have been groping their way to a right classification in the ichthyic
department of their science, and how it has been that identical
perception of resemblance, having _cartilage_ for its general idea,
to which the author of the “Vestiges” attaches so much importance,
that has served mainly to retard their progress. Not a few of the more
distinguished among their number deemed it too important a distinction
to be regarded as merely secondary; and so long as it was retained as
a primary characteristic, the fishes failed to range themselves in the
natural order;—dissimilar tribes were brought into close neighborhood,
while tribes nearly allied were widely separated. It failed, as has
been shown, to influence Linnæus; and though he no doubt pressed his
peculiar views too far when he degraded the glutinous hag into a worm,
and elevated the Sharks and Rays into reptiles, it is certainly worthy
of remark, that, in the scheme of classification which is now regarded
as the _most natural_,—that of Professor Muller, modified by Professor
Owen,—the ichthyic worms of the Swede are placed in the first and
lowest order of fishes,—the _Dermopteri_,—and the greater part of his
ichthyic reptiles, in the eleventh and highest,—the _Plagiostomi_. Cuvier
yielded, as has been shown, to the idea of resemblance founded on the
_material_ of the ichthyic framework, and so ranged his fishes into two
parallel lines. Professor Oken, after first enunciating as law that “the
characteristic _organ_ of fishes is the osseous system,” confessed the
“great difficulty” which attaches to the question of skeletal “texture
or substance,” and finally gave up the distinction founded on it as
obstinately irreducible to the purposes of a natural classification.
“The cartilaginous fishes,” he says, “appear to belong to each other,
and are also usually arranged together; yet amongst them we find those
species, such as the Lampreys, which obviously occupy the lowest grade
of all fishes, while the Sharks and Rays remind us of the Reptilia.”
And so, sinking the consideration of texture altogether, he placed the
family of the Lamprey, including the glutinous hag, at the bottom of the
scale, and the Sharks and Rays at the top. Agassiz’s system, peculiarly
his own, has had the rare merit, as I have shown, of furnishing a key
to the history of the fish in its several dynasties, which we may in
vain seek in any other. His divisions,—if, retaining his strongly-marked
Placoids and Ganoids, as orders stamped in the mint of nature, we throw
his perhaps less obviously divisible Ctenoids and Cycloids into one
order,—the corneous or horn-covered,—are scarcely less representative
of periods than those great classes of the vertebrata, mammals, birds,
reptiles, and fishes, which we find not less regularly ranged in their
order of succession in the geologic record than in the “Animal Kingdom”
of Cuvier,—a shrewd corroboration, in both cases, I am disposed to
think, of the rectitude of the arrangement. What seems to be the special
defect of his system is, that having erected his four orders, and then
finding a certain number of residuary families that, on his principle
of cuticular character, stubbornly refused to fall into any determinate
place, he distributed them among the others, with reference chiefly to
the totally distinct principle of Cuvier. Thus the _Suctorii_, soft,
smooth, slimy-skinned fishes, that do not possess a single placoid
character, and are not true Placoids, he has yet placed in his Placoid
order, influenced, apparently, by the “perception of resemblance that
has _cartilage_ for its central idea;” and the effect has been a massing
into one anomalous and entangled group the fishes of the first period
of geologic history, with fishes of which we do not find a trace save
in the existing scene of things, and of the highest families of their
class with families that occupy the lowest place. But we live in an age
in which even the benefactors of the world of mind cannot make false
steps with impunity; and so, while Agassiz’s _three_ ichthyic orders will
continue to be recognized by the palæontologist as the orders of three
great geologic periods, the _Suctorii_ have already been struck from off
his higher fishes by the classification of Muller and Owen, and carried
to that lowest point in the scale (indicated by Linnæus and Oken) which
their inferior standing renders so obviously the natural one. Some of
my readers may perhaps remember how finely Bacon, in his “Wisdom of the
Ancients,” interprets the old mythologic story of Prometheus. Prometheus,
says the philosopher, had conferred inestimable favors on men, by
moulding their forms into shape, and bringing them fire from heaven; and
yet they complained of him and his teachings to Jupiter. And the god,
instead of censuring their ingratitude, was pleased with the complaint,
and rewarded them with gifts. In putting nature to the question, it is
eminently wholesome to be doubting, cross-examining, complaining; ever
demanding of our masters and benefactors the philosophers, that they
should reign over us, not arbitrarily and despotically,
“Like the old kings, with high exacting looks,
Sceptred and globed,”
but like our modern constitutional monarchs, who govern by law; and,
further, that an appeal from their decisions on all subjects within the
jurisdiction of Nature should for ever be open to Nature herself. The
seeming ingratitude of such a course, if the “complaints” be made in a
right spirit and on proper grounds, Jupiter always rewards with gifts.
Let us now see for ourselves, in this spirit, whether there may not be
something absolutely derogatory, in the existence of a cartilaginous
skeleton, to the creatures possessing it; or whether a deficit of
internal bone may not be greatly more than neutralized, as it assuredly
must have been in the view of Linnæus, Muller, and Owen, by a larger than
ordinary share of a vastly more important substance.
THE PLACOID BRAIN. EMBRYONIC CHARACTERISTICS NOT NECESSARILY OF A LOW
ORDER.
That special substance, according to whose mass and degree of development
all the creatures of this world take rank in the scale of creation,
is not _bone_, but _brain_. Were animals to be ranged according to
the solidity of their bones, the class of birds would be assigned the
first place; the family of the _Felidæ_, including the tiger and lion,
the second; and the other terrestrial carnivora the third. Man and the
herbivorous animals, though tolerably low in the scale, would be in
advance of at least the reptiles. Most of these, however, would take
precedence of the sagacious _Delphinidæ_; the osseous fishes would
come next in order; the true Placoids would follow, succeeded by the
_Sturiones_; and the _Suctorii_, _i. e._ Cyclostomi or Lampreys, would
bring up the rear. There would be evidently no order here: the utter
confusion of such an arrangement, like that of the bits of a dissected
map flung carelessly out of its box by a child, would of itself
demonstrate the inadequacy and erroneousness of the regulating principle.
But how very different the appearance presented, when for _solidity of
bone_ we substitute _development of brain_! Man takes his proper place
at the head of creation; the lower mammalia follow,—each species in
due order, according to its modicum of intelligence; the birds succeed
the mammalia; the reptiles succeed the birds; the fishes succeed the
reptiles; next in the long procession come the invertebrate animals; and
these, too, take rank, if not according to their development of brain
proper, at least according to their development of the _substance_ of
brain. The occipital nervous ganglion of the scorpion greatly exceeds
in size that of the earthworm; and the occipital nervous ring of the
lobster, that of the intestinal Ascaris. At length, when we reach the
lowest or _acrite_ division of the animal kingdom, the substance of
brain altogether disappears. It has been calculated by naturalists, that
in the vertebrata, the brain in the class of fishes bears an average
proportion to the spinal cord of about two to one; in the class of
reptiles, of about two and a half to one; in the class of birds, of about
three to one; in the class of mammals, of about four to one; and in the
high-placed, sceptre-bearing human family, a proportion of not less than
_twenty-three_ to one. It is palpably according to development of brain,
not development of bone, that we are to determine points of precedence
among the animals,—a fact of which no one can be more thoroughly aware
than the author of the “Vestiges” himself. Of this let me adduce a
striking instance, of which I shall make further use anon.
“All life,” says Oken, “is from the sea; none from the continent.
Man also is a child of the warm and shallow parts of the sea in the
neighborhood of the land.” Such also was the hypothesis of Lamarck and
Maillet. In following up the view of his masters, the author of the
“Vestiges” fixes on the _Delphinidæ_ as the sea-inhabiting progenitors
of the simial family, and, through the simial family, of man For that
highest order of the mammalia to which the _Simiadæ_ (monkeys) belong,
“there remains,” he says, “a basis in the _Delphinidæ_, the last and
smallest of the cetacean tribes. This affiliation has a special support
in the brain of the dolphin family, which is distinctly allowed to be,
in proportion to general bulk, the greatest among mammalia next to the
orang-outang and man. We learn from Tiedemann, that each of the cerebral
hemispheres is composed, as in man and the monkey tribe, of three
lobes,—an anterior, a middle, and a posterior; and these hemispheres
present much more numerous circumvolutions and grooves than those of any
other animal. Here it might be rash to found any thing upon the ancient
accounts of the dolphin,—its familiarity with man, and its helping him
in shipwreck and various marine disasters; although it is difficult to
believe these stories to be altogether without some basis in fact. There
is no doubt, however, that the dolphin evinces a predilection for human
society, and charms the mariner by the gambols which it performs beside
his vessel.”
Here, then, the author of the “Vestiges” palpably founds on a large
development of brain in the dolphin, and on the manifestation of a
correspondingly high order of instincts,—and this altogether irrespective
of the structure or composition of the creature’s internal skeleton. The
substance to which he looks as all-important in the case is _brain_, not
_bone_. For were he to estimate the standing of the dolphin, not by its
brain, but by its skeleton, he would have to assign to it a place, not
only _not_ in advance of its brethren the _mammalia_ of the sea, but
even in the rear of the _reptiles_ of the sea, the marine tortoises, or
turtles,—and scarce more than abreast of the osseous fishes. “Fishes,”
says Professor Owen, in his “Lectures on the Vertebrate Animals,”
“have the least proportion of earthy matter in their bones; birds the
largest. The mammalia, especially the active, predatory species, have
more earth, or harder bones, than reptiles. In each class, however, there
are differences in the density of bone among its several members. For
example, in the fresh-water fishes, the bones are lighter, and retain
more animal matter, than in those which swim in the denser sea. And in
the _dolphin_, a warm-blooded marine animal, they differ little in this
respect from those of the sea-fish.” Such being the fact, it is surely
but fair to inquire of the author of the “Vestiges,” why he should
determine the rank and standing of the _Delphinidæ_ according to one set
of principles, and the rank and standing of the Placoids according to
another and entirely different set? If the _Delphinidæ_ are to be placed
high in the scale, notwithstanding the softness of their skeletons,
simply because their brains are large, why are the Placoids to be
placed low in the scale, notwithstanding the largeness of their brains,
simply because their skeletons are soft? It is not too much to demand,
that on the principle which he himself recognizes as just, he should
either degrade the dolphin or elevate the Placoid. For it is altogether
inadmissible that he should reason on one set of laws when the exigencies
of his hypothesis require that creatures with soft skeletons should be
raised in the scale, and on another and entirely different set when its
necessities demand that they should be depressed.
But do the Placoids possess in reality a large development of brain? I
have examined the brains of almost all the common fish of our coast, both
osseous and cartilaginous, not, I fear, with the skill of a Tiedemann,
but all the more intelligently in consequence of what Tiedemann had
previously done and written: and so I can speak with some little
confidence on the subject, so far at least as my modicum of experience,
thus acquired, extends. Of all the common fish of the Scottish seas,
the spotted or lesser dog-fish bears, in proportion to its size, the
largest brain; the gray or picked dog-fish ranks next in its degree
of development; the Rays, in their various species, follow after; and
the osseous fishes compose at least the great body of the rear; while
still further behind, there lags a hapless class—the _Suctorii_, one of
which, the glutinous hag, has scarce any brain, and one, the _Amphioxus_
or lancelet, wants brain altogether. I have compared the brain of the
spotted dog-fish with that of a young alligator, and have found that in
scarce any perceptible degree was it inferior, in point of bulk, and very
slightly indeed in point of organization, to the brain of the reptile.
And the instincts of this Placoid family,—one of the truest existing
representatives of the Placoids of the Silurian System[25] to which we
can appeal,—correspond, we invariably find, with their superior cerebral
development. I have seen the common dog-fish, _Spinax Acanthias_,
hovering in packs in the Moray Frith, some one or two fathoms away from
the side of the herring boat from which, when the fishermen were engaged
in hauling their nets, I have watched them, and have admired the caution
which, with all their ferocity of disposition, they rarely failed to
manifest;—how they kept aloof from the net, even more warily than the
cetacea themselves,—though both dog-fish and cetacea are occasionally
entangled;—and how, when a few herrings were shaken loose from the
meshes they at once darted upon them, exhibiting for a moment, through
the green depths, the pale gleam of their abdomens, as they turned upon
their sides to seize the desired morsels,—a motion rendered necessary
by the position of the mouth in this family; and how next, their object
accomplished, they fell back into their old position, and waited on as
before. And I have been assured by intelligent fishermen, that at the
deep-sea white-fishing, in which baited hooks, not nets, are employed,
the degree of shrewd caution exercised by these creatures seems more
extraordinary still. The hatred which the fisher bears to them arises not
more from the actual amount of mischief which they do him, than from the
circumstance that in most cases they persist in doing it with complete
impunity to themselves. I have seen, said an observant Cromarty fisherman
to the writer of these chapters, a pack of dog-fish watching beside
our boat, as we were hauling our lines, and severing the hooked fish,
as they passed them, at a bite, just a little above the vent, so that
they themselves escaped the swallowed hook; and I have frequently lost,
in this way, no inconsiderable portion of a fishing. I have observed,
however, he continued, that when a fresh pack of hungry dog-fish came
up, and joined the pack that had been robbing us so coolly, and at their
leisure, a sudden rashness would seize the whole,—the united packs would
become a mere heedless mob, and, rushing forward, they would swallow
our fish entire, and be caught themselves by the score and the hundred.
We may see something very similar to this taking place among even the
shrewder mammalia. When pig refuses to take his food, his mistress
straightway calls upon the cat, and, quickened by the dread of the coming
rival, he gobbles up his rations at once. With the comparatively large
development of brain, and the corresponding manifestations of instinct,
which the true Placoids exhibit, we find other unequivocal marks of a
general superiority to their class. In their reproductive organs they
rank not with the common fishes, nor even with the lower reptiles,
but with the Chelonians and the Sauria. Among the Rays, as among the
higher animals, there are individual attachments formed between male and
female: their eggs unlike the mere spawn of the osseous fishes, or of
even the Batrachians, are, like those of the tortoise and the crocodile,
comparatively few in number, and of considerable size: their young,
too, like the young of birds and of the higher reptiles, pass through
no such metamorphosis as those of the toad and frog, or of the amphibia
generally. And some of their number—the common dog-fish for instance—are
ovoviviparous, bringing forth their young, like the common viper and the
viviparous lizard, alive and fully formed.
“But such features,” says the author of the “Vestiges,” referring
chiefly to certain provisions connected with the reproductory system
in the Placoids, “are partly partaken of by families in inferior
sub-kingdoms, showing that they cannot truly be regarded as marks of
grade in their own class.” Nay, single features do here and there occur
in the inferior sub-kingdoms, which very nearly resemble single features
in the placoid character and organization, which even very nearly
resemble single features in the _human_ character and organization; but
is there any of the inferior sub-kingdoms in which there occurs such
a _collocation_ of features? or does such a collocation occur in any
class of animals—setting the Placoids wholly out of view—which is not
a high class? Nay, further, does there occur in any of the inferior
sub-kingdoms—existing even as a single feature—that most prominent,
leading characteristic of this series of fishes,—a large brain?
But is not the “cartilaginous structure” of the Placoids analogous to
the embryonic state of vertebrated animals in general? Do not the other
placoid peculiarities to which the author of the “Vestiges” refers,—such
as the heterocercal or one-sided tail, the position of the mouth on the
under side of the head, and the rudimental state of the maxillaries
and intermaxillaries,—bear further analogies with the embryonic state
of the higher animals? And is not “embryonic progress the grand key to
the theory of development?” Let us examine this matter. “These are the
characters,” says this ingenious writer, “which, above all, I am chiefly
concerned in looking to; for they are features of embryonic progress, and
embryonic progress is the grand key to the theory of development.” Bold
assertion, certainly; but, then, assertion is not argument! The statement
is not a reason for the faith that is in the author of the “Vestiges,”
but simply an avowal of it; it is simply a confession, not a defence,
of the Lamarckian creed; and, instead of being admitted as embodying a
first principle, it must be put stringently to the question, in order to
determine whether it contain a principle at all.
In the first place, let us remark, that the cartilaginous structure
of the Placoids bears no very striking analogy to the cartilaginous
structure of the higher vertebrata in the embryonic state. In the case
of the _Delphinidæ_, with their soft skeletons, the analogy is greatly
more close. Bone consists of animal matter, chiefly gelatinous, hardened
by a diffusion of inorganic earth. In the bones of young and fœtal
mammalia, inhabitants of the land, the gelatinous prevails; in the old
and middle-aged there is a preponderance of the earth. Now, in the bones
of the dolphin there is comparatively little earth. The analogies of
its internal skeleton bear, not on the skeletons of its brethren the
mature full-grown mammals of the land, but on the skeletons of their
immature or fœtal offspring. But in the case of the true Placoids that
analogy is faint indeed. Their skeletons contain true bone;—the vertebral
joints of the Sharks and Rays possess each, as has been shown, an osseous
nucleus, which retains, when subjected to the heat of a common fire,
the complete form of the joint; and their cranial framework has its
surface always covered over with hard osseous points. But though their
skeletons possess thus their modicum of bone, unlike those of embryonic
birds or mammals, they contain, in what is properly their cartilage, no
gelatine. The analogy signally fails in the very point in which it has
been deemed specially to exist. The cartilage of the _Chondropterygii_
is a substance so essentially different from that of young or embryonic
birds and mammals, and so unique in the animal kingdom, that the heated
water in which the one readily dissolves has no effect whatever upon the
other. It is, however, a curious circumstance, exemplified in some of the
Shark family,[26] though it merely serves, in its exceptive character,
to establish the general fact, that while the rays of the double fins,
which answer to the phalanges, are all formed of this _indissoluble_
cartilage, those rays which constitute their outer framework, with the
rays which constitute the framework of all the single fins, are composed
of a _mucoidal_ cartilage, which boils into glue. At certain definite
lines a change occurs in the texture of the skeleton; and it is certainly
suggestive of thought, that the difference of substance which the change
involves distinguishes that part of the skeleton which is homologically
representative of the skeletons of the higher vertebrata, from that part
of it which is peculiar to the creature as a fish, viz. the dorsal and
caudal rays, and the extremities of the double fins. These emphatically
ichthyic portions of the animal may be dissipated by boiling, whereas
what Linnæus would perhaps term its _reptilian_ portion abides the heat
without reduction.
But is not the one-sided tail, so characteristic of the sharks, and of
almost all the ancient Ganoids, also a characteristic of the young salmon
just burst from the egg? Yes, assuredly; and, so far as research on the
subject has yet extended, of not only the salmon, but of _all_ the other
osseous fishes in their fœtal state. The salmon, on its escape from the
egg, is a little monster of about three quarters of an inch in length,
with a huge heart-shaped bag, as bulky as all the rest of its body,
depending from its abdomen. In this bag provident nature has packed up
for it, in lieu of a nurse, food for five weeks; and, moving about every
where in its shallow pool, with its provision knapsack slung fast to it,
it reminds one disposed to be fanciful, save that its burden is on the
wrong side, of Scottish soldiers of the olden time summoned to attend
their king in war,—
“Each on his _back_, a slender store,
His forty days’ provision bore,
As ancient statutes tell.”
Around that terminal part of the creature’s body traversed by the
caudal portion of the vertebral column, which commences in the salmon
immediately behind the ventrals, there runs at this period, and for the
ensuing five weeks in which it does not feed, a membranous fringe or fin,
which exactly resembles that of the tadpole, and which, existing simply
as an expansion of the skin, exhibits no mark or rays. In the place of
the true caudal fin, however, we may detect with the assistance of a
lens, an internal framework with two well-marked lobes, and ascertain,
further, that this tail is set on awry,—the effect of a slight upward
bend in the creature’s body. And when viewed in a strong light as a
transparency, we perceive that the spinal cord takes the same upward
bend, and, as in the sturgeon, passes in an exceedingly attenuated
form into the upper lobe. What may be regarded as the _design_ of the
arrangement is probably to be found in the peculiar form given to the
little creature by the protuberan bag in front. A wise instinct teaches
it, from the moment of its exclusion from the egg, to avoid its enemies.
In the instant the human shadow falls upon its pool, we see it darting
into some recess at the side or bottom, with singular alacrity; and in
order to enable it to do so, and to steer itself aright,—as, like an
ill-trimmed vessel, deep in the water ahead, the balance of its body
is imperfect,—there is, if I may so express myself, a heterocercal
peculiarity of helm required. It has got an irregularly-developed tail to
balance an irregularly-developed body, as skiffs _lean_ on the one beam
and _full_ on the other require, in rowing, a cast of the rudder to keep
them straight in their course.