scale? On this head we can, of course, determine merely by the analogies
which their structure exhibits to that of fishes of the existing
period; and these point in three several directions;—in two of the
number, directly on genera of the high Ganoid order; and in the third,
on the still higher Placoids and Enaliosaurs. No trace of vertebræ
has yet been found; and so we infer—lodging, however, a precautionary
protest, as the evidence is purely negative, and therefore it some
degree inconclusive—that the vertebral column of the _Asterolepis_
was, like that of the sturgeon, cartilaginous. Respecting its external
covering, we positively know, as has been already shown, that, like
the _Lepidosteus_ of America and the _Polypterus_ of the Nile, it was
composed of strong plates and scales of solid bone; and, regarding its
dentition, that, as in these last genera, and even more decidedly than
in these, it was of the mixed ichthyic-reptilian character,—an outer row
of thickly-set fish-teeth being backed by an inner row of thinly-set
reptile-teeth. And its form of coprolite indicates the spiral disposition
of intestine common to the Rays and Sharks of the existing period, and
of the Ichthyosauri of the Secondary ages. Instead of being, as the
development hypothesis would require, a fish low in its organization,
it seems to have ranged on the level of the highest ichthyic-reptilian
families ever called into existence. Had an intelligent being, ignorant
of what was going on upon earth during the week of creation, visited
Eden on the morning of the sixth day, he would have found in it many
of the inferior animals, but no trace of man. Had he returned again in
the evening, he would have seen, installed in the office of keepers of
the garden, and ruling with no tyrant sway as the humble monarchs of
its brute inhabitants, two mature human creatures, perfect in their
organization, and arrived at the full stature of their race. The entire
evidence regarding them, in the absence of all such information as that
imparted to Adam by Milton’s angel, would amount simply to this, that
in the morning man _was not_, and that in the evening he _was_. There,
of course, could not exist, in the circumstances, a single appearance
to sanction the belief that the two human creatures whom he saw walking
together among the trees at sunset had been “developed from infusorial
points,” not created mature. The evidence would, on the contrary,
lie all the other way. And in no degree does the geologic testimony
respecting the earliest Ganoids differ from what, in the supposed case,
would be the testimony of Eden regarding the earliest men. Up to a
certain point in the geologic scale we find that the Ganoids _are not_;
and when they at length make their appearance upon the stage, they enter
large in their stature and high in their organization.
FISHES OF THE SILURIAN ROCKS—UPPER AND LOWER. THEIR RECENT HISTORY,
ORDER, AND SIZE.
But the system of the Old Red Sandstone represents the _second_, not
the _first_, great period of the world’s history. There was a preceding
period at least equally extended, perhaps greatly more so, represented
by the Upper and Lower Silurian formations. And what is the testimony
of this morning period of organic existence, in which, so far as can
yet be shown, vitality, in the planet which man inhabits, and of whose
history or productions he knows anything, was first associated with
matter? May not the development hypothesis find a standing in the system
representative of this earliest age of creation, which it fails to find
in the system of the Old Red Sandstone?
It has been confidently asserted, not merely that it _may_, but that it
_does_. Ever since the publication, in 1839, of Sir Roderick Murchison’s
great work on the Silurian System, it had been known that the remains
of fishes occur in a bed of the “Ludlow Rock,”—one of the most modern
deposits of the _Upper_ Silurian division; and subsequent discoveries
both in England and America, had shown that even the _base_ of this
division has its ichthyic organisms. But for year after year, the
lower half of the system,—a division more than three thousand feet in
thickness,—had failed, though there were hands and eyes busy among its
deposits, to yield any vertebrate remains. During the earlier half of
the first great period of organic existence, though the polyparia,
radiata, articulata, and mollusca, existed, as their remains testified,
by myriads, fish had, it was held, not yet entered upon the scene; and
the assertors of the development theory founded largely on the presumed
fact of their absence. “It is still customary,” says the author of the
“Vestiges of Creation,” in his volume of “Explanations,” “to speak of
the earliest fauna as one of an elevated kind. When rigidly examined, it
is not found to be so. IN THE FIRST PLACE, IT CONTAINS NO FISH. There
were seas supporting crustacean and molluscan life, but _utterly devoid
of a class of tenants who seem able to live in every example of that
element which supports meaner creatures_. This single fact, that only
invertebrated animals now lived, is surely in itself a strong proof that,
in the course of nature, _time_ was necessary for the creation of the
superior creatures. And if so, it undoubtedly is a powerful evidence of
such a theory of development as that which I have presented. If not, let
me hear an equally plausible reason for the great and amazing fact, that
seas were for numberless ages destitute of fish. I fix my opponents down
to the consideration of this fact, so that no diversion respecting high
molluscs shall avail them.” And how is this bold challenge to be met?
Most directly, and after a fashion that at once discomfits the challenger.
It might be rationally enough argued in the case, that the author of
the “Vestiges” was building greatly more on a piece of purely negative
evidence,—the presumed absence of fish from the Lower Silurian
formations,—than purely negative evidence is, from its nature as such,
suited to bear; that only a very few years had passed since it was known
that vertebrate remains occurred in the _Upper_ Silurian, and only a
few more since they had been detected in the Old Red Sandstone; nay,
that within the present century their frequent occurrence in even the
Coal Measures was scarce suspected; and that, as his argument, had it
been founded twelve years ago on the supposed absence of fishes from
the Upper Silurian, or twenty years ago on the supposed absence of
fishes from the Old Red Sandstone, would have been quite as plausible
in reference to its negative data then as in reference to its negative
data now, so it might now be quite as erroneous as it assuredly would
have been then. Or it might be urged, that the fact of the absence of
fish from the Lower Silurians, even were it really a fact, would be in
no degree less reconcilable with the theory of creation by direct act,
than with the hypothesis of gradual development. The fact that Adam
did not exist during the first, second, third, fourth, and fifth days
of the introductory week of Scripture narrative, furnishes no argument
whatever against the fact of his creation on the sixth day. And the
remark would of course equally apply to the non-existence of fishes
during the Lower Silurian period, had they been really non-existent
at the time, and to their sudden appearance in that of the Upper. But
the objection admits of a greatly more conclusive answer. “I fix my
opponents down,” says the author of the “Vestiges,” “to the consideration
of this fact,” _i. e._ that of the absence of fishes from the earliest
fossiliferous formations. And I, in turn, fix you down, I reply, to the
consideration of the antagonist fact, not negative, but positive, and
now, in the course of geological discovery, fully established, that
fishes were _not_ absent from the earliest fossiliferous formations.
From none of the great geological formations were fishes absent,—not
even from the formations of the Cambrian division. “The Lower Silurian,”
says Sir Roderick Murchison, in a communication with which, in 1847, he
honored the writer of these chapters, “is no longer to be viewed as an
invertebrate period; for the _Onchus_ (species not yet decided) has been
found in the Llandeilo Flags and in the Lower Silurian rocks of Bala.
In one respect I am gratified by the discovery; for the form is so very
like that of the _Onchus Murchisoni_ of the Upper Ludlow rock, that it
is clear the Silurian system is one great natural-history series, as is
proved, indeed, by all its other organic remains.” It may be mentioned
further, in addition to this interesting statement, that the Bala spine
was detected in its calcareous matrix by the geologists of the Government
Survey, and described to Sir Roderick as that of an _Onchus_, by a very
competent authority in such matters,—Professor Edward Forbes, and that
the annunciation of the existence of spines of fishes in the Llandeilo
Flags we owe to one of the most cautious and practised geologists of the
present age,—Professor Sedgwick of Cambridge.
So much for the _fact_ of the existence of vertebrata in the Lower
Silurian formations, and the _argument_ founded on their presumed
absence. Let me now refer—their presence being determined—to the tests
of size and organization. Were these Silurian fishes of a bulk so
inconsiderable as in any degree to sanction the belief that they had
been developed shortly before from microscopic points? Or were they of a
structure so low as to render it probable that their development was at
the time incomplete? Were they, in other words, the embryos and fœtuses
of their class? or did they, on the contrary, rank with the higher and
larger fishes of the present time?
It is of importance that not only the direct _bearing_, but also the
actual _amount_, of the evidence in this case, should be fairly stated.
So far as it extends, the testimony is clear; but it does not extend
far. All the vertebrate remains yet detected in the Silurian System,
if we except the debris of the Upper Ludlow bone-bed, might be sent
through the Post-Office in a box scarcely twice the size of a copy of
the “Vestiges.” The naturalist of an exploring party, who, in crossing
some unknown lake, had looked down over the side of his canoe, and seen
a few fish gliding through the obscure depths of the water, would be but
indifferently qualified, from what he had witnessed, to write a history
of _all_ its fish. Nor, were the some six or eight individuals of which
he had caught a glimpse to be of small size, would it be legitimate for
him to infer that only small-sized fish lived in the lake; though, were
there to be some two or three large ones among them, he might safely
affirm the contrary. Now, the evidence regarding the fishes of the
Silurian formation very much resembles what that of the naturalist would
be, in the supposed case, regarding the fishes of the unexplored lake;
with, however, this difference, that as the deposits of the ancient
system in which they occur have been examined for years in various parts
of the world, and all its characteristic organisms, save the ichthyic
ones, found in great abundance and fine keeping, we may conclude that
the fish of the period were comparatively few. The palæontologist, so
far as the question of number is involved, is in the circumstances, not
of the naturalist who has only once crossed the unknown lake, but of the
angler who, day after day, casts his line into some inland sea abounding
in shell-fish and crustacea, and, after the lapse of months, can scarce
detect a nibble, and, after the lapse of years, can reckon up all the
fish which he has caught as considerably under a score. The existence
of this great division of the animal kingdom, like that of the earlier
reptiles during the Carboniferous period, did not form a prominent
characteristic of those ages of the earth’s history in which they began
to be.
The earliest discovered vertebral remains of the system—those of the
Upper Ludlow rock—were found in digging the foundations of a house at
Ludford, on the confines of Shropshire, and submitted, in 1838, by
Sir Roderick Murchison to Agassiz, through the late Dr. Malcolmson of
Madras. I used at the time to correspond on geological subjects with Dr.
Malcolmson,—an accomplished geologist and a good man, too early lost to
science and his friends,—and still remember the interest which attached
on this occasion to his communication bearing the Paris post-mark,
from which I learned for the first time that there existed ichthyic
fragments greatly older than even the ichthyolites of the Lower Old Red
Sandstone, and which made me acquainted with Agassiz’s earliest formed
decision regarding them. Though existing in an exceedingly fragmentary
condition,—for the materials of the thin dark-colored layer in which
they had lain seemed as if they had been triturated in a mortar,—the
ichthyologist succeeded in erecting them into six genera; though it may
be very possible,—as some of these were formed for the reception of
detached spines, and others for the reception of detached teeth,—that,
as in the case of _Dipterus_ and _Asterolepis_, the fragments of but
a single genus may have been multiplied into two genera or more. And
minute scale-like markings, which mingled with the general mass, and
were at first regarded as the impressions of real scales, have been
since recognized as of the same character with the scale-like markings
of the _Seraphim_ of Forfarshire, a huge crustacean. Even admitting,
however, that a set of teeth and spines, with perhaps the shagreen points
represented in page 54, fig. 2, _b_, in addition, may have all belonged
to but a single species of fish, there seem to be materials enough,
among the remains found, for the erection of two species more. And we
have evidence that at least two of the three kinds were fishes of the
Placoid order, (_Onchus Murchisoni_ and _Onchus tenuistriatus_,) and—as
the supposed scales must be given up—no good evidence that the other kind
was not. The ichthyic remains of the Silurian System next discovered
were first introduced to the notice of geologists by Professor Phillips,
at the meeting of the British Association in 1842.[22] They occurred,
he stated, in a quarry near Hales End, at the base of the Upper Ludlow
rock, immediately over the Aymestry Limestone, and were so exceedingly
diminutive, that they appeared to the naked eye as mere discolored spots;
but resolved under the microscope into scattered groupes of minute
spines, like those of the _Cheiracanthus_, with what seemed to be still
more minute _scales_, or, perhaps,—what in such circumstances could
scarce be distinguished from scales,—shagreen points of the scale-like
type. The next ichthyic organism detected in the Silurian rocks occurred
in the Wenlock Limestone, a considerably lower and older deposit, and was
first described in the “Edinburgh Review” for 1845 by a vigorous writer
and masterly geologist, (generally understood to be Professor Sedgwick of
Cambridge,) as “a characteristic portion of a fish undoubtedly belonging
to the Cestraciont family of the Placoid order.” In the “American
Journal of Science” for 1846, Professor Silliman figured, from a work
of the States’ Surveyors, the defensive spine of a Placoid found in the
Onondago Limestone of New York,—a rock which occurs near the base of the
Upper Silurian System, as developed in the western world;[23] and in
the same passage he made reference to a mutilated spine detected in a
still lower American deposit,—the Oriskany Sandstone. In the Geological
Journal for 1847, it was announced by Professor Sedgwick, that he had
found “_defences of fishes_” in the Upper Llandeilo Flags, and by Sir
Roderick Murchison, that the “defence of an _Onchus_” had been detected
by the geologists of the Government survey, in the Limestone near Bala.
Sir Roderick referred in the same number to the remains of a fish found
by Professor Phillips in the Wenlock Shale. And such, up to the present
time, is the actual _amount_ of the evidence with which we have to deal,
and the dates of its piecemeal production. Let us next consider the
_order of its occurrence in the geologic scale_.
{ +-----+
{ Upper | | Fish, 1838,
{ Ludlow. | 1 | (Murchison.)
{ | | Fish, 1842,
{ +-----+ (Phillips.)
{ Aymestry | |
{ Limestone. | 2 |
{ +-----+
UPPER SILURIAN ROCKS. { Lower | |
{ Ludlow. | 3 |
{ +-----+
{ Wenlock | | Fish, 1845,
{ Limestone. | 4 | (Sedgwick.)
{ | | Fish, 1846,
{ +-----+ (Silliman.)
{ Wenlock | 5 | Fish, 1847.
{ Shale. | | (Phillips.)
+-----+
-----
{ +-----+
{ Caradoc | |
{ Sandstone, | 6 |
{ &c. | |
LOWER SILURIAN ROCKS. { +-----+
{ Llandeilo | | Fish, 1847,
{ Flags, &c. | 7 | (Sedgwick.)
{ +-----+
-----
{ +-----+
{ Plynlimmon | |
{ Group. | _a_ |
{ +-----+
{ Bala | | Fish, 1847,
CAMBRIAN ROCKS. { Limestone. | _b_ | (Geologists of
{ | | Government
{ +-----+ Survey.)
{ Snowdon | |
{ Group. | _c_ | Fucoids.
{ | |
{ +-----+
The better marked sub-divisions of the Silurian System, as described
in the great work specially devoted to it, may be regarded as seven
in number. An eight has since been added, by the transference of the
Tilestones from the lower part of the Old Red Sandstone group, to the
upper part of the Silurian group underneath; but in order the better to
show how ichthyic discovery has in its slow course penetrated into the
depths, I shall retain the divisions recognized as those of the system
when that course began. The highest or most modern Silurian deposit,
then, (No. 1 of the accompanying diagram,) is the _Upper_ Ludlow Rock;
and it is in the superior strata of this division that the bone-bed
discovered in 1838 occurs; while the exceedingly minute vertebrate
remains described by Professor Phillips in 1842 occur in its base. The
division next in the descending order is the Aymestry Limestone, (No. 2;)
the next (No. 3.) the _Lower_ Ludlow rock; then (No. 4.) the Wenlock or
Dudley Limestone occurs; and then, last and oldest deposit of the _Upper_
Silurian formation, the Wenlock shale, (No. 5.) It is in the fourth, or
Wenlock Limestone division, that the defensive spine described in the
“Edinburgh Review” for 1845 as the oldest vertebrate organism known at
the time, was found;[24] while the vertebrate organism found by Professor
Phillips belongs to the fifth, or base deposit of the Upper Silurian.
Further, the American spines of Onondago and Oriskany, described in 1846,
occurred in rocks deemed contemporary with those of the Wenlock division.
We next cross the line which separates the base of the Upper from the top
of the Lower Silurian deposits, and find a great arenaceous formation,
(No. 6,) known as the Caradoc Sandstones; while the Llandeilo Flags, (No.
7,) the formation upon which the sandstones rest, compose, according to
the sections of Sir Roderick, published in 1839, the lowest deposit of
the Lower Silurian rocks. And it is in the upper part of this lowest
member of the system that the ichthyic defences, announced in 1847 by
Professor Sedgwick, occur. Vertebrate remains have now been detected in
the same relative position in the _seventh_ and _most ancient_ member
of the system, that they were found to occupy in its _first_ and _most
modern_ member ten years ago. But this is not all. Beneath the Lower
Silurian division there occur vast fossiliferous deposits, to which the
name “Cambrian System” was given, merely provisionally, by Sir Roderick,
but which Professor Sedgwick still retains as representative of a
distinct geologic period; and it is in these, greatly below the Lower
Silurian base line, as drawn in 1839, that the Bala Limestones occur.
The Plynlimmon rocks (_a_)—a series of conglomerate, grauwacke, and
slate beds, several thousand yards in thickness—intervene between the
Llandeilo Flags and the Limestones of Bala, (_b_.) And, of consequence,
the defensive spine of the _Onchus_, announced in 1847 as detected in
these limestones by the geologists of the Government Survey, must have
formed part of a fish that perished many ages ere the oldest of the Lower
Silurian formations _began_ to be deposited.
Let us now, after this survey of both the amount of our materials, and
the order and time of their occurrence, pass on to the question of size,
as already stated. Did the ichthyic remains of the Silurian System,
hitherto examined and described, belong to large or to small fishes? The
question cannot be altogether so conclusively answered as in the case
of those Ganoids of the Lower Old Red Sandstone whose dermal skeletons
indicate their original dimensions and form. In fishes of the Placoid
order, such as the Sharks and Rays, the dermal skeleton is greatly less
continuous and persistent than in such Ganoids as the Dipterians and
Cœlacanths; and when their remains occur in the fossil state, we can
reason, in most instances, regarding the bulk of the individuals of which
they formed part, merely from that of detached teeth or spines, whose
proportion to the entire size of the animals that bore them cannot be
strictly determined. We can, indeed, do little more than infer, that
though a large Placoid may have been armed with but small spines or
teeth, a small Placoid could not have borne very large ones. And to this
Placoid order all the Silurian fish, from the Aymestry Limestone to the
Cambrian deposits of Bala inclusive, unequivocally belong. Nor, as has
been already said, is there sufficient evidence to show that any of the
ichthyic remains of the Upper Ludlow rocks do _not_ belong to it. It is
peculiarly the order of the system. The Ludlow bone-bed contains not
only defensive spines, but also teeth, fragments of jaws, and shagreen
points; whereas, in all the inferior deposits which yield any trace of
the vertebrata, the remains are those of defensive spines exclusively.
Let us, then, take the defensive spine as the part on which to found our
comparison.