
existed; it may be certain that, in so cooling, its contracting crust
must have undergone sudden convulsions, which were to our earthquakes as
an earthquake is to the vibration caused by the periodical eruption of a
Geyser; but in that case, the earth must, like other respectable parents,
have sowed her wild oats, and got through her turbulent youth, before we,
her children, have any knowledge of her.
So far as the evidence afforded by the superficial crust of the earth
goes, the modern geologist can, _ex animo_, repeat the saying of Hutton,
"We find no vestige of a beginning--no prospect of an end." However, he
will add, with Hutton, "But in thus tracing back the natural operations
which have succeeded each other, and mark to us the course of time past,
we come to a period in which we cannot see any further." And if he seek
to peer into the darkness of this period, he will welcome the light
proffered by physics and mathematics.
IV
YEAST
[1871]
It has been known, from time immemorial, that the sweet liquids which may
be obtained by expressing the juices of the fruits and stems of various
plants, or by steeping malted barley in hot water, or by mixing honey
with water--are liable to undergo a series of very singular changes, if
freely exposed to the air and left to themselves, in warm weather.
However clear and pellucid the liquid may have been when first prepared,
however carefully it may have been freed, by straining and filtration,
from even the finest visible impurities, it will not remain clear. After
a time it will become cloudy and turbid; little bubbles will be seen
rising to the surface, and their abundance will increase until the liquid
hisses as if it were simmering on the fire. By degrees, some of the solid
particles which produce the turbidity of the liquid collect at its
surface into a scum, which is blown up by the emerging air-bubbles into a
thick, foamy froth. Another moiety sinks to the bottom, and accumulates
as a muddy sediment, or "lees."