Discourses - Thomas H. Huxley
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In 1810, Risso, in his work on the Ichthyology of Nice, laid the
foundation of what has since been termed "bathymetrical" distribution, or
distribution in depth, by showing that regions of the sea bottom of
different depths could be distinguished by the fishes which inhabit them.
There was the _littoral region_ between tide marks with its sand-eels,
pipe fishes, and blennies: the _seaweed region_, extending from low-
water-mark to a depth of 450 feet, with its wrasses, rays, and flat fish;
and the _deep-sea region_, from 450 feet to 1500 feet or more, with
its file-fish, sharks, gurnards, cod, and sword-fish.
More than twenty years later, M.M. Audouin and Milne Edwards carried out
the principle of distinguishing the Faunae of different zones of depth
much more minutely, in their "Recherches pour servir a l'Histoire
Naturelle du Littoral de la France," published in 1832.
They divide the area included between highwater-mark and lowwater-mark of
spring tides (which is very extensive, on account of the great rise and
fall of the tide on the Normandy coast about St. Malo, where their
observations were made) into four zones, each characterized by its
peculiar invertebrate inhabitants. Beyond the fourth region they
distinguish a fifth, which is never uncovered, and is inhabited by
oysters, scallops, and large starfishes and other animals. Beyond this
they seem to think that animal life is absent.[3]
[Footnote 3: "Enfin plus has encore, c'est-a-dire alors loin des cotes,
le fond des eaux ne parait plus etre habite, du moms dans nos mers, par
aucun de ces animaux" (1. c. tom. i. p. 237). The "ces animaux" leaves
the meaning of the authors doubtful.]
Audouin and Milne Edwards were the first to see the importance of the
bearing of a knowledge of the manner in which marine animals are
distributed in depth, on geology. They suggest that, by this means, it
will be possible to judge whether a fossiliferous stratum was formed upon
the shore of an ancient sea, and even to determine whether it was
deposited in shallower or deeper water on that shore; the association of
shells of animals which live in different zones of depth will prove that
the shells have been transported into the position in which they are
found; while, on the other hand, the absence of shells in a deposit will
not justify the conclusion that the waters in which it was formed were
devoid of animal inhabitants, inasmuch as they might have been only too
deep for habitation.
The new line of investigation thus opened by the French naturalists was
followed up by the Norwegian, Sars, in 1835, by Edward Forbes, in our own
country, in 1840,[4] and by Oersted, in Denmark, a few years later. The
genius of Forbes, combined with his extensive knowledge of botany,
invertebrate zoology, and geology, enabled him to do more than any of his
compeers, in bringing the importance of distribution in depth into
notice; and his researches in the Aegean Sea, and still more his
remarkable paper "On the Geological Relations of the existing Fauna and
Flora of the British Isles," published in 1846, in the first volume of
the "Memoirs of the Geological Survey of Great Britain," attracted
universal attention.
[Footnote 4: In the paper in the _Memoirs of the Survey_ cited further
on, Forbes writes:--
"In an essay 'On the Association of Mollusca on the British Coasts,
considered with reference to Pleistocene Geology,' printed in [the
_Edinburgh Academic Annual_ for] 1840, I described the mollusca, as
distributed on our shores and seas, in four great zones or regions,
usually denominated 'The Littoral zone,' 'The region of Laminariae,' 'The
region of Coral-lines,' and 'The region of Corals.' An extensive series
of researches, chiefly conducted by the members of the committee
appointed by the British Association to investigate the marine geology of
Britain by means of the dredge, have not invalidated this classification,
and the researches of Professor Loven, in the Norwegian and Lapland seas,
have borne out their correctness The first two of the regions above
mentioned had been previously noticed by Lamoureux, in his account of the
distribution (vertically) of sea-weeds, by Audouin and Milne Edwards in
their _Observations on the Natural History of the coast of France_, and
by Sars in the preface to his _Beskrivelser og Jagttayelser_."]
On the coasts of the British Islands, Forbes distinguishes four zones or
regions, the Littoral (between tide marks), the Laminarian (between
lowwater-mark and 15 fathoms), the Coralline (from 15 to 50 fathoms), and
the Deep sea or Coral region (from 50 fathoms to beyond 100 fathoms).
But, in the deeper waters of the Aegean Sea, between the shore and a depth
of 300 fathoms, Forbes was able to make out no fewer than eight zones of
life, in the course of which the number and variety of forms gradually
diminished until, beyond 300 fathoms, life disappeared altogether. Hence
it appeared as if descent in the sea had much the same effect on life, as
ascent on land. Recent investigations appear to show that Forbes was
right enough in his classification of the facts of distribution in depth
as they are to be observed in the Aegean; and though, at the time he
wrote, one or two observations were extant which might have warned him
not to generalize too extensively from his Aegean experience, his own
dredging work was so much more extensive and systematic than that of any
other naturalist, that it is not wonderful he should have felt justified
in building upon it. Nevertheless, so far as the limit of the range of
life in depth goes, Forbes' conclusion has been completely negatived, and
the greatest depths yet attained show not even an approach to a "zero of
life":--
"During the several cruises of H.M. ships _Lightning_ and _Porcupine_ in
the years 1868, 1869, and 1870," says Dr. Wyville Thomson, "fifty-seven
hauls of the dredge were taken in the Atlantic at depths beyond 500
fathoms, and sixteen at depths beyond 1,000 fathoms, and, in all cases,
life was abundant. In 1869, we took two casts in depths greater than
2,000 fathoms. In both of these life was abundant; and with the deepest
cast, 2,435 fathoms, off the month of the Bay of Biscay, we took living,
well-marked and characteristic examples of all the five invertebrate sub-
kingdoms. And thus the question of the existence of abundant animal life
at the bottom of the sea has been finally settled and for all depths, for
there is no reason to suppose that the depth anywhere exceeds between
three and four thousand fathoms; and if there be nothing in the
conditions of a depth of 2,500 fathoms to prevent the full development of
a varied Fauna, it is impossible to suppose that even an additional
thousand fathoms would make any great difference."[5]
[Footnote 5: _The Depths of the Sea_, p. 30. Results of a similar kind,
obtained by previous observers, are stated at length in the sixth
chapter, pp. 267-280. The dredgings carried out by Count Pourtales, under
the authority of Professor Peirce, the Superintendent of the United
States Coast Survey, in the years 1867, 1868, and 1869, are particularly
noteworthy, and it is probably not too much to say, in the words of
Professor Agassiz, "that we owe to the coast survey the first broad and
comprehensive basis for an exploration of the sea bottom on a large
scale, opening a new era in zoological and geological research."]
As Dr. Wyville Thomson's recent letter, cited above, shows, the use of
the trawl, at great depths, has brought to light a still greater
diversity of life. Fishes came up from a depth of 600 to more than 1,000
fathoms, all in a peculiar condition from the expansion of the air
contained in their bodies. On their relief from the extreme pressure,
their eyes, especially, had a singular appearance, protruding like great
globes from their heads. Bivalve and univalve mollusca seem to be rare at
the greatest depths; but starfishes, sea urchins and other echinoderms,
zoophytes, sponges, and protozoa abound.
It is obvious that the _Challenger_ has the privilege of opening a new
chapter in the history of the living world. She cannot send down her
dredges and her trawls into these virgin depths of the great ocean
without bringing up a discovery. Even though the thing itself may be
neither "rich nor rare," the fact that it came from that depth, in that
particular latitude and longitude, will be a new fact in distribution,
and, as such, have a certain importance.
But it may be confidently assumed that the things brought up will very
frequently be zoological novelties; or, better still, zoological
antiquities, which, in the tranquil and little-changed depths of the
ocean, have escaped the causes of destruction at work in the shallows,
and represent the predominant population of a past age.
It has been seen that Audouin and Milne Edwards foresaw the general
influence of the study of distribution in depth upon the interpretation
of geological phenomena. Forbes connected the two orders of inquiry still
more closely; and in the thoughtful essay "On the connection between the
distribution of the existing Fauna and Flora of the British Isles, and
the geological changes which have affected their area, especially during
the epoch of the Northern drift," to which reference has already been
made, he put forth a most pregnant suggestion.
In certain parts of the sea bottom in the immediate vicinity of the
British Islands, as in the Clyde district, among the Hebrides, in the
Moray Firth, and in the German Ocean, there are depressed areas, forming a
kind of submarine valleys, the centres of which are from 80 to 100
fathoms, or more, deep. These depressions are inhabited by assemblages of
marine animals, which differ from those found over the adjacent and
shallower region, and resemble those which are met with much farther
north, on the Norwegian coast. Forbes called these Scandinavian
detachments "Northern outliers."
How did these isolated patches of a northern population get into these
deep places? To explain the mystery, Forbes called to mind the fact that,
in the epoch which immediately preceded the present, the climate was much
colder (whence the name of "glacial epoch" applied to it); and that the
shells which are found fossil, or sub-fossil, in deposits of that age are
precisely such as are now to be met with only in the Scandinavian, or
still more Arctic, regions. Undoubtedly, during the glacial epoch, the
general population of our seas had, universally, the northern aspect
which is now presented only by the "northern outliers"; just as the
vegetation of the land, down to the sea-level, had the northern character
which is, at present, exhibited only by the plants which live on the tops
of our mountains. But, as the glacial epoch passed away, and the present
climatal conditions were developed, the northern plants were able to
maintain themselves only on the bleak heights, on which southern forms
could not compete with them. And, in like manner, Forbes suggested that,
after the glacial epoch, the northern animals then inhabiting the sea
became restricted to the deeps in which they could hold their own against
invaders from the south, better fitted than they to flourish in the
warmer waters of the shallows. Thus depth in the sea corresponded in its
effect upon distribution to height on the land.
The same idea is applied to the explanation of a similar anomaly in the
Fauna of the Aegean:--
"In the deepest of the regions of depth of the Aegean, the representation
of a Northern Fauna is maintained, partly by identical and partly by
representative forms.... The presence of the latter is essentially due to
the law (of representation of parallels of latitude by zones of depth),
whilst that of the former species depended on their transmission from
their parent seas during a former epoch, and subsequent isolation. That
epoch was doubtless the newer Pliocene or Glacial Era, when the _Mya
truncata_ and other northern forms now extinct in the Mediterranean, and
found fossil in the Sicilian tertiaries, ranged into that sea. The
changes which there destroyed the _shallow water_ glacial forms, did not
affect those living in the depths, and which still survive."[6]
[Footnote 6: _Memoirs of the Geological Survey of Great Britain_, Vol. i.
p. 390.]
The conception that the inhabitants of local depressions of the sea
bottom might be a remnant of the ancient population of the area, which
had held their own in these deep fastnesses against an invading Fauna, as
Britons and Gaels have held out in Wales and in Scotland against
encroaching Teutons, thus broached by Forbes, received a wider
application than Forbes had dreamed of when the sounding machine first
brought up specimens of the mud of the deep sea. As I have pointed out
elsewhere,[7] it at once became obvious that the calcareous sticky mud of
the Atlantic was made up, in the main, of shells of _Globigerina_ and
other _Foraminifera_, identical with those of which the true chalk is
composed, and the identity extended even to the presence of those
singular bodies, the Coccoliths and Coccospheres, the true nature of
which is not yet made out. Here then were organisms, as old as the
cretaceous epoch, still alive, and doing their work of rock-making at the
bottom of existing seas. What if _Globigerina_ and the Coccoliths should
not be the only survivors of a world passed away, which are hidden
beneath three miles of salt water? The letter which Dr. Wyville Thomson
wrote to Dr. Carpenter in May, 1868, out of which all these expeditions
have grown, shows that this query had become a practical problem in Dr.
Thomson's mind at that time; and the desirableness of solving the problem
is put in the foreground of his reasons for urging the Government to
undertake the work of exploration:--
[Footnote 7: See above, "On a Piece of Chalk," p. 13.]
"Two years ago, M. Sars, Swedish Government Inspector of Fisheries, had
an opportunity, in his official capacity, of dredging off the Loffoten
Islands at a depth of 300 fathoms. I visited Norway shortly after his
return, and had an opportunity of studying with his father, Professor
Sars, some of his results. Animal forms were _abundant_; many of them
were new to science; and among them was one of surpassing interest, the
small crinoid, of which you have a specimen, and which we at once
recognised as a degraded type of the _Apiocrinidoe_, an order hitherto
regarded as extinct, which attained its maximum in the Pear Encrinites of
the Jurassic period, and whose latest representative hitherto known was
the _Bourguettocrinus_ of the chalk. Some years previously, Mr.
Absjornsen, dredging in 200 fathoms in the Hardangerfjord, procured
several examples of a Starfish (_Brisinga_), which seems to find its
nearest ally in the fossil genus _Protaster_. These observations place it
beyond a doubt that animal life is abundant in the ocean at depths
varying from 200 to 300 fathoms, that the forms at these great depths
differ greatly from those met with in ordinary dredgings, and that, at
all events in some cases, these animals are closely allied to, and would
seem to be directly descended from, the Fauna of the early tertiaries.
"I think the latter result might almost have been anticipated; and,
probably, further investigation will largely add to this class of data,
and will give us an opportunity of testing our determinations of the
zoological position of some fossil types by an examination of the soft
parts of their recent representatives. The main cause of the destruction,
the migration, and the extreme modification of animal types, appear to be
change of climate, chiefly depending upon oscillations of the earth's
crust. These oscillations do not appear to have ranged, in the Northern
portion of the Northern Hemisphere, much beyond 1,000 feet since the
commencement of the Tertiary Epoch. The temperature of deep waters seems
to be constant for all latitudes at 39 deg.; so that an immense area of the
North Atlantic must have had its conditions unaffected by tertiary or
post-tertiary oscillations."[8]
[Footnote 8: The Depths of the Sea, pp. 51-52.]
As we shall see, the assumption that the temperature of the deep sea is
everywhere 39 deg. F. (4 deg. Cent.) is an error, which Dr. Wyville Thomson
adopted from eminent physical writers; but the general justice of the
reasoning is not affected by this circumstance, and Dr. Thomson's
expectation has been, to some extent, already verified.
Thus besides _Globigerina_, there are eighteen species of deep-sea
_Foraminifera_ identical with species found in the chalk. Imbedded in the
chalky mud of the deep sea, in many localities, are innumerable cup-
shaped sponges, provided with six-rayed silicious spicula, so disposed
that the wall of the cup is formed of a lacework of flinty thread. Not
less abundant, in some parts of the chalk formation, are the fossils
known as _Ventriculites_, well described by Dr. Thomson as "elegant vases
or cups, with branching root-like bases, or groups of regularly or
irregularly spreading tubes delicately fretted on the surface with an
impressed network like the finest lace"; and he adds, "When we compare
such recent forms as _Aphrocallistes, Iphiteon, Holtenia_, and
_Askonema_, with certain series of the chalk _Ventriculites_, there
cannot be the slightest doubt that they belong to the same family--in
some cases to very nearly allied genera."[9]
[Footnote 9: _The Depths of the Sea_, p. 484.]
Professor Duncan finds "several corals from the coast of Portugal more
nearly allied to chalk forms than to any others."
The Stalked Crinoids or Feather Stars, so abundant in ancient times, are
now exclusively confined to the deep sea, and the late explorations have
yielded forms of old affinity, the existence of which has hitherto been
unsuspected. The general character of the group of star fishes imbedded
in the white chalk is almost the same as in the modern Fauna of the deep
Atlantic. The sea urchins of the deep sea, while none of them are
specifically identical with any chalk form, belong to the same general
groups, and some closely approach extinct cretaceous genera.
Taking these facts in conjunction with the positive evidence of the
existence, during the Cretaceous epoch, of a deep ocean where now lies
the dry land of central and southern Europe, northern Africa, and western
and southern Asia; and of the gradual diminution of this ocean during the
older tertiary epoch, until it is represented at the present day by such
teacupfuls as the Caspian, the Black Sea, and the Mediterranean; the
supposition of Dr. Thomson and Dr. Carpenter that what is now the deep
Atlantic, was the deep Atlantic (though merged in a vast easterly
extension) in the Cretaceous epoch, and that the _Globigerina_ mud has
been accumulating there from that time to this, seems to me to have a
great degree of probability. And I agree with Dr. Wyville Thomson against
Sir Charles Lyell (it takes two of us to have any chance against his
authority) in demurring to the assertion that "to talk of chalk having
been uninterruptedly formed in the Atlantic is as inadmissible in a
geographical as in a geological sense."
If the word "chalk" is to be used as a stratigraphical term and
restricted to _Globigerina_ mud deposited during the Cretaceous epoch, of
course it is improper to call the precisely similar mud of more recent
date, chalk. If, on the other hand, it is to be used as a mineralogical
term, I do not see how the modern and the ancient chalks are to be
separated--and, looking at the matter geographically, I see no reason to
doubt that a boring rod driven from the surface of the mud which forms
the floor of the mid-Atlantic would pass through one continuous mass of
_Globigerina_ mud, first of modern, then of tertiary, and then of
mesozoic date; the "chalks" of different depths and ages being
distinguished merely by the different forms of other organisms associated
with the _Globigerinoe_.
On the other hand, I think it must be admitted that a belief in the
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 deg. F. and got lighter as
it approached 32 deg. F., the water of the bottom of the deep sea could not
be colder than 39 deg.. 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 deg. 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 deg.-81 deg.
Fahr., the temperature at the bottom, at a depth of 2270 to 2656 fathoms,
was only from 34 deg. to 32 deg. Fahr.
As the mean temperature of the superficial layer of the crust of the
earth may be taken at about 50 deg. 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.