history before the first vestige of a valve was formed. It had passed
through several metamorphoses; in its pupa stage it had the form of a
_Cypris_, and in this condition it first became adherent to the timber:
before this it was a larva, having a general resemblance to another
Waterflea, the _Cyclops_, especially in its younger stages: in this
state it moulted several times. Nor was this the beginning of its life;
for there was the still earlier condition common to all these classes of
animals, viz. that of the egg, which was laid and carried for some time
by the parent Barnacle, and at length hatched while within the valves of
her shell.
Thus, through a course of several years we are able to trace back the
existence of this Cirriped, to its parent of a former generation. But
our conclusions are altogether vitiated by the simple fact that this
individual is the first of its species; it never had a parent; it never
was an egg.
From the rocky pool before us I have picked up a rough pebble, the
surface of which is incrusted with a delicate work of stony lace. This
fabric, too fine to be resolved by the unassisted eye, consists of the
oval cells of a species of _Lepralia_. There are some hundreds of cells
in this patch, which altogether does not cover a square inch of the
pebble; and they are all made after one pattern, and set in a very
regular manner, in quincunx. Each is a minute slipper-shaped box of
stone, with the orifice set round with spines for the protection of the
inmate, a transparent, elegant, and sensitive Polypide, which bears on
its head a coronet of ciliated tentacles.
I am not going to describe the interesting structure and economy of this
atom of life; but merely wish to direct your attention to one
point,--the evidence which it affords of the lapse of past time.
Every one of these hundreds of stony cells, together with its living
tenant, was normally produced by a process of gemmation; each having
budded forth from the side of its predecessor as a knob of clear
gelatinous flesh, in the midst of which was developed, first the cell,
and then the polypide,--the latter appearing in a rudimentary condition,
and gradually acquiring its proper organs, before the orifice of the
cell was opened.
I said every one of the cells was thus formed; but I ought to have
excepted a single cell, which, though in nowise differing from the rest
in form or structure, had a very different origin. This was the primal
cell, and its beginning was as follows:
A minute atom of a scarlet hue, and of a semi-elliptical shape, was one
day whirling round and round with rapid gyrations in the open sea. It
was of soft consistence, covered with strongly vibrating cilia, and
furnished with some stouter setae. After enjoying its motile instincts
awhile, it settled down on this pebble, and became stationary. Presently
it secreted and deposited calcareous matter around at, like a coating of
the thinnest glass, the red parenchyma receding from the hyaline wall
towards the centre.
Soon an orifice with thickened edges appeared on the upper side, and
minute spines grew from the edges, which quickly lengthened. It was now
a _Lepralia_ cell, and now the polypide was developed, and protruded its
mouth from the orifice, surrounded by its elegant bell of ciliate
tentacles. This solitary cell became the parent of hundreds more, by the
gemmative process which I have already described.
But the red swimming atom;--whence came that? Well, it was shot out from
the interior of a previous _Lepralia_, the result not of a gemmative but
of a generative act. It originated in another patch similar to the one
which incrusts this pebble, and that, in like manner, and by exactly
similar stages, looked back to an anterior patch, and so on.
Plausible as this inference is, it is false; for the little aggregation
of cells and polypides has been called into existence by the Divine
_fiat_, this very instant.
We are still at the sea-shore. Within the long and narrow crevices into
which these low-lying ledges of shale are split, innumerable tufts of
sea-weed,--olive, purple, and green,--are perpetually waving in the wash
of the sea. On one of these branching shrubs of _Phyllophora_, there is
adhering, apparently cast there by accident, an irregular mass of
pellucid jelly. It firmly cleaves to the alga, enclosing the bases of
several branches within its firm but gelatinous substance.
This knob of jelly is a compound animal of the genus _Botryllus_, and it
has just been created as we see it. In order to understand its nature,
look at it more closely.
Enclosed in the clear purplish-grey jelly, in the midst of scattered
lighter specks, we see several star-like figures of bright hues, in
which yellow and red are predominant; the symmetrical arrangement of
which pleases the eye, and reminds us of some ornamental pattern
designed by human art. Each star is composed of several (three, seven,
ten or more) pear-shaped animals, with their smaller ends meeting in
the centre around a common orifice, from which a current of water is
discharged.
Now this assemblage of animals bears evidence of progressive
development. Some time ago a tiny egg was discharged from a parent
_Botryllus_, which presently produced a little active tadpole-like
larva, called a "spinule." This swam actively by means of its wriggling
tail; but at length it settled head downward on this piece of sea-weed.
Immediately the head adhered, by an effused cement, to its support; the
tail now gradually disappeared; and the round head, in the midst of a
mass of jelly-like cement, began to display two orifices on its surface.
It soon assumed a pear-like shape, and thus the first _Botryllus_ was
formed.
From the side of this "pear," another was developed by gemmation, and a
third on the opposite side; the smaller ends of all were in contact, and
the orifices of these extremities began to merge into one; while the
large ends diverged. A fourth and a fifth "pear" were successively
produced in the same mode, until a star or "system" was formed.
Meanwhile the surrounding mass of living jelly had been commensurately
enlarging, and a new _Botryllus_, separate from the other star, had been
produced in the jelly, which was the commencing point of a second
system; and thus, by degrees, the compound mass of systems has grown to
its present state of development.
[Illustration: BOTRYLLUS.
_a_, portion of one system and of a mass, on _Phyllophora rubens_; _b_,
an egg _c_, spinule; _d_, the same, attached; _e_, the tail absorbed;
_f_, the young _Botryllus_. All magnified.]
This process has been one of time: the adhesion of the "spinule" took
place in about sixteen hours after its escape from the egg. The
appearance of the two orifices was when the little animal was four days
old; and by the end of a week a second "pear" had budded. The attainment
of the present condition may have occupied about six months.
Nay; time has been no element in this development; it is prochronic
development; it is the development of creation, not of nature.
Behold that ruffling of the smooth surface of the water; it is caused
evidently by the forcible ejection of a current from some source a
little way beneath the surface. Yes, it proceeds from the orifice in
this mass of calcareous grit; where the protruding pipe of shell
indicates the snug fortress of a _Clavagella_. I will carefully break
away a little of the soft stone, and we shall see the curious structure
more clearly. Ha! I have split off a piece which nicely exposes the
whole burrow, without having materially injured the creature or his
shell.
You see it is a bivalve Mollusk with one valve firmly imbedded and
cemented into the stony wall of its chamber. But the hinder end of this
valve is continued into a shelly tube, intended to protect the siphons,
which is carried through the gallery forming the entrance into the
chamber, and opens by a wide orifice in the free water outside. It is
to this tube that I call your attention.
[Illustration: CLAVAGELLA.]
You observe that on its outer surface there are several foliated
expansions of the shelly substance, surrounding it like so many frills
at pretty regular intervals. Each of these foliations is a permanent
record of a certain epoch. The terminal one is the margin of the
tube-wall everted. The one below this was at some past period the
eversion of the margin at what was at that time the extremity. The third
frill had in like manner terminated the tube still earlier; and so with
the fourth and fifth. It is impossible to look at these expansions, and
not to believe that they have been formed in succession, in this way, by
the periodic growth of the tube.
There was a time when, the first frill was not commenced; when the
creature was a Mollusk with simple valves. But even this was not the
beginning of its history. It was as a swimming Infusory with a broad
ciliated disk, and a lashing _flagellum_, that the creature commenced
its independent career; and it was doubtless in this condition[74] that
it found its way into the burrow of some _Saxicava_. Here its tiny
transparent valves were secreted; the left valve was soon cemented to
the chamber; and then the creature began to secrete and form the tube
around its siphons, which was progressively enlarged, and adorned at
every stage of elongation by these witnessing frills--whose testimony is
recorded in imperishable stone.
What can be more irresistible than such evidence as this? And yet we
must take exception to it on the ground that this is the very hour of
the animal's creation.
[Illustration: DIONE VENERIS.]
The elegant spinous shell-fish that we discern yonder, half-buried in
the sandy floor of the sea--I mean that lilac-tinted Prickly Venus
(_Dione Veneris_) needs no shelly protection for its siphons, which, as
you may observe, are protruded to a great length. But a lesson not less
instructive than that taught by the tube-frills of the _Clavagella_, is
inculcated by the valves of the _Dione_. Near the hinder margin of each
valve there is a ridge which runs from the beak to the front edge, a
ridge which bears the series of long slender shelly spines, that imparts
such a charm to this shell.
Each of these spines records an interval in the growth of the shell.
There are sixteen distinctly enumerable; each of which may possibly mark
a year's growth. The increase of bivalves, however, is slow; and it may
be that a longer interval than a year has intervened between spine and
spine. For if we look more closely at this beautiful shell, we see that
the whole exterior of both valves is marked with concentric foliated
ridges, which are also indubitable lines of growth; and that these are
twice or thrice as numerous as the spines, from one to five being
intercalated between those which support the prolongations of the shelly
substance.
Each of these concentric lines has a history. Every line, as well as
every spine, has been produced by a protrusion and eversion of the
glanduligerous edge of the mantle, which then secreted and poured out a
copious deposit of calcareous matter along the margin of the previously
existing valve. In this species each periodic deposit took the form of a
ridge slightly elevated above the general surface; and, because the
turned up margin of the mantle invested the edge of the valve already
formed, therefore the new layer, with its elevated ridge, was concentric
with the last edge, which was concentric with the previous one, and so
on, the common centre of all being the beak (_umbo_) at the back of the
valve.
The spines were formed in a manner essentially similar. At every second
or third period of increase, the margin of the mantle, which is very
versatile and protrusile, was thrust out, at the point which corresponds
to the spines, into a long fleshy groove, by the reduplication of its
edge. Within this groove the calcareous secretion was poured out; and
after it had been allowed a few moments to harden or "_set_," the
mantle-groove was cautiously withdrawn, and a new spine was exposed, as
a produced end to the foliated ridge.
Yet, though this is the normal and natural mode of production, both of
the concentric line and of the spines, it would be illusory to conclude
that they have been so produced in the present example. The entire
formation of the _Dione_ before us has been ab-normal and
preter-natural: it has been _created_, not _born_: the whole development
so legibly written on the shell has been prochronic.
There goes the Scorpion Stromb (_Pteroceras scorpio_), crawling over the
rocks with protruded head and tentacles, and bearing his massive house
on his back. This shelly house of his will afford us a good example of
structural development.
The great dilated lip, and the long finger-like processes of its edge,
had no existence in the youthful days of the shell; they are marks of
adult age: when young, the shell was simply spiral, with a thin straight
lip bounding a narrow aperture.
Observe also a far more beautiful creature by its side, the Tiger Cowry
(_Cypraea tigris_). Its shell is now entirely enveloped in the meeting
wings of the great fleshy mantle, which is mottled with changing hues;
and its foot or crawling disk covers a space three or four times as
large as the shell. On lifting it in our hand, the whole of this array
of soft flesh has been rapidly retracted, and has wholly disappeared
within that very narrow orifice, bordered with toothed projections, on
the under side of the shell, which we can hardly believe capable of
receiving a twentieth part of the bulk that has vanished within it. And
now we see nothing but the shell, with its smooth rounded back, marked
with dark spots, its white inferior surface cleft by this longitudinal
denticulate aperture, and its brilliant porcellanous varnish over the
whole.
Now here is evidence of change and progress again. This Cowry-shell is
very unlike that of an Olive, with a simple spire, an oval body, a
smooth thin lip, and a wide orifice; and as unlike that of a Nautilus.
Yet it has passed through both of these stages before it was disguised
as we see it now. When it escaped from the egg-shell, it was a minute
Pteropod, with two great ciliated disks, inhabiting a transparent
nautiloid shell, and swimming giddily about in a revolving fashion. By
and by, the tiny shell increased, and the outer whorl lengthened,
putting on a long-oval figure. Then--that is, after a considerable
period occupied in increasing the dimensions of the shell in this
form--it began to assume the adult appearance. The outer lip, which had
hitherto been thin, gradually thickened and encroached upon the spire,
and the mantle began to secrete and deposit on the outer surface the
coat of glassy enamel.
At length the thickening of the lips proceeded to such an extent as
almost to conceal the spire, and to reduce the aperture to a narrow
line, the edges of which were now thickly plaited with the tooth-like
ridges so characteristic of the genus. The lobes of the mantle now
protrude through this aperture; and, expanding on each side, have
deposited all over the exterior of the shell a coat of glassy enamel,
studded with dark round spots or clouds, which entirely conceals the
surface with the markings that were formerly visible upon it.
[Illustration: MUREX TENUISPINA.]
Yonder Thorny Woodcock (_Murex tenuispina_) is a still more striking
shell than either, and one whose periodic growths are peculiarly well
marked. It is covered at regular intervals with rows of shelly spines,
still longer and more numerous than those we lately admired in the
_Dione_. Each series crowns a thickened ridge, which runs across the
whorl, as regards the direction of its growth, but longitudinally as
regards the general figure of the shell.
Now, the increase of the shell in the Univalves is performed almost
exactly as in the Bivalves; namely, by the protrusion and eversion of
the mantle on the existing edge. And, therefore, each of these thorny
ridges, separated as they are by an interval of just two-thirds of a
whorl, marks the termination of a new growth, the shelly matter rising
up at the margin in this thickened ridge, which bristles with elongated
points.
In this specimen we can trace ten such ridges, whence we legitimately
infer ten distinct periods through which this animal has passed, besides
the nautiloid stage under which all the creatures of this Class commence
existence.
Yet, since each of these three univalves has been this day created,
these inferences are deceptive. The Scorpion-shell was _never_ otherwise
than dilated and digitated. The Cowry has _never_ had a lip that was
not thickened, nor an exterior that was not porcellanous. The Woodcock
has _never_ known a moment in which its thorns were less numerous than
they are now.
Notice that fine round shell carried along the floor of the sea, by
means of a great fleshy tortoiseshell-coloured[75] body, which, with a
head of many spreading tentacles applied to the ground, crawls with a
tolerably quick progress.[76] It is the Pearly Nautilus.
The amplitude of the beautiful nacreous shell is by no means a measure
of the dimensions of the animal; for this merely sits within the shallow
mouth, like a Welsh fisherman in his coracle. If we remove the creature,
we shall find the cavity bounded by a pearly floor, in the centre of
which is a slender tube running down from it. On breaking away this
floor, we expose an empty chamber, with a similar pearly floor, through
which passes the shelly tube, continued through the middle of the
chamber, and running down to the next. Thus we should find the whole
interior of the shell occupied by a series of these empty chambers,
fifty or upwards in number, each less than its predecessor (rather
_successor_, if we regard them in the order of development), until we
can trace them no longer in the minute centre of the spire.
Without dwelling on the function of these chambers, farther than to say
that they appear admirably contrived to make the animal with its shell
either heavier or lighter than the surrounding fluid, by forcing water
into them through the tube, and thus condensing the contained air, or by
relaxing the pressure, and allowing the elasticity of the air to exclude
the water,--our business is just with the formation of the septa, as an
evidence of periodic development.[77]
"The septa are formed periodically, but it must not be supposed that the
shell-muscles ever become detached, or that the animal moves the
distance of a chamber all at once. It is most likely that the
_adductors_ grow only in front, and that a constant waste takes place
behind, so that they are always moving onward, except when a new septum
is to be formed; the _septa_ indicate periodic _rests_."[78]
These periodic alternations of rest and action, however, it is obvious,
can never have really existed in an organism which has but this instant
been created. The appearances, therefore, which indicate them, are
illusory, considered as testimonies to actual time.
You are aware that what is often spoke of as the "bone" in this
Cuttlefish (_Sepia officinalis_), is only a concealed shell; and I need
not to dissect the animal to acquaint you that it is a highly
interesting structure. A deservedly eminent physiologist shall describe
it for us.
"The outer shelly portion of this body consists of horny layers,
alternating with calcified layers, in which last may be seen a hexagonal
arrangement. The soft, friable substance, that occupies the hollow of
this boat-shaped shell, is formed of a number of delicate plates,
running across it from one side to the other in parallel directions, but
separated by intervals several times wider than the thickness of the
plates; and these intervals are in great part filled up by what appear
to be fibres, or slender pillars, passing from one plate or floor to
another. A more careful examination shows, however, that instead of a
large number of detached pillars, there exists a comparatively small
number of very thin, sinuous laminae, which pass from one surface to the
other, winding and doubling upon themselves, so that each lamina
occupies a considerable space. Their precise arrangement is best seen by
examining the parallel plates, after the sinuous laminae have been
detached from them; the lines of junction being distinctly indicated
upon these. By this arrangement, each layer is most effectually
supported by those with which it is connected above and below; and the
sinuosity of the thin intervening laminae, answering exactly the same
purpose as the "corrugation" given to iron plates for the sake of
diminishing their flexibility, adds greatly to the strength of this
curious texture, which is at the same time lightened by the large amount
of space between the parallel plates that intervenes between the
sinuosities of the laminae."[79]
Now the delicately thin calcareous plates have all been formed in
succession, "the first formed being at the outer part and posterior
termination of the shell, and the succeeding new layers extending always
more forwards than the edges of the old."[80] They exhibit then many
hundreds of distinct deposits, each the result of a separate process,
each the work of a definite period of time. The "cuttle-bone" is an
autographic record, indubitably genuine, of the Cuttlefish's history.
Yes, it is certainly genuine; it is as certainly autographic: but it is
_not true_. That Cuttle has been this day created.
IX.
PARALLELS AND PRECEDENTS.
(_Vertebrate Animals._)
"The organisation of the body at each epoch may be truly
said to be the _resultant_ of all the material changes which
it has undergone during the preceding periods."--_Dr.
Carpenter; Human Physiology_, p. 903.
The _Invertebrata_ then agree in one story, and that story is the same
as what the plants had told us before. Let us try if the Vertebrate
creatures bear them out.
From this promontory we can look far down into the clear profundity of
the still and smooth sea. What is that large object that plays hither
and thither yonder, now shooting ahead, now resting on his oars, now
turning on his course, now cutting the surface, now descending to the
depths? It is a full-grown Sword-fish, some ten feet long. We are
sufficiently near him to discern that he has one short but high dorsal
fin, near the head, and a minute one close to the caudal, the whole
intermediate region being smooth. But this is a mark of adult age; for
in early life this same species is furnished with one long and high
dorsal, which is continuous from the occiput to the vicinity of the
tail-fin. The remotely divided dorsal here tells of many years of life;
but tells deceitfully, for the Sword-fish is but just created.
Ha! the Sword-fish has darted away, like lightning, after a finny
victim. See with what doublings and windings he pursues it, and how the
terrified prey uses all its powers to escape from its gigantic enemy!
Now they near the shore; and now the frightened quarry has leaped out of
the sea upon yonder flat shelf of rock, where it lies gasping and
floundering, delivered indeed from its pursuer, but only to die by being
drowned _in the air_. We will descend from the cliffs, and look at it.
It is a Gilt-head (_Chrysophrys aurata_). Life is extinct now; but the
brilliant colours and fine metallic reflections are scarcely dimmed--the
silvery belly--the azure fins--the sides that gleam like polished steel,
inlaid with bands of burnished gold!