Young Folks' Library, Volume XI (of 20) by Various

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There is more curious evidence, again, that the process of covering
up, or, in other words, the deposit of Globigerina skeletons, did not
go on very fast. It is demonstrable that an animal of the cretaceous
sea might die, that its skeleton might lie uncovered upon the
sea-bottom long enough to lose all its outward coverings and
appendages by putrefaction; and that, after this had happened, another
animal might attach itself to the dead and naked skeleton, might grow
to maturity, and might itself die before the calcareous mud had buried
the whole.

Cases of this kind are admirably described by Sir Charles Lyell. He
speaks of the frequency with which geologists find in the chalk a
fossilized sea-urchin to which is attached the lower valve of a
Crania. This is a kind of shell-fish, with a shell composed of two
pieces, of which, as in the oyster, one is fixed and the other free.

"The upper valve is almost invariably wanting, though occasionally
found in a perfect state of preservation in the white chalk at some
distance. In this case, we see clearly that the sea-urchin first lived
from youth to age, then died and lost its spines, which were carried
away. Then the young Crania adhered to the bared shell, grew and
perished in its turn; after which, the upper valve was separated from
the lower, before the Echinus became enveloped in chalky mud."

A specimen in the Museum of Practical Geology, in London, still
further prolongs the period which must have elapsed between the death
of the sea-urchin and its burial by the Globigeringae. For the outward
face of the valve of a Crania, which is attached to a sea-urchin
(Micrastor), is itself overrun by an incrusting coralline, which
spreads thence over more or less of the surface of the sea-urchin. It
follows that, after the upper valve of the Crania fell off, the
surface of the attached valve must have remained exposed long enough
to allow of the growth of the whole coralline, since corallines do not
live imbedded in the mud.

The progress of knowledge may, one day, enable us to deduce from such
facts as these the maximum rate at which the chalk can have
accumulated, and thus to arrive at the minimum duration of the chalk
period. Suppose that the valve of the Crania upon which a coralline
has fixed itself in the way just described is so attached to the
sea-urchin that no part of it is more than an inch above the face upon
which the sea-urchin rests. Then, as the coralline could not have
fixed itself if the Crania had been covered up with chalk-mud, and
could not have lived had itself been so covered, it follows, that an
inch of chalk mud could not have accumulated within the time between
the death and decay of the soft parts of the sea-urchin and the growth
of the coralline to the full size which it has attained. If the decay
of the soft parts of the sea-urchin; the attachment, growth to
maturity, and decay of the Crania; and the subsequent attachment and
growth of the coralline, took a year (which is a low estimate enough),
the accumulation of the inch of chalk must have taken more than a
year: and the deposit of a thousand feet of chalk must, consequently,
have taken more than twelve thousand years.

The foundation of all this calculation is, of course, a knowledge of
the length of time the Crania and the coralline needed to attain their
full size; and, on this head, precise knowledge is at present wanting.
But there are circumstances which tend to show that nothing like an
inch of chalk has accumulated during the life of a Crania; and, on any
probable estimate of the length of that life, the chalk period must
have had a much longer duration than that thus roughly assigned to
it.

Thus, not only is it certain that the chalk is the mud of an ancient
sea-bottom; but it is no less certain that the chalk sea existed
during an extremely long period, though we may not be prepared to give
a precise estimate of the length of that period in years. The relative
duration is clear, though the absolute duration may not be definable.
The attempt to affix any precise date to the period at which the chalk
sea began or ended its existence, is baffled by difficulties of the
same kind. But the relative age of the cretaceous epoch may be
determined with as great ease and certainty as the long duration of
that epoch.

You will have heard of the interesting discoveries recently made, in
various parts of Western Europe, of flint implements, obviously worked
into shape by human hands, under circumstances which show conclusively
that man is a very ancient denizen of these regions.

It has been proved that the old populations of Europe, whose existence
has been revealed to us in this way, consisted of savages, such as the
Esquimaux are now; that, in the country which is now France, they
hunted the reindeer, and were familiar with the ways of the mammoth
and the bison. The physical geography of France was in those days
different from what it is now--the river Somme, for instance, having
cut its bed a hundred feet deeper between that time and this; and it
is probable that the climate was more like that of Canada or Siberia
than that of Western Europe.

The existence of these people is forgotten even in the traditions of
the oldest historical nations. The name and fame of them had utterly
vanished until a few years back; and the amount of physical change
which has been effected since their day renders it more than probable
that, venerable as are some of the historical nations, the workers of
the chipped flints of Hoxne or of Amiens are to them, as they are to
us, in point of antiquity.

But, if we assign to these hoar relics of long-vanished generations of
men the greatest age that can possibly be claimed for them, they are
not older than the drift, or boulder clay, which, in comparison with
the chalk, is but a very juvenile deposit. You need go no further than
your own seaboard for evidence of this fact. At one of the most
charming spots on the coast of Norfolk, Cromer, you will see the
boulder clay forming a vast mass, which lies upon the chalk, and must
consequently have come into existence after it. Huge boulders of chalk
are, in fact, included in the clay, and have evidently been brought to
the position they now occupy by the same agency as that which has
planted blocks of syenite from Norway side by side with them.

The chalk, then, is certainly older than the boulder clay. If you ask
how much, I will again take you no further than the same spot upon
your own coasts for evidence. I have spoken of the boulder clay and
drift as resting upon the chalk. That is not strictly true. Interposed
between the chalk and the drift is a comparatively insignificant
layer, containing vegetable matter. But that layer tells a wonderful
history. It is full of stumps of trees standing as they grew.
Fir-trees are there with their cones, and hazel-bushes with their
nuts; there stand the stools of oak and yew trees, beeches and
alders. Hence this stratum is appropriately called the "forest-bed."

It is obvious that the chalk must have been upheaved and converted
into dry land before the timber trees could grow upon it. As the bolls
of some of these trees are from two to three feet in diameter, it is
no less clear that the dry land thus formed remained in the same
condition for long ages. And not only do the remains of stately oaks
and well-grown firs testify to the duration of this condition of
things, but additional evidence to the same effect is afforded by the
abundant remains of elephants, rhinoceroses, hippopotamuses, and other
great wild beasts, which it has yielded to the zealous search of such
men as the Rev. Mr. Gunn.

When you look at such a collection as he has formed, and bethink you
that these elephantine bones did veritably carry their owners about,
and these great grinders crunch, in the dark woods of which the
forest-bed is now the only trace, it is impossible not to feel that
they are as good evidence of the lapse of time as the annual rings of
the tree-stumps.

Thus there is a writing upon the wall of cliffs at Cromer, and whoso
runs may read it. It tells us, with an authority which cannot be
impeached, that the ancient sea-bed of the chalk sea was raised up,
and remained dry land, until it was covered with forest, stocked with
the great game whose spoils have rejoiced your geologists. How long it
remained in that condition cannot be said; but "the whirligig of time
brought its revenges" in those days as in these. That dry land, with
the bones and teeth of generations of long-lived elephants, hidden
away among the gnarled roots and dry leaves of its ancient trees, sank
gradually to the bottom of the icy sea, which covered it with huge
masses of drift and boulder clay. Sea-beasts, such as the walrus, now
restricted to the extreme north, paddled about where birds had
twittered among the topmost twigs of the fir-trees. How long this
state of things endured we know not, but at length it came to an end.
The upheaved glacial mud hardened into the soil of modern Norfolk.
Forests grew once more, the wolf and the beaver replaced the reindeer
and the elephant; and at length what we call the history of England
dawned.

Thus you have, within the limits of your own county, proof that the
chalk can justly claim a very much greater antiquity than even the
oldest physical traces of mankind. But we may go further and
demonstrate, by evidence of the same authority as that which testifies
to the existence of the father of men, that the chalk is vastly older
than Adam himself.

The Book of Genesis informs us that Adam, immediately upon his
creation, and before the appearance of Eve, was placed in the garden
of Eden. The problem of the geographical position of Eden has greatly
vexed the spirits of the learned in such matters, but there is one
point respecting which, so far as I know, no commentator has ever
raised a doubt. This is, that of the four rivers which are said to run
out of it, Euphrates and Hiddekel are identical with the rivers now
known by the names of Euphrates and Tigris.

But the whole country in which these mighty rivers take their origin,
and through which they run, is composed of rocks which are either of
the same age as the chalk, or of later date. So that the chalk must
not only have been formed, but, after its formation, the time required
for the deposit of these later rocks, and for their upheaval into dry
land, must have elapsed, before the smallest brook which feeds the
swift stream of "the great river, the river of Babylon," began to
flow.

* * * * *

Thus, evidence which cannot be rebutted, and which need not be
strengthened, though if time permitted I might indefinitely increase
its quantity, compels you to believe that the earth, from the time of
the chalk to the present day, has been the theatre of a series of
changes as vast in their amount as they were slow in their progress.
The area on which we stand has been first sea and then land, for at
least four alternations; and has remained in each of these conditions
for a period of great length.

Nor have these wonderful metamorphoses of sea into land, and of land
into sea, been confined to one corner of England. During the chalk
period, or "cretaceous epoch," not one of the present great physical
features of the globe was in existence. Our great mountain ranges,
Pyrenees, Alps, Himalayas, Andes, have all been upheaved since the
chalk was deposited, and the cretaceous sea flowed over the sites of
Sinai and Ararat.

All this is certain, because rocks of cretaceous or still later date
have shared in the elevatory movements which gave rise to these
mountain chains; and may be found perched up, in some cases, many
thousand feet high upon their flanks. And evidence of equal cogency
demonstrates that, though in Norfolk the forest-bed rests directly
upon the chalk, yet it does so, not because the period at which the
forest grew immediately followed that at which the chalk was formed,
but because an immense lapse of time, represented elsewhere by
thousands of feet of rock, is not indicated at Cromer.

I must ask you to believe that there is no less conclusive proof that
a still more prolonged succession of similar changes occurred before
the chalk was deposited. Nor have we any reason to think that the
first term in the series of these changes is known. The oldest
sea-beds preserved to us are sands, and mud, and pebbles, the wear and
tear of rocks which were formed in still older oceans.

But, great as is the magnitude of these physical changes of the world,
they have been accompanied by a no less striking series of
modifications in its living inhabitants.

All the great classes of animals, beasts of the field, fowls of the
air, creeping things, and things which dwell in the waters, flourished
upon the globe long ages before the chalk was deposited. Very few,
however, if any, of these ancient forms of animal life were identical
with those which now live. Certainly not one of the higher animals was
of the same species as any of those now in existence. The beasts of
the field, in the days before the chalk, were not our beasts of the
field, nor the fowls of the air such as those which the eye of man has
seen flying, unless his antiquity dates infinitely further back than
we at present surmise. If we could be carried back into those times,
we should be as one suddenly set down in Australia before it was
colonized. We should see mammals, birds, reptiles, fishes, insects,
snails, and the like, clearly recognizable as such, and yet not one of
them would be just the same as those with which we are familiar, and
many would be extremely different.

From that time to the present, the population of the world has
undergone slow and gradual, but incessant, changes. There has been no
grand catastrophe--no destroyer has swept away the forms of life of
one period, and replaced them by a totally new creation; but one
species has vanished and another has taken its place; creatures of one
type of structure have diminished, those of another have increased, as
time has passed on. And thus, while the differences between the living
creatures of the time before the chalk and those of the present day
appear startling, if placed side by side, we are led from one to the
other by the most gradual progress, if we follow the course of Nature
through the whole series of those relics of her operations which she
has left behind.

[Illustration: SKELETON OF THE PTERODACTYL.]

And it is by the population of the chalk sea that the ancient and the
modern inhabitants of the world are most completely connected. The
groups which are dying out flourish, side by side, with the groups
which are now the dominant forms of life.

Thus the chalk contains remains of those flying and swimming
reptiles, the pterodactyl, the ichthyosaurus, and the plesiosaurus,
which are found in no later deposits, but abounded in preceding ages.
The chambered shells called ammonites and belemnites, which are so
characteristic of the period preceding the cretaceous, in like manner
die with it.

[Illustration: THE SKELETON OF THE ICHTHYOSAURUS.]

[Illustration: THE SKELETON OF THE PLESIOSAURUS.]

[Illustration: AMMONITES.]

But, among these fading remainders of a previous state of things, are
some very modern forms of life, looking like Yankee peddlers among a
tribe of red Indians. Crocodiles of modern type appear; bony fishes,
many of them very similar to existing species, almost supplant the
forms of fish which predominate in more ancient seas; and many kinds
of living shell-fish first become known to us in the chalk. The
vegetation acquires a modern aspect. A few living animals are not even
distinguishable as species from those which existed at that remote
epoch. The Globigerina of the present day, for example, is not
different specifically from that of the chalk; and the same may be
said of many other Foraminifera. I think it probable that critical and
unprejudiced examination will show that more than one species of much
higher animals have had a similar longevity; but the only example
which I can at present give confidently is the snake's-head lamp-shell
(_Terebratulina caput serpentis_), which lives in our English seas and
abounded (as _Terebratulina striata_ of authors) in the chalk.

[Illustration: BELEMNITES.]

[Illustration: TEREBRATULINA.]

The longest line of human ancestry must hide its diminished head
before the pedigree of this insignificant shell-fish. We Englishmen
are proud to have an ancestor who was present at the Battle of
Hastings. The ancestors of _Terebratulina caput serpentis_ may have
been present at a battle of Ichthyosauria in that part of the sea
which, when the chalk was forming, flowed over the site of Hastings.
While all around has changed, this Terebratulina has peacefully
propagated its species from generation to generation, and stands to
this day as a living testimony to the continuity of the present with
the past history of the globe.

* * * * *

Up to this moment I have stated, so far as I know, nothing but
well-authenticated facts, and the immediate conclusions which they
force upon the mind.

But the mind is so constituted that it does not willingly rest in
facts and immediate causes, but seeks always after a knowledge of the
remoter links in the chain of causation.

Taking the many changes of any given spot of the earth's surface, from
sea to land, and from land to sea, as an established fact, we cannot
refrain from asking ourselves how these changes have occurred. And
when we have explained them--as they must be explained--by the
alternate slow movements of elevation and depression which have
affected the crusts of the earth, we go still further back, and ask,
Why these movements?

I am not certain that any one can give you a satisfactory answer to
that question. Assuredly I cannot. All that can be said for certain
is, that such movements are part of the ordinary course of nature,
inasmuch as they are going on at the present time. Direct proof may be
given, that some parts of the land of the northern hemisphere are at
this moment insensibly rising and others insensibly sinking; and there
is indirect but perfectly satisfactory proof, that an enormous area
now covered by the Pacific has been deepened thousands of feet since
the present inhabitants of that sea came into existence.

Thus there is not a shadow of a reason for believing that the
physical changes of the globe, in past times, have been effected by
other than natural causes.

Is there any more reason for believing that the concomitant
modifications in the forms of the living inhabitants of the globe have
been brought about in any other ways?

Before attempting to answer this question, let us try to form a
distinct mental picture of what has happened in some special case.

The crocodiles are animals which, as a group, have a very vast
antiquity. They abounded ages before the chalk was deposited; they
throng the rivers in warm climates at the present day. There is a
difference in the form of the joints of the backbone, and in some
minor particulars, between the crocodiles of the present epoch and
those which lived before the chalk; but, in the cretaceous epoch, as I
have already mentioned, the crocodiles had assumed the modern type of
structure. Notwithstanding this, the crocodiles of the chalk are not
identically the same as those which lived in the times called "older
tertiary," which succeeded the cretaceous epoch; and the crocodiles of
the older tertiaries are not identical with those of the newer
tertiaries, nor are these identical with existing forms. I leave open
the question whether particular species may have lived on from epoch
to epoch. But each epoch has had its peculiar crocodiles; though all,
since the chalk, have belonged to the modern type, and differ simply
in their proportions and in such structural particulars as are
discernible only to trained eyes.

How is the existence of this long succession of different species of
crocodiles to be accounted for?

Only two suppositions seem to be open to us--either each species of
crocodile has been specially created, or it has arisen out of some
pre-existing form by the operation of natural causes.

Choose your hypothesis; I have chosen mine. I can find no warranty for
believing in the distinct creation of a score of successive species of
crocodiles in the course of countless ages of time. Science gives no
countenance to such a wild fancy; nor can even the perverse ingenuity
of a commentator pretend to discover this sense, in the simple wrords
in which the writer of Genesis records the proceeding of the fifth and
sixth days of the Creation.

On the other hand, I see no good reason for doubting the necessary
alternative, that all these varied species have been evolved from
pre-existing crocodilian forms by the operation of causes as
completely a part of the common order of nature as those which have
effected the changes of the inorganic world.

Few will venture to affirm that the reasoning which applies to
crocodiles loses its force among other animals or among plants. If one
series of species has come into existence by the operation of natural
causes, it seems folly to deny that all may have arisen in the same
way.

* * * * *

A small beginning has led us to a great ending. If I were to put the
bit of chalk with which we started into the hot but obscure flame of
burning hydrogen, it would presently shine like the sun. It seems to
me that this physical metamorphosis is no false image of what has been
the result of our subjecting it to a jet of fervent, though nowise
brilliant, thought to-night. It has become luminous, and its clear
rays, penetrating the abyss of the remote past, have brought within
our ken some stages of the evolution of the earth. And in the shifting
"without haste, but without rest" of the land and sea, as in the
endless variation of the forms assumed by living beings, we have
observed nothing but the natural product of the forces originally
possessed by the substance of the universe.

[Illustration]

A BIT OF SPONGE

(Written on Scotland.)

(FROM GLIMPSES OF NATURE.)

BY A. WILSON.

[Illustration]

This morning, despite the promise of rain over-night, has broken with
all the signs and symptoms of a bright July day. The Firth is bathed
in sunlight, and the wavelets at full tide are kissing the strand,
making a soft musical ripple as they retire, and as the pebbles run
down the sandy slope on the retreat of the waves. Beyond the farthest
contact of the tide is a line of seaweed dried and desiccated, mixed
up with which, in confusing array, are masses of shells, and such
_olla podrida_ of the sea.

Tossed up at our very feet is a dried fragment of sponge, which
doubtless the unkind waves tore from its rocky bed. It is not a large
portion of sponge this, but its structure is nevertheless to be fairly
made out, and some reminiscences of its history gleaned, for the sake
of occupying the by no means "bad half-hour" before breakfast. "What
is a sponge?" is a question which you may well ask as a necessary
preliminary to the understanding of its personality.

[Illustration: A SPONGE ATTACHED TO ITS ROCKY BED.]

The questionings of childhood and the questionings of science run in
precisely similar grooves. "What is it?" and "How does it live?" and
"Where does it come from?" are equally the inquiries of childhood, and
of the deepest philosophy which seeks to determine the whole history
of life. This morning, we cannot do better than follow in the
footsteps of the child, and to the question, "What is a sponge?" I
fancy science will be able to return a direct answer. First of all, we
may note that a sponge, as we know it in common life, is the horny
skeleton or framework which was made by, and which supported, the
living parts. These living parts consist of minute masses of that
living jelly to which the name of _protoplasm_ has been applied. This,
in truth, is the universal matter of life. It is the one substance
with which life everywhere is associated, and as we see it simply in
the sponge, so also we behold it (only in more complex guise) in the
man. Now, the living parts of this dried cast-away sponge were found
both in its interior and on its surface. They lined the canals that
everywhere permeate the sponge-substance, and microscopic examination
has told us a great deal about their nature.

[Illustration: FIG. 1. DEVELOPMENT OF A SPONGE (_Olynthus_). 1. The
egg. 2, 3, and 4. The process of egg-division. 5 and 6. The
gastrula-stage. 7. The perfect sponge.]

For, whether found in the canals of the sponge themselves, or embedded
in the sponge-substance, the living sponge-particles are represented
each by a semi-independent mass of protoplasm. So that the first view
I would have you take of the sponge as a living mass, is, that it is a
colony and not a single unit. It is composed, in other words, of
aggregated masses of living particles, which bud out one from the
other, and manufacture the supporting skeleton we know as "the sponge
of commerce" itself. Under the microscope, these living sponge-units
appear in various guises and shapes. Some of them are formless, and,
as to shape, ever-altering masses, resembling that familiar animalcule
of our pools we know as the _Amoeba_. These members of the
sponge-colony form the bulk of the population. They are embedded in
the sponge substance; they wander about through the meshes of the
sponge; they seize food and flourish and grow; and they probably also
give origin to the "eggs" from which new sponges are in due course
produced.

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