Atlantic Monthly, Vol. XI., April, 1863, No. LXVI. by Various

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What we do find--and these, as I shall endeavor to show my readers, in
such profusion that it would seem as if God, in the joy of creation, had
compensated Himself for a less variety of forms in the greater richness
of the early types--is an immense number of beings belonging to the four
primary divisions of the Animal Kingdom, but only to those classes whose
representatives are marine, whose home then, as now, was either in the
sea or along its shores. In other words, the first organic creation
expressed in its totality the structural conception since carried out in
such wonderful variety of details, and purposely limited then, because
the world, which was to be the home of the higher animals, was not yet
made ready to receive them.

I am fully aware that the intimate relations between the organic and
physical world are interpreted by many as indicating the absence, rather
than the presence, of an intelligent Creator. They argue, that the
dependence of animals on material laws gives us the clue to their origin
as well as to their maintenance. Were this influence as absolute and
unvarying as the purely mechanical action of physical circumstances
must necessarily be, this inference might have some pretence to
logical probability,--though it seems to me unnecessary, under any
circumstances, to resort to climatic influences or the action of any
physical laws to explain the thoughtful distribution of the organic and
inorganic world, so evidently intended to secure for all beings what
best suits their nature and their needs. But the truth is, that, while
these harmonious relations underlie the whole creation in such a manner
as to indicate a great central plan, of which all things are a part,
there is at the same time a freedom, an arbitrary element in the mode of
carrying it out, which seems to point to the exercise of an individual
will; for, side by side with facts, apparently the direct result of
physical laws, are other facts, the nature of which shows a complete
independence of external influences.

Take, for instance, the similarity above alluded to between the fauna of
the Arctics and that of the Alps, certainly showing a direct relation
between climatic conditions and animal and vegetable life. Yet even
there, where the shades of specific difference between many animals
and plants of the same class are so slight as to battle the keenest
investigators, we have representative types both in the Animal and
Vegetable Kingdoms as distinct and peculiar as those of widely removed
and strongly contrasted climatic conditions. Shall we attribute the
similarities and the differences alike to physical causes? Compare, for
example, the Reindeer of the Arctics with the Ibex and the Chamois,
representing the same group in the Alps. Even on mountain-heights of
similar altitudes, where not only climate, but other physical conditions
would suggest a recurrence of identical animals, we do not find the
same, but representative types. The Ibex of the Alps differs, for
instance, from that of the Pyrenees, that of the Pyrenees from those of
the Caucasus and Himalayas, these again from each other and from that of
the Altai.

But perhaps the most conclusive proof that we must seek for the origin
of organic life outside of physical causes consists in the permanence of
the fundamental types, while the species representing these types have
differed in every geological period. Now what we call typical features
of structure are in themselves no more stable or permanent than specific
features. If physical causes, such as light, heat, moisture, food,
habits of life, etc., acting upon individuals, have gradually in
successive generations changed the character of the species to which
they belong, why not that of the class and the branch also? If we judge
this question from the material side at all, we must, in order to judge
it fairly, look at it wholly from that point of view. If these specific
changes are brought about in this way, it is because external causes
have positive permanent effects upon the substances of which animals are
built: they have power to change their hair, to change their skin, to
change certain external appendages or ornamentations, and any other of
those ultimate features which naturalists call specific characters.
Now I would ask what there is in the substances out of which class
characters are built that would make them less susceptible to such
external influences than these specific characters. In many instances
the former are more delicate, more sensitive, far more fragile and
transient in their material nature than the latter. And yet never, in
all the chances and changes of time, have we seen any alteration in the
mode of respiration, of reproduction, of circulation, or in any of the
systems of organs which characterize the more comprehensive groups of
the Animal Kingdom, although they are quite as much under the immediate
influence of physical causes as those structural features which have
been constantly changing.

The woody fibre of the Pine-trees has had the same structure from the
Carboniferous age to this day, while their mode of branching and the
forms of their cones and leaves have been different in each period
according to their respective species. The combination of rings, the
structure of the wings, and the articulations of the legs are the same
in the Cockroaches of the Carboniferous age as in those which infest our
ships and our dwellings to-day, while the proportion of their parts is
on quite another scale. The tissue of the Corals in the Silurian age is
identical in chemical combination and organic structure with that of the
Corals of our modern reefs, and yet the extensive researches upon this
class for which we are indebted to Milne Edwards and Haime have not
revealed a single species extending through successive geological ages,
but show us, on the contrary, that every age has had its own kinds,
differing among themselves in the same way as those of the Gulf of
Mexico differ now from those of the Indian Ocean and the Pacific. The
scales of the oldest known fishes in the Silurian beds have the same
microscopic structure as those of their representative types today, and
yet I have never seen a single fossil fish presenting the same specific
characters in the successive geological epochs. The teeth of the oldest
Sharks show the same microscopic structure as those of the present time,
and we do not lack opportunities for comparison, since the former are as
common in the mountain-limestone of Ireland as are those of the living
Sharks on any beach where our fishermen boil them for the sake of their
oil, and yet the Sharks appear under different generic and specific
forms in each geological age.

But without multiplying examples, which might be adduced _ad infinitum_,
to show permanence of type combined with repeated changes of species,
suffice it to say, that, while the general features in the framework of
the organic world and the materials of which that framework is
built, though quite as subject to the influence of physical external
circumstances as any so-called specific-features, have remained
perfectly intact from the beginning of Creation till now, so that not
the smallest difference is to be discerned in these respects between the
oldest representatives of the oldest types in the oldest Silurian rocks
and their successors through all the geological ages up to the present
day, the species have been different in each epoch. It is surely a fair
question to ask the advocates of the transmutation theory, whether they
attribute to physical laws the discernment that would lead them to
change the specific features, but to respect all those characters by
which the higher structural combinations of the Animal Kingdom are
preserved without alteration,--in other words, to maintain the organic
plan, while constantly diversifying the mode of expressing it. If so, it
would perhaps be as well to call them by another name, since they show
all the comprehensive wisdom of an intelligent Creator. Until they can
tell us why certain features of animals and plants are permanent under
conditions which, according to their view, have power to change certain
other features no more perishable or transient in themselves, the
supporters of the development theory will have failed to substantiate
their peculiar scientific doctrine.

But this discussion has led us far away from our starting point, and
interrupted our walk along the Silurian beach; let us return to gather a
few specimens there, and compare them with the more familiar ones of our
own shores. I have said that the beach was a shelving one, and covered
of course with shoal waters; but as I have no desire to mislead my
readers, or to present truths as generally accepted which are still
subject to dispute, I would state here that the parallel ridges across
the State of New York, considered by some geologists as the successive
shores of a receding ocean, are believed by others to be the
inequalities on the bottom of a shallow sea. Not only, however, does the
general character of these successive terraces suggest the idea that
they must have been shores, but the ripple-marks upon them are as
distinct as upon any modern beach. The regular rise and fall of the
water is registered there in waving, undulating lines as clearly as on
the sand-beaches of Newport or Nahant; and we can see on any one of
those ancient shores the track left by the waves as they rippled back at
ebb of the tide thousands of centuries ago. One can often see where some
obstacle interrupted the course of the water, causing it to break around
it; and such an indentation even retains the soft, muddy, plastic look
that we observe on the present beaches, where the resistance made by any
pebble or shell to the retreating wave has given it greater force at
that point, so that the sand around the spot is soaked and loosened.
There is still another sign, equally familiar to those who have watched
the action of water on a beach. Where a shore is very shelving and flat,
so that the waves do not recede in ripples from it, but in one unbroken
sheet, the sand and small pebbles are dragged and form lines which
diverge whenever the water meets an obstacle, thus forming sharp angles
on the sand. Such marks are as distinct on the oldest Silurian rocks as
if they had been made yesterday. Nor are these the only indications of
the same fact. There are certain animals living always upon sandy or
muddy shores, which require for their well-being that the beach should
be left dry a part of the day. These animals, moving about in the sand
or mud from which the water has retreated, leave their tracks there; and
if, at such a time, the wind is blowing dust over the beach, and the sun
is hot enough to bake it upon the impressions so formed, they are left
in a kind of mould. Such trails and furrows, made by small Shells or
Crustacea, are also found in plenty on the oldest deposits.

Admitting it, then, to be a beach, let us begin with the lowest type of
the Animal Kingdom, and see what Radiates are to be found there. There
are plenty of Corals, but they are not the same kinds of Corals as those
that build up our reefs and islands now. The modern Coral animals are
chiefly Polyps, but the prevailing Corals of the Silurian age were
Acalephian Hydroids, animals which indeed resemble Polyps in certain
external features, and have been mistaken for them, but which are
nevertheless Acalephs by their internal structure; for, instead of
having the vertical partitions dividing the body into chambers, so
characteristic of the Polyps, they are divided by tubes corresponding to
the radiating tubes of the Acalephs proper, these tubes being themselves
divided at regular distances by horizontal floors, so that they never
run uninterruptedly from top to bottom of the body. I subjoin a woodcut
of a Silurian Coral, which does not, however, show the peculiar internal
structure, but gives some idea of the general appearance of the old
Hydroid Corals. We have but one Acalephian Coral now living, the
Millepore; and it was by comparing that with these ancient ones that I
first detected their relation to the Acalephs. For the true Acalephs or
Jelly-Fishes we shall look in vain; but the presence of the Acalephian
Corals establishes the existence of the type, and we cannot expect to
find those kinds preserved which are wholly destitute of hard parts. I
do not attempt any description of the Polyps proper, because the early
Corals of that class are comparatively few, and do not present features
sufficiently characteristic to attract the notice of the casual
observer.

[Illustration]

[Illustration]

Of the Echinoderms, the class of Radiates represented now by our
Star-Fishes and Sea-Urchins, we may gather any quantity, though the old
fashioned forms are very different from the living ones. I have dwelt at
such length in a former article[A] on the wonderful beauty and variety
of the Crinoids, or "Stone Lilies," as they have been called, from their
resemblance to flowers, that I will only briefly allude to them here.
The subjoined wood-cut represents one with a closed cup; but the number
of their different patterns is hardly to be counted, and I would invite
any one who questions the abundant expression of life in those days to
look at some slabs of ancient limestone in the Zooelogical Museum at
Cambridge, where the stems of the Crinoids are tangled together as
thickly as sea-weed on the shore. Indeed, some of our rock-deposits
consist chiefly of the fragments of their remains.

[Footnote A: See _Methods of Study in Natural History, Atlantic
Monthly_, No. LVII., July, 1862.]

[Illustration]

The Mollusks were also represented then, as now, by their three
classes,--Acephala, Gasteropoda, and Cephalopoda. The Acephala or
Bivalves we shall find in great numbers, but of a very different pattern
from the Oysters, Clams, and Mussels of recent times. The annexed
wood-cut represents one of these Brachiopods, which form a very
characteristic type of the Silurian deposits. The square cut of the
upper edge, where the two valves meet along the back and are united by
a hinge, is altogether old-fashioned, and unknown among our modern
Bivalves. The wood-cut does not show the inequality of the two valves,
also a very characteristic feature of this group,--one valve being flat
and fitting closely into the other, which is more spreading and much
fuller. These, also, were represented by a great variety of species, and
we find them crowded together as closely in the ancient rocks as Oysters
or Clams or Mussels on any of our modern shores. Besides these, there
were the Bryozoa, a small kind of Mollusk allied to the Clams, and very
busy then in the ancient Coral work. They grew in communities, and the
separate individuals are so minute that a Bryozoan stock looks like
some delicate moss. They still have their place among the Reef-Building
Corals, but play an insignificant part in comparison with that of their
predecessors.

Of the Silurian Univalves or Gasteropods there is not much to tell, for
their spiral shells were so brittle that scarcely any perfect specimens
are known, though their broken remains are found in such quantities as
to show that this class also was very fully represented in the earliest
creation. But the highest class of Mollusks, the Cephalopods or
Chambered Shells, or Cuttle-Fishes, as they are called when the animal
is unprotected by a shell, are, on the contrary, very well preserved,
and they are very numerous. Of these I will speak somewhat more in
detail, because their geological history is a very curious one.

[Illustration]

The Chambered Nautilus is familiar to all, since, from the exquisite
beauty of its shell, it is especially sought for by conchologists;
but it is nevertheless not so common in our days as the Squids and
Cuttle-Fishes, which are the most numerous modern representatives of the
class. In the earliest geological days, on the contrary, those with a
shell predominated, differing from the later ones, however, in having
the shell perfectly straight instead of curved, though its internal
structure was the same as it is now and has ever been. Then, as now, the
animal shut himself out from his last year's home, building his annual
wall behind him, till his whole shell was divided into successive
chambers, all of which were connected by a siphon. Some of the shells of
this kind belonging to the Silurian deposits are enormous: giants of the
sea they must have been in those days. They have been found fifteen
feet long, and as large round as a man's body. One can imagine that the
Cuttle-Fish inhabiting such a shell must have been a formidable animal.
These straight-chambered shells of the Silurian and Devonian seas are
called Orthoceratites (see wood-cut below). We shall meet them again
hereafter, under another name and with a different form; for, as they
advance in the geological ages, they not only assume the curved outline
with ever closer whorls till it culminates in the compact coil of the
Ammonites of the middle periods, but the partitions, which are perfectly
plain walls in these earlier forms, become scalloped and involuted along
the edges in the later ones, making the most delicate and exquisite
tracery on the surface of the shell.

Of Articulates we find only two classes, Worms and Crustacea. Insects
there were none,--for, as we have seen, this early world was wholly
marine. There is little to be said of the Worms, for their soft bodies,
unprotected by any hard covering, could hardly be preserved; but,
like the marine Worms of our own times, they were in the habit of
constructing envelopes for themselves, built of sand, or sometimes from
a secretion of their own bodies, and these cases we find in the earliest
deposits, giving us assurance that the Worms were represented there.
I should add, however, that many impressions described as produced by
Worms are more likely to have been the tracks of Crustacea.

But by far the most characteristic class of Articulates in ancient times
were the Crustaceans. The Trilobites stand in the same relation to the
modern Crustacea as the Crinoids do to the modern Echinoderms. They were
then the sole representatives of the class, and the variety and richness
of the type are most extraordinary. They were of nearly equal breadth
for the whole length of the body, and rounded at the two ends, so as
to form an oval outline. To give any adequate idea of the number and
variety of species would fill a volume, but I may enumerate some of
the more striking differences: as, for instance, the greater or less
prominence of the anterior shield,--the preponderance of the posterior
end in some, while in others the two ends are nearly equal,--the
presence or absence of prongs on the shield and of spines along the
sides of the body,--appendages on the head in some species, of which
others are entirely destitute,--and the smooth outline of some, while
in others the surface is broken by a variety of external ornamentation.
Such are a few of the more prominent differences among them. But the
general structural features are the same in all. The middle region of
the body is always divided in uniform rings, lobed in the middle so as
to make a ridge along the back with a slight depression on either side
of it. It is from this three-lobed division that they receive their
name. The subjoined wood-cut represents a characteristic Silurian
Trilobite.

[Illustration]

There is no group more prominent in the earliest creations than this one
of the Trilobites, and so exclusively do they belong to them, that, as
we shall see, in proportion as the later representatives of the class
come in, these old-world Crustaceans drop out of the ranks, fall behind,
as it were, in the long procession of animals, and are left in the
ancient deposits. Even in the Carboniferous period but few are to
be found: they had their day in the Silurian and Devonian ages. In
consequence of their solid exterior, the preservation of these animals
is very complete; and their attitudes are often so natural, and the
condition of all their parts so perfect, that one would say they had
died yesterday rather than countless centuries ago.

Their geological history has been very thoroughly studied; not only are
we familiar with all their adult characters, but even their embryology
is well known to naturalists. It is, indeed, wonderful that the mode of
growth of animals which died out in the Carboniferous period should
be better known to us than that of many living types. But it is
nevertheless true that their embryonic forms have been found perfectly
preserved in the rocks, and Barrande, in his "Systeme Silurien de la
Boheme," gives us all the stages of their development, from the time
when the animal is merely sketched out as a simple furrow in the embryo
to its mature condition. So complete is the sequence, that the plate on
which their embryonic changes are illustrated contains more than thirty
figures, all representing different phases of their growth. There is not
a living Crab represented so fully in any of our scientific works as is
that one species of Trilobite whose whole story Barrande has traced from
the egg to its adult size. Such facts should make those who rest
their fanciful theories of the origin and development of life on the
imperfection of the geological record, filling up the supposed lapses to
suit themselves, more cautious as to their results.

We have found, then, Radiates, Mollusks, and Articulates in plenty;
and now what is to be said of Vertebrates in these old times,--of the
highest and most important division of the Animal Kingdom, that to which
we ourselves belong? They were represented by Fishes alone; and the Fish
chapter in the history of the early organic world is a curious, and,
as it seems to me, a very significant one. We shall find no perfect
specimens; and he would be a daring, not to say a presumptuous thinker,
who would venture to reconstruct a fish of the Silurian age from any
remains that are left to us. But still we find enough to indicate
clearly the style of those old fishes, and to show, by comparison with
the living types, to what group of modern times they belong. We should
naturally expect to find the Vertebrates introduced in their simplest
form; but this is by no means the case: the common fishes, as Cod,
Herring, Mackerel, and the like, were unknown in those days.

But there are two groups of so-called fishes, differing from these by
some marked features, among which we may find the modern representatives
of these earliest Vertebrates. Of these two groups one consists chiefly
now of the Gar-Pikes of our Western waters, though the Sturgeons share
also in some of their features. In these fishes there is a singular
union of reptilian with fish-like characters. The systems of circulation
and of respiration in them are more complicated than in the common
fishes; the structure of the skull resembles that of the skull in
reptiles, and they have other reptilian characters, such as their
ability to move the head upon the neck independently of the body, and
the connection of the vertebrae by ball-and-socket joint, instead of
by inverted cones, as in the ordinary fishes. Their scales are also
peculiar, being covered by enamel so hard, that, if struck with steel,
they will emit sparks like flint. It is on account of this peculiarity
that the whole group has been called Ganoid. Now, though we have
not found as yet any complete specimens of Silurian fishes, their
disconnected remains are scattered profusely in the early deposits. The
scales, parts of the backbone, parts of the skull, the teeth, are found
in a tolerable state of preservation; and these indications, fragmentary
as they are, give us the clue to the character of the most ancient
fishes. A large proportion of them were no doubt Ganoids; for they had
the same peculiar articulation of the vertebrae, the flexibility of the
neck, and the hard scales so characteristic of our Gar-Pikes.

There is another type of these ancient Vertebrates, which has also
its representatives among our modern fishes. These are the Sharks and
Skates, or, as the Greeks used to call them, the Selachians,--making a
very appropriate distinction between them and common fishes, on account
of the difference in the structure of the skeleton. In Selachians the
quality of the bones is granular, instead of fibrous, as in fishes; the
arches above and below the backbone are formed by flat plates, instead
of the spines so characteristic of all the fish proper; and the skull
consists of a solid box, instead of being built of overlapping pieces
like the true fish-skull. They differ also in their teeth, which,
instead of being implanted in the bone by a root, as in fishes, are
loosely set in the gum without any connection with the bone, and are
movable, being arranged in several rows one behind another, the back
rows moving forward to take the place of the front ones when the latter
are worn off. They are unlike the common fishes also in having the
backbone continued to the very end of the tail, which is cut in uneven
lobes, the upper lobe being the longer of the two, while the terminal
fin, so constant a feature in fishes, is wanting. The Selachians
resemble higher Vertebrate types not only in the small number of their
eggs, and in the closer connection of the young with the mother, but
also in their embryological development, which has many features in
common with that of birds and turtles. Of this group, also, we find
numerous remains in the ancient geological deposits; and though we have
not the means of distinguishing the species, we have ample evidence for
determining the type.

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