Among the astounding discoveries of modern science is that of the
immense periods which have passed in the gradual formation of our earth.
So vast were the cycles of time preceding even the appearance of man on
the surface of our globe, that our own period seems as yesterday when
compared with the epochs that have gone before it. Had we only the
evidence of the deposits of rock heaped above each other in regular
strata by the slow accumulation of materials, they alone would convince
us of the long and slow maturing of God's work on the earth but when we
add to these the successive populations of whose life this world has
been the theatre, and whose remains are hidden in the rocks into which
the mud or sand or soil of whatever kind on which they lived has
hardened in the course of time,--or the enormous chains of mountains
whose upheaval divided these periods of quiet accumulation by great
convulsions,--or the changes of a different nature in the configuration
of our globe, as the sinking of lands beneath the ocean, or the gradual
rising of continents and islands above it,--or the wearing of great
river-beds, or the filling of extensive water-basins, till marshes first
and then dry land succeeded to inland seas,--or the slow growth of coral
reefs, those wonderful sea-walls raised by the little ocean-architects
whose own bodies furnish both the building-stones and the cement that
binds them together, and who have worked so busily during the long
centuries, that there are extensive countries, mountain-chains, islands,
and long lines of coast consisting solely of their remains,--or the
countless forests that must have grown up, flourished, died, and
decayed, to fill the storehouses of coal that feed the fires of the
human race to-day,--if we consider all these records of the past, the
intellect fails to grasp a chronology for which our experience furnishes
no data, and the time that lies behind us seems as much an eternity to
our conception as the future that stretches indefinitely before us.
The physical as well as the human history of the world has its mythical
age, lying dim and vague in the morning mists of creation, like that of
the heroes and demigods in the early traditions of man, defying all
our ordinary dates and measures. But if the succession of periods that
prepared the earth for the coming of man, and the animals and plants
that accompany him on earth, baffles our finite attempts to estimate its
duration, have we any means of determining even approximately the length
of the period to which we ourselves belong? If so, it may furnish us
with some data for the further solution of these wonderful mysteries of
time, and it is besides of especial importance with reference to the
question of permanence of Species. Those who maintain the mutability of
Species, and account for all the variety of life on earth by the gradual
changes wrought by time and circumstances, do not accept historical
evidence as affecting the question at all. The monuments of those oldest
nations, all whose history is preserved in monumental records, do not
indicate the slightest variation of organic types from that day to this.
The animals that were preserved within their tombs or carved upon their
walls by the ancient Egyptians were the same as those that have their
home in the valley of the Nile today; the negro, whose peculiar features
are unmistakable even in their rude artistic attempts to represent them,
was the same woolly-haired, thick-lipped, flat-nosed, dark-skinned being
in the days of the Rameses that he is now. The Apis, the Ibis, the
Crocodiles, the sacred Beetles, have brought down to us unchanged all
the characters that superstition hallowed in those early days. The
stony face of the Sphinx is not more true to its past, nor the massive
architecture of the Pyramids more unchanged, than they are. But the
advocates of the mutability of Species say truly enough that the most
ancient traditions are but as yesterday in the world's history, and that
what six thousand years could not do sixty thousand years might effect.
Leaving aside, then, all historical chronology, how far back can we
trace our own geological period, and the Species belonging to it? By
what means can we determine its duration? Within what limits, by what
standard, may it be measured? Shall hundreds, or thousands, or hundreds
of thousands, or millions of years be the unit from which we start?
I will begin this inquiry with a series of facts which I myself have
had an opportunity of investigating with especial care respecting the
formation and growth of the Coral Reefs of Florida. But first a few
words on Coral Reefs in general. They are living limestone walls that
are built up from certain depths in the ocean by the natural growth of a
variety of animals, but limited by the level of high-water, beyond which
they cannot rise, since the little beings that compose them die as soon
as they are removed from the vitalizing influence of the pure sea-water.
These walls have a variety of outlines: they may be straight, circular,
semicircular, oblong, according to the form of the coast along which
the little Reef-Builders establish themselves; and their height is, of
course, determined by the depth of the bottom on which they rest. If
they settle about an island on all sides of which the conditions for
their growth are equally favorable, they will raise a wall all around
it, thus encircling it with a ring of Coral growth. The Athols in the
Pacific Ocean, those circular islands inclosing sometimes a fresh-water
lake in mid-ocean, are Coral walls of this kind, that have formed a ring
around a central island. This is easily understood, if we remember that
the bottom of the Pacific Ocean is by no means a stable foundation
for such a structure. On the contrary, over a certain area, which has
already been surveyed with some accuracy by Professor Dana, during the
United States Exploring Expedition, it is subsiding; and if an island
upon which the Reef-Builders have established themselves be situated
in that area of subsidence, it will, of course, sink with the floor on
which it rests, carrying down also the Coral wall to a greater depth in
the sea. In such instances, if the rate of subsidence be more rapid than
the rate of growth in the Corals, the island and the wall itself will
disappear beneath the ocean. But whenever, on the contrary, the rate of
increase in the wall is greater than that of subsidence in the island,
while the latter gradually sinks below the surface, the former rises
in proportion, and by the time it has completed its growth the central
island has vanished, and there remains only a ring of Coral Reef, with
here and there a break, perhaps, at some spot where the more prosperous
growth of the Corals has been checked. If, however, as sometimes
happens, there is no such break, and the wall is perfectly
uninterrupted, the sheet of sea-water so inclosed may be changed to
fresh water by the rains that are poured into it. Such a water-basin
will remain salt, it is true, in its lower part, and the fact that it is
affected by the rise and fall of the tides shows that it is not entirely
secluded from communication with the ocean outside; but the salt water,
being heavier, sinks, while the lighter rain-water remains above, and it
is to all appearance actually changed into a fresh-water lake.
I need not dwell here on the further history of such a Coral island, or
follow it through the changes by which the summit of its circular wall
becomes covered with a fertile soil, a tropical vegetation springs up on
it, and it is at last perhaps inhabited by man. There is something very
attractive in the idea of these green rings inclosing sheltered harbors
and quiet lakes in mid-ocean, and the subject has lost none of its
fascination since the mystery of their existence has been solved by the
investigations of several contemporary naturalists who have enabled us
to trace the whole story of their structure. I would refer all who wish
for a more detailed account of them to Charles Darwin's charming
little volume on "Coral Reefs," where their mode of formation is fully
described, and also to James D. Dana's "Geological Report of the United
States Exploring Expedition."
Coral Reefs are found only in tropical regions: although Polyps, animals
of the same class as those chiefly instrumental in their formation,
are found in all parts of the globe, yet the Reef-Building Polyps are
limited to the Tropics. We are too apt to forget that the homes of
animals are as definitely limited in the water as on the land. Indeed,
the subject of the geographical distribution of animals according to
laws that are established by altitude, by latitude and longitude, by
pressure of atmosphere or pressure of water, already alluded to in
a previous article, is exceedingly interesting, and presents a most
important field of investigation. The climatic effect of different
degrees of altitude upon the growth of animals and plants is the same as
that of different degrees of latitude; and the slope of a high mountain
in the Tropics, from base to summit, presents, in a condensed form, an
epitome, as it were, of the same kind of gradation in vegetable growth
that may be observed from the Tropics to the Arctics. At the base of
such a mountain we have all the luxuriance of growth characteristic
of the tropical forest,--the Palms, the Bananas, the Bread-trees, the
Mimosas; higher up, these give way to a different kind of growth,
corresponding to our Oaks, Chestnuts, Maples, etc.; as these wane, on
the loftier slopes comes in the Pine forest, fading gradually, as it
ascends, into a dwarfish growth of the same kind; and this at last gives
way to the low creeping Mosses and Lichens of the greater heights, till
even these find a foothold no longer, and the summit of the mountain is
clothed in perpetual snow and ice. What have we here but the same series
of changes through which we pass, if, travelling northward from the
Tropics, we leave Palms and Pomegranates and Bananas behind, where the
Live-Oaks and Cypresses, the Orange-trees and Myrtles of the warmer
Temperate Zone come in, and these die out as we reach the Oaks,
Chestnuts, Maples, Elms, Nut-trees, Beeches, and Birches of the colder
Temperate Zone, these again waning as we enter the Pine forests of
the Arctic borders, till, passing out of these, nothing but a dwarf
vegetation, a carpet of Moss and Lichen, fit food for the Reindeer and
the Esquimaux, greets us, and beyond that lies the region of the snow
and ice fields, impenetrable to all but the daring Arctic voyager?
I have thus far spoken of the changes in the vegetable growth alone as
influenced by altitude and latitude, but the same is equally true of
animals. Every zone of the earth's surface has its own animals, suited
to the conditions under which they are meant to live; and with the
exception of those that accompany man in all his pilgrimages, and are
subject to the same modifying influences by which he adapts his home and
himself to all climates, animals are absolutely bound by the laws of
their nature within the range assigned to them. Nor is this the case
only on land, where river-banks, lake-shores, and mountain-ranges might
be supposed to form the impassable boundaries that keep animals within
certain limits; but the ocean as well as the land has its faunae and
florae bound within their respective zoölogical and botanical provinces;
and a wall of granite is not more impassable to a marine animal than
that ocean-line, fluid and flowing and ever-changing though it be, on
which is written for him, "Hitherto shalt thou come, but no farther."
One word as to the effect of pressure on animals will explain this.
We all live under the pressure of the atmosphere. Now thirty-two feet
under the sea doubles that pressure, since a column of water of that
height is equal in weight to the pressure of one atmosphere. At the
depth of thirty-two feet, then, any marine animal is under the pressure
of two atmospheres,--that of the air which surrounds our globe, and of a
weight of water equal to it; at sixty-four feet he is under the pressure
of three atmospheres, and so on,--the weight of one atmosphere being
always added for every thirty-two feet of depth. There is a great
difference in the sensitiveness of animals to this pressure. Some fishes
live at a great depth and find the weight of water genial to them, while
others would be killed at once by the same pressure, and the latter
naturally seek the shallow waters. Every fisherman knows that he must
throw a long line for a Halibut, while with a common fishing-rod he will
catch plenty of Perch from the rocks near the shore; and the differently
colored bands of sea-weed revealed by low tide, from the green line of
the Ulvas through the brown zone of the common Fucas to the rosy and
purple hued sea-weeds of the deeper water show that the florae as well
as the faunae of the ocean have their precise boundaries. This wider
or narrower range of marine animals is in direct relation to their
structure, which enables them to bear a greater or less pressure of
water. All fishes, and, indeed, all animals having a wide range of
distribution in ocean-depths, have a special apparatus of water-pores,
so that the surrounding element penetrates their structure, thus
equalizing the pressure of the weight, which is diminished from without
in proportion to the quantity of water they can admit into their bodies.
Marine animals differ in their ability to sustain this pressure, just
as land animals differ in their power of enduring great variations of
climate and of atmospheric pressure.
Of all air-breathing animals, none exhibits a more surprising power of
adapting itself to great and rapid changes of external influences than
the Condor. It may be seen feeding on the sea-shore under a burning
tropical sun, and then, rising from its repast, it floats up among the
highest summits of the Andes and is lost to sight beyond them, miles
above the line of perpetual snow, where the temperature must be lower
than that of the Arctics. But even the Condor, sweeping at one flight
from tropic heat to arctic cold, although it passes through greater
changes of temperature, does not undergo such changes of pressure as a
fish that rises from a depth of sixty-four feet to the surface of the
sea; for the former remains within the air that surrounds our globe,
and therefore the increase or diminution of pressure to which it is
subjected must be confined within the limits of one atmosphere, while
the latter, at a depth of sixty-four feet, is under a weight equal to
that of three such atmospheres, which is reduced to one when it reaches
the sea-level. The change is even much greater for those fishes that
come from a depth of several hundred feet. These laws of limitation in
space explain many facts in the growth of Coral Reefs that would be
otherwise inexplicable, and which I will endeavor to make clear to my
readers.
For a long time it was supposed that the Coral animals inhabited very
deep waters, for they were sometimes brought up on sounding-lines from a
depth of many hundreds or even thousands of feet, and it was taken for
granted that they must have had their home where they were found;
but the facts recently ascertained respecting the subsidence of
ocean-bottoms have shown that the foundation of a Coral wall may have
sunk far below the place where it was laid, and it is now proved beyond
a doubt that no Reef-Building Coral can thrive at a depth of more than
fifteen fathoms, though Corals of other kinds occur far lower, and that
the dead Reef-Corals sometimes brought to the surface from much greater
depths are only broken fragments of some Reef that has subsided with
the bottom on which it was growing. But though fifteen fathoms is the
maximum depth at which any Reef-Builder can prosper, there are many
which will not sustain even that degree of pressure, and this fact has,
as we shall see, an important influence on the structure of the Reef.
Imagine now a sloping shore on some tropical coast descending gradually
below the surface of the sea. Upon that slope, at a depth of from ten
to twelve or fifteen fathoms, and two or three or more miles from the
main-land, according to the shelving of the shore, we will suppose that
one of those little Coral animals to whom a home in such deep waters is
genial has established itself. How it happens that such a being, which
we know is immovably attached to the ground and forms the foundation of
a solid wall, was ever able to swim freely about in the water till it
found a suitable resting-place, I shall explain hereafter, when I say
something of the mode of reproduction of these animals. Accept, for the
moment, my unsustained assertion, and plant our little Coral on this
sloping shore some twelve or fifteen fathoms below the surface of the
sea. The internal structure of such a Coral corresponds to that of the
Sea-Anemone: the body is divided by vertical partitions from top to
bottom, leaving open chambers between, while in the centre hangs the
digestive cavity connecting by an opening in the bottom with all these
chambers; at the top is an aperture which serves as a mouth, surrounded
by a wreath of hollow tentacles, each one connecting at its base with
one of the chambers, so that all parts of the animal communicate freely
with each other. But though the structure of the Coral is identical in
all its parts with that of the Sea-Anemone, it nevertheless presents one
important difference. The body of the Sea-Anemone is soft, while that of
the Coral is hard. It is well known that all animals and plants have the
power of appropriating to themselves and assimilating the materials they
need, each selecting from the surrounding elements whatever contributes
to its well-being. The plant takes carbon, the animal takes oxygen, each
rejecting what the other requires. We ourselves build our bones with
the lime that we find unconsciously in the world around us; much of our
nourishment supplies us with it, and the very vegetables we eat have,
perhaps, themselves been fed from some old lime strata deposited
centuries ago. We all represent materials that have contributed to
construct our bodies. Now Corals possess, in an extraordinary degree,
the power of assimilating to themselves the lime contained in the salt
water around them; and as soon as our little Coral is established on a
firm foundation, a lime deposit begins to form in all the walls of its
body, so that its base, its partitions, and its outer wall, which in
the Sea-Anemone remain always soft, become perfectly solid in the Polyp
Coral and form a frame as hard as bone. It may naturally be asked
where the lime comes from in the sea which the Corals absorb in such
quantities. As far as the living Corals are concerned the answer is
easy, for an immense deal of lime is brought down to the ocean by
rivers that wear away the lime deposits through which they pass. The
Mississippi, whose course lies through extensive lime regions, brings
down yearly lime enough to supply all the animals living in the Gulf of
Mexico. But behind this lies a question not so easily settled, as to
the origin of the extensive deposits of limestone found at the very
beginning of life upon earth. This problem brings us to the threshold of
astronomy, for limestone is metallic in character, susceptible therefore
of fusion, and may have formed a part of the materials of our earth,
even in an incandescent state, when the worlds were forming. But though
this investigation as to the origin of lime does not belong either to
the naturalist or the geologist, its suggestion reminds us that the
time has come when all the sciences and their results are so intimately
connected that no one can be carried on independently of the others.
Since the study of the rocks has revealed a crowded life whose records
are hoarded within them, the work of the geologist and the naturalist
has become one and the same, and at that border-land where the first
crust of the earth condensed out of the igneous mass of materials which
formed its earliest condition their investigation mingles with that of
the astronomer, and we cannot trace the limestone in a little Coral
without going back to the creation of our solar system, when the worlds
that compose it were thrown off from a central mass in a gaseous
condition.
When the Coral has become in this way permeated with lime, all parts of
the body are rigid, with the exception of the upper margin, the stomach,
and the tentacles. The tentacles are soft and waving, projected or drawn
in at will, and they retain their flexible character through life, and
decompose when the animal dies. For this reason the dried specimens of
Corals preserved in museums do not give us the least idea of the living
Corals, in which every one of the millions of beings composing such
a community is crowned by a waving wreath of white or green or
rose-colored tentacles.
As soon as the little Coral is fairly established and solidly attached
to the ground, it begins to bud. This may take place in a variety of
ways, dividing at the top or budding from the base or from the sides,
till the primitive animal is surrounded by a number of individuals like
itself, of which it forms the nucleus, and which now begin to bud in
their turn, each one surrounding itself with a numerous progeny, all
remaining, however, attached to the parent. Such a community increases
till its individuals are numbered by millions; and I have myself counted
no less than fourteen millions of individuals in a Coral mass measuring
not more than twelve feet in diameter. These are the so-called Coral
heads which form the foundation of a Coral wall, and their massive
character and regular form seem to be especially adapted to give a
strong, solid base to the whole structure. They are known in our
classifications as the Astraeans, so named on account of the star-shaped
form of the little pits that are crowded upon the surface, each one
marking the place of a single individual in such a community.
Thus firmly and strongly is the foundation of the reef laid by the
Astraeans; but we have seen that for their prosperous growth they
require a certain depth and pressure of water, and when they have
brought the wall so high that they have not more than six fathoms of
water above them, this kind of Coral ceases to grow. They have, however,
prepared a fitting surface for different kinds of Corals that could not
live in the depths from which the Astraeans have come, but find their
genial home nearer the surface; such a home being made ready for them
by their predecessors, they now establish themselves on the top of the
Coral wall and continue its growth for a certain time. These are the
Mandrinas, or the so-called Brain-Corals, and the Porites. The Mandrinas
differ from the Astraeans by their less compact and definite pits. In
the Astraeans the place occupied by the animal in the community is
marked by a little star-shaped spot, in the centre of which all the
partition-walls meet. But in the Mandrinas, although all the partitions
converge toward the central opening, as in the Astraeans, these central
openings elongate, run into each other, and form waving furrows all over
the surface, instead of the small round pits so characteristic of the
Astraeans. The Porites resemble the Astraeans, but the pits are smaller,
with fewer partitions and fewer tentacles, and their whole substance is
more porous.
But these also have their bounds within the sea: they in their turn
reach the limit beyond which they are forbidden by the laws of their
nature to pass, and there they also pause. But the Coral wall continues
its steady progress; for here the lighter kinds set in,--the Madrepores,
the Millepores, and a great variety of Sea-Fans and Corallines, and the
reef is crowned at last with a many-colored shrubbery of low feathery
growth. These are all branching in form, and many of them are simple
calciferous plants, though most of them are true animals, resembling,
however, delicate Algae more than any marine animals; but, on
examination of the latter, one finds them to be covered with myriads of
minute dots, each representing one of the little beings out of which the
whole is built.
I would add here one word on the true nature of the Millepores, long
misunderstood by naturalists, because it throws light not only on some
interesting facts respecting Coral Reefs, especially the ancient ones,
but also because it tells us something of the early inhabitants of the
globe, and shows us that a class of Radiates supposed to be missing in
that primitive creation had its representatives then as now. In the
diagram of the geological periods introduced in a previous article, I
have represented all the three classes of Radiates, Polyps, Acalephs,
and Echinoderms, as present on the first floor of our globe that was
inhabited at all. But it is only recently that positive proofs have been
found of the existence of Acalephs or Jelly-Fishes, as they are
called, at that early period. Their very name indicates their delicate
structure; and were there no remains preserved in the rocks of these
soft, transparent creatures, it would yet be no evidence that they did
not exist. Fragile as they are, however, they have left here and there
some faint record of themselves, and in the Museum at Carlsruhe, on a
slab from Solenhofen, I have seen a very perfect outline of one which
remains undescribed to this day. This, however, does not carry them
farther back than the Jurassic period, and it is only lately that I have
satisfied myself that they not only existed, but were among the most
numerous animals in the first representation of organic life.
The earliest Corals correspond in certain features of their structure to
the Millepores. They differ from them as all early animals differ from
the succeeding ones, every geological period having its special set of
representatives. But still they are always true to their class, and have
a certain general correspondence with animals of like kind that follow
them in later periods. In this sense the Millepores are in our epoch the
representatives of those early Corals called by naturalists Tabulata and
Rugosa,--distinguished from the Polyp Corals by the horizontal floors,
waving in some, straight in others, which divide the body transversely
at successive heights through its whole length, and also by the absence
of the vertical partitions, extending from top to bottom of each animal,
so characteristic of the true Polyps. As I have said, they were for a
long time supposed, notwithstanding these differences, to be Polyps, and
I had shared in this opinion, till, during the winter of 1857, while
pursuing my investigations on the Coral Reefs of Florida, one of these
Millepores revealed itself to me in its true character of Acaleph. It is
by its soft parts alone--those parts which are seen only in its living
state, and when the animal is fully open--that its Acalephian character
can be perceived, and this accounts for its being so long accepted as
a Polyp, when studied in the dry Coral stock. Nothing could exceed
my astonishment when for the first time I saw such an animal fully
expanded, and found it to be a true Acaleph. It is exceedingly difficult
to obtain a view of them in this state, for, at any approach, they draw
themselves in, and remain closed to all investigation. Only once, for a
short hour, I had this opportunity; during that time one of these little
creatures revealed to me its whole structure, as if to tell me, once for
all, the story of its existence through all the successive epochs from
the dawn of Creation till now, and then withdrew. With my most patient
watching, I have never been able to see one of them open again. But to
establish the fact that one of the Corals represented from the earliest
period till now, and indeed far more numerous in the beginning than any
other, was in truth no Polyp, but an Acaleph, the glimpse I had was
all-sufficient. It came out as if to bear witness of its class,--as if
to say, "We, too, were among the hosts of living beings with which God
first peopled His earth."
With these branching Corals the reef reaches the level of high-water,
beyond which, as I have said, there can be no further growth, for want
of the action of the fresh sea-water. This dependence upon the vivifying
influence of the sea accounts for one unfailing feature in the Coral
walls. They are always abrupt and steep on the seaward side, but have a
gentle slope towards the land. This is accounted for by the circumstance
that the Corals on the outer side of the reef are in immediate contact
with the pure ocean-water, while by their growth they partially exclude
the inner ones from the same influence,--the rapid growth of the latter
being also impeded by any impurity or foreign material washed away from
the neighboring shore and mingling with the water that fills the channel
between the main-land and the reef. Thus the Coral Reefs, whether built
around an island, or concentric to a rounding shore, or along a straight
line of coast, are always shelving toward the land, while they
are comparatively abrupt and steep toward the sea. This should be
remembered, for, as we shall see hereafter, it has an important bearing
on the question of time as illustrated by Coral Reefs.
I have spoken of the budding of Corals, by which each one becomes the
centre of a cluster; but this is not the only way in which they multiply
their kind. They give birth to eggs also, which are carried on the inner
edge of their partition-walls, till they drop into the sea, where they
float about, little, soft, transparent, pear-shaped bodies, as unlike as
possible to the rigid stony structure they are to assume hereafter. In
this condition they are covered with vibratile cilia or fringes, that
are always in rapid, uninterrupted motion, and keep them swimming about
in the water. It is by means of these little germs of the Corals,
swimming freely about during their earliest stages of growth, that the
reef is continued, at the various heights where special kinds die
out, by those that prosper at shallower depths; otherwise it would be
impossible to understand how this variety of building material, as it
were, is introduced wherever it is needed. This point, formerly a puzzle
to naturalists, has become quite clear since it has been found that
myriads of these little germs are poured into the water surrounding a
reef. There they swim about till they find a genial spot on which to
establish themselves, when they become attached to the ground by one
end, while a depression takes place at the opposite end, which gradually
deepens to form the mouth and inner cavity, while the edges expand to
form the tentacles, and the productive life of the little Coral begins:
it buds from every side, and becomes the foundation of a new community.
I should add, that, beside the Polyps and the Acalephs, Mollusks also
have their representatives among the Corals. There is a group of small
Mollusks called Bryozoa, allied to the Clams by their structure, but
excessively minute when compared to the other members of their class,
which, like the other Corals, harden in consequence of an absorption of
solid materials, and contribute to the formation of the reef. Besides
these, there are certain plants, limestone Algae,--Corallines, as they
are called,--which have their share also in the work.
I had intended to give some account of the Coral Reefs of Florida,
and to show what bearing they have upon the question of time and the
permanence of Species; but this cursory sketch of Coral Reefs in general
has grown to such dimensions that I must reserve a more particular
account of the Florida Reefs and Keys for a future article.
* * * * *