Our task of detecting the extinct ancestors of our race among the vast
numbers of animals known to us encounters very different difficulties
in the various sections of man's stem-history. These were very great
in the series of our invertebrate ancestors; they are much slighter in
the subsequent series of our vertebrate ancestors. Within the
vertebrate stem there is, as we have already seen, so complete an
agreement in structure and embryology that it is impossible to doubt
their phylogenetic unity. In this case the evidence is much clearer
and more abundant.
The characteristics that distinguish the Vertebrates as a whole from
the Invertebrates have already been discussed in our description of
the hypothetical Primitive Vertebrate (Chapter 1.11, Figure 1.98 to
1.102). The chief of these are: (1) The evolution of the primitive
brain into a dorsal medullary tube; (2) the formation of the chorda
between the medullary tube and the gut; (3) the division of the gut
into branchial (gill) and hepatic (liver) gut; and (4) the internal
articulation or metamerism. The first three features are shared by the
Vertebrates with the ascidia-larvae and the Prochordonia; the fourth
is peculiar to them. Thus the chief advantage in organisation by which
the earliest Vertebrates took precedence of the unsegmented Chordonia
consisted in the development of internal segmentation.
The whole vertebrate stem divides first into the two chief sections of
Acrania and Craniota. The Amphioxus is the only surviving
representative of the older and lower section, the Acrania
("skull-less"). All the other vertebrates belong to the second
division, the Craniota ("skull-animals"). The Craniota descend
directly from the Acrania, and these from the primitive Chordonia. The
exhaustive study that we made of the comparative anatomy and ontogeny
of the Ascidia and the Amphioxus has proved these relations for us.
(See Chapters 2.16 and 2.17.) The Amphioxus, the lowest Vertebrate,
and the Ascidia, the nearest related Invertebrate, descend from a
common extinct stem-form, the Chordaea; and this must have had,
substantially, the organisation of the chordula.
However, the Amphioxus is important not merely because it fills the
deep gulf between the Invertebrates and Vertebrates, but also because
it shows us to-day the typical vertebrate in all its simplicity. We
owe to it the most important data that we proceed on in reconstructing
the gradual historical development of the whole stem. All the Craniota
descend from a common stem-form, and this was substantially identical
in structure with the Amphioxus. This stem-form, the Primitive
Vertebrate (Prospondylus, Figures 1.98 to 1.102), had the
characteristics of the vertebrate as such, but not the important
features that distinguish the Craniota from the Acrania. Though the
Amphioxus has many peculiarities of structure and has much
degenerated, and though it cannot be regarded as an unchanged
descendant of the Primitive Vertebrate, it must have inherited from it
the specific characters we enumerated above. We may not say that
"Amphioxus is the ancestor of the Vertebrates"; but we can say:
"Amphioxus is the nearest relation to the ancestor of all the animals
we know." Both belong to the same small family, or lowest class of the
Vertebrates, that we call the Acrania. In our genealogical tree this
group forms the twelfth stage, or the first stage among the vertebrate
ancestors (Chapter 2.20). From this group of Acrania both the
Amphioxus and the Craniota were evolved.
The vast division of the Craniota embraces all the Vertebrates known
to us, with the exception of the Amphioxus. All of them have a head
clearly differentiated from the trunk, and a skull enclosing a brain.
The head has also three pairs of higher sense-organs (nose, eyes, and
ears). The brain is very rudimentary at first, a mere bulbous
enlargement of the fore end of the medullary tube. But it is soon
divided by a number of transverse constrictions into, first three,
then five successive cerebral vesicles. In this formation of the head,
skull, and brain, with further development of the higher sense-organs,
we have the advance that the Craniota made beyond their skull-less
ancestors. Other organs also attained a higher development; they
acquired a compact centralised heart with valves and a more advanced
liver and kidneys, and made progress in other important respects.
We may divide the Craniota generally into Cyclostoma ("round-mouthed")
and Gnathostoma ("jaw-mouthed"). There are only a few groups of the
former in existence now, but they are very interesting, because in
their whole structure they stand midway between the Acrania and the
Gnathostoma. They are much more advanced than the Acrania, much less
so than the fishes, and thus form a very welcome connecting-link
between the two groups. We may therefore consider them a special
intermediate group, the fourteenth and fifteenth stages in the series
of our ancestors.
(FIGURE 2.247. The large marine lamprey (Petromyzon marinus), much
reduced. Behind the eye there is a row of seven gill-clefts visible on
the left, in front the round suctorial mouth.)
The few surviving species of the Cyclostoma are divided into two
orders--the Myxinoides and the Petromyzontes. The former, the
hag-fishes, have a long, cylindrical, worm-like body. They were
classed by Linne with the worms, and by later zoologists, with the
fishes, or the amphibia, or the molluscs. They live in the sea,
usually as parasites of fishes, into the skin of which they bore with
their round suctorial mouths and their tongues, armed with horny
teeth. They are sometimes found alive in the body cavity of fishes
(such as the torsk or sturgeon); in these cases they have passed
through the skin into the interior. The second order consists of the
Petromyzontes or lampreys; the small river lamprey (Petromyzon
fluviatilis) and the large marine lamprey (Petromyzon marinus, Figure
2.247). They also have a round suctorial mouth, with horny teeth
inside it; by means of this they attach themselves by sucking to
fishes, stones, and other objects (hence the name Petromyzon =
stone-sucker). It seems that this habit was very widespread among the
earlier Vertebrates; the larvae of many of the Ganoids and frogs have
suctorial disks near the mouth.
The class that is formed of the Myxinoides and Petromyzontes is called
the Cyclostoma (round-mouthed), because their mouth has a circular or
semi-circular aperture. The jaws (upper and lower) that we find in all
the higher Vertebrates are completely wanting in the Cyclostoma, as in
the Amphioxus. Hence the other Vertebrates are collectively opposed to
them as Gnathostoma (jaw-mouthed). The Cyclostoma might also be called
Monorhina (single-nosed), because they have only a single nasal
passage, while all the Gnathostoma have two nostrils (Amphirhina =
double-nosed). But apart from these peculiarities the Cyclostoma
differ more widely from the fishes in other special features of their
structure than the fishes do from man. Hence they are obviously the
last survivors of a very ancient class of Vertebrates, that was far
from attaining the advanced organisation of the true fish. To mention
only the chief points, the Cyclostoma show no trace of pairs of limbs.
Their mucous skin is quite naked and smooth and devoid of scales.
There is no bony skeleton. A very rudimentary skull is developed at
the foremost end of their chorda. At this point a soft membranous
(partly turning into cartilage), small skull-capsule is formed, and
encloses the brain.
The brain of the Cyclostoma is merely a very small and comparatively
insignificant swelling of the spinal marrow, a simple vesicle at
first. It afterwards divides into five successive cerebral vesicles,
like the brain of the Gnathostoma. These five primitive cerebral
vesicles, that are found in the embryos of all the higher vertebrates
from the fishes to man, and grow into very complex structures, remain
at a very rudimentary stage in the Cyclostoma. The histological
structure of the nerves is also less advanced than in the rest of the
vertebrates. In these the auscultory organ always contains three
circular canals, but in the lampreys there are only two, and in the
hag-fishes only one. In most other respects the organisation of the
Cyclostoma is much simpler--for instance, in the structure of the
heart, circulation, and kidneys. We must especially note the absence
of a very important organ that we find in the fishes, the
floating-bladder, from which the lungs of the higher Vertebrates have
been developed.
When we consider all these peculiarities in the structure of the
Cyclostoma, we may formulate the following thesis: Two divergent lines
proceeded from the earliest Craniota, or the primitive Craniota
(Archicrania). One of these lines is preserved in a greatly modified
condition: these are the Cyclostoma, a very backward and partly
degenerate side-line. The other, the chief line of the Vertebrate
stem, advanced straight to the fishes, and by fresh adaptations
acquired a number of important improvements.
(FIGURE 2.248. Fossil Permian primitive fish (Pleuracanthus Dechenii),
from the red sandstone of Saarbrucken. (From Doderlein.) I Skull and
branchial skeleton: o eye-region, pq palatoquadratum, nd lower jaw, hm
hyomandibular, hy tongue-bone, k gill-radii, kb gill-arches, z
jaw-teeth, sz gullet-teeth, st neck-spine. II Vertebral column: ob
upper arches, ub lower arches, hc intercentra, r ribs. III Single
fins: d dorsal fin, c tail-fin (tail-end wanting), an anus-fin, ft
supporter of fin-rays. IV Breast-fin: sg shoulder-zone, ax fin-axis,
ss double lines of fin-rays, bs additional rays, sch plates. V Ventral
fin: p pelvis, ax fin-axis, ss single row of fin-rays, bs additional
rays, sch scales, cop penis.
FIGURE 2.249. Embryo of a shark (Scymnus lichia), seen from the
ventral side, v breast-fins (in front five pairs of gill-clefts), h
belly-fins, a anus, s tail-fin, k external gill-tuft, d yelk-sac
(removed for most part), g eye, n nose, m mouth-cleft.)
The Cyclostoma are almost always classified by zoologists among the
fishes; but the incorrectness of this may be judged from the fact that
in all the chief and distinctive features of organisation they are
further removed from the fishes than the fishes are from the Mammals,
and even man. With the fishes we enter upon the vast division of the
jaw-mouthed or double-nosed Vertebrates (Gnathostoma or Amphirhina).
We have to consider the fishes carefully as the class which, on the
evidence of palaeontology, comparative anatomy, and ontogeny, may be
regarded with absolute certainty as the stem-class of all the higher
Vertebrates or Gnathostomes. Naturally, none of the actual fishes can
be considered the direct ancestor of the higher Vertebrates. But it is
certain that all the Vertebrates or Gnathostomes, from the fishes to
man, descend from a common, extinct, fish-like ancestor. If we had
this ancient stem-form before us, we would undoubtedly class it as a
true fish. Fortunately the comparative anatomy and classification of
the fishes are now so far advanced that we can get a very clear idea
of these interesting and instructive features.
In order to understand properly the genealogical tree of our race
within the vertebrate stem, it is important to bear in mind the
characteristics that separate the whole of the Gnathostomes from the
Cyclostomes and Craniota. In these respects the fishes agree entirely
with all the other Gnathostomes up to man, and it is on this that we
base our claim of relationship to the fishes. The following
characteristics of the Gnathostomes are anatomic features of this
kind: (1) The internal gill-arch apparatus with the jaw arches; (2)
the pair of nostrils; (3) the floating bladder or lungs; and (4) the
two pairs of limbs.
The peculiar formation of the frame work of the branchial (gill)
arches and the connected maxillary (jaw) apparatus is of importance in
the whole group of the Gnathostomes. It is inherited in rudimentary
form by all of them, from the earliest fishes to man. It is true that
the primitive transformation (which we find even in the Ascidia) of
the fore gut into the branchial gut can be traced in all the
Vertebrates to the same simple type; in this respect the gill-clefts,
which pierce the walls of the branchial gut in all the Vertebrates and
in the Ascidia, are very characteristic. But the EXTERNAL, superficial
branchial skeleton that supports the gill-crate in the Cyclostoma is
replaced in the Gnathostomes by an INTERNAL branchial skeleton. It
consists of a number of successive cartilaginous arches, which lie in
the wall of the gullet between the gill-clefts, and run round the
gullet from both sides. The foremost pair of gill-arches become the
maxillary arches, from which we get our upper and lower jaws.
The olfactory organs are at first found in the same form in all the
Gnathostomes, as a pair of depressions in the fore part of the skin of
the head, above the mouth; hence, they are also called the Amphirhina
("double-nosed"). The Cyclostoma are "one-nosed" (Monorhina); their
nose is a single passage in the middle of the frontal surface. But as
the olfactory nerve is double in both cases, it is possible that the
peculiar form of the nose in the actual Cyclostomes is a secondary
acquisition (by adaptation to suctorial habits).
A third essential character of the Gnathostomes, that distinguishes
them very conspicuously from the lower vertebrates we have dealt with,
is the formation of a blind sac by invagination from the fore part of
the gut, which becomes in the fishes the air-filled floating-bladder.
This organ acts as a hydrostatic apparatus, increasing or reducing the
specific gravity of the fish by compressing or altering the quantity
of air in it. The fish can rise or sink in the water by means of it.
This is the organ from which the lungs of the higher vertebrates are
developed.
(FIGURE 2.250. Fully developed man-eating shark (Carcharias
melanopterus), left view. r1 first, r2 second dorsal fin, s tail-fin,
a anus-fin, v breast-fins, h belly-fins.)
Finally, the fourth character of the Gnathostomes in their simple
embryonic form is the two pairs of extremities or limbs--a pair of
fore legs (breast-fins in the fish, Figure 2.250 v) and a pair of hind
legs (ventral fins in the fish, Figure 2.250 h). The comparative
anatomy of these fins is very interesting, because they contain the
rudiments of all the skeletal parts that form the framework of the
fore and hind legs in all the higher vertebrates right up to man.
There is no trace of these pairs of limbs in the Acrania and
Cyclostomes.
Turning, now, to a closer inspection of the fish class, we may first
divide it into three groups or sub-classes, the genealogy of which is
well known to us. The first and oldest group is the sub-class of the
Selachii or primitive fishes; the best-known representatives of which
to-day are the orders of the sharks and rays (Figures 2.248 to 2.252).
Next to this is the more advanced sub-class of the plated fishes or
Ganoids (Figures 2.253 to 2.255). It has been long extinct for the
most part, and has very few living representatives, such as the
sturgeon and the bony pike; but we can form some idea of the earlier
extent of this interesting group from the large numbers of fossils.
From these plated fishes the sub-class of the bony fishes or Teleostei
was developed, to which the great majority of living fishes belong
(especially nearly all our river fishes). Comparative anatomy and
ontogeny show clearly that the Ganoids descended from the Selachii,
and the Teleostei from the Ganoids. On the other hand, a collateral
line, or rather the advancing chief line of the vertebrate stem, was
developed from the earlier Ganoids, and this leads us through the
group of the Dipneusta to the important division of the Amphibia.
(FIGURE 2.251. Fossil angel-shark (Squatina alifera), from the upper
Jurassic at Eichstatt. (From Zittel.) The cartilaginous skull is
clearly seen in the broad head, and the gill-arches behind. The wide
breast-fin and the narrower belly-fin have a number of radii; between
these and the vertebral column are a number of ribs.)
The earliest fossil remains of Vertebrates that we know were found in
the Upper Silurian (Chapter 2.18), and belong to two groups--the
Selachii and the Ganoids. The most primitive of all known
representatives of the earliest fishes are probably the remarkable
Pleuracanthida, the genera Pleuracanthus, Xenacanthus, Orthocanthus,
etc. (Figure 2.248). These ancient cartilaginous fishes agree in most
points of structure with the real sharks (Figures 2.249 and 2.250);
but in other respects they seem to be so much simpler in organisation
that many palaeontologists separate them altogether, and regard them
as Proselachii; they are probably closely related to the extinct
ancestors of the Gnathostomes. We find well-preserved remains of them
in the Permian period. Well-preserved impressions of other sharks are
found in the Jurassic schist, such as of the angel-fish (Squatina,
Figure 2.251). Among the extinct earlier sharks of the Tertiary period
there were some twice as large as the biggest living fishes;
Carcharodon was more than 100 feet long. The sole surviving species of
this genus (C. Rondeleti) is eleven yards long, and has teeth two
inches long; but among the fossil species we find teeth six inches
long (Figure 2.252).
From the primitive fishes or Selachii, the earliest Gnathostomes, was
developed the legion of the Ganoids. There are very few genera now of
this interesting and varied group--the ancient sturgeons (Accipenser),
the eggs of which are eaten as caviare, and the stratified pikes
(Polypterus, Figure 2.255) in African rivers, and bony pikes
(Lepidosteus) in the rivers of North America. On the other hand, we
have a great variety of specimens of this group in the fossil state,
from the Upper Silurian onward. Some of these fossil Ganoids approach
closely to the Selachii; others are nearer to the Dipneusts; others
again represent a transition to the Teleostei. For our genealogical
purposes the most interesting are the intermediate forms between the
Selachii and the Dipneusts. Huxley, to whom we owe particularly
important works on the fossil Ganoids, classed them in the order of
the Crossopterygii. Many genera and species of this order are found in
the Devonian and Carboniferous strata (Figure 2.253); a single,
greatly modified survivor of the group is still found in the large
rivers of Africa (Polypterus, Figure 2.255, and the closely related
Calamichthys). In many impressions of the Crossopterygii the floating
bladder seems to be ossified, and therefore well preserved--for
instance, in the Undina (Figure 2.254, immediately behind the head).
Part of these Crossopterygii approach very closely in their chief
anatomic features to the Dipneusts, and thus represent
phylogenetically the transition from the Devonian Ganoids to the
earliest air-breathing vertebrates. This important advance was made in
the Devonian period. The numerous fossils that we have from the first
two geological sections, the Laurentian and Cambrian periods, belong
exclusively to aquatic plants and animals. From this paleontological
fact, in conjunction with important geological and biological
indications, we may infer with some confidence that there were no
terrestrial animals at that time. During the whole of the vast
archeozoic period--many millions of years--the living population of
our planet consisted almost exclusively of aquatic organisms; this is
a very remarkable fact, when we remember that this period embraces the
larger half of the whole history of life. The lower animal-stems are
wholly (or with very few exceptions) aquatic. But the higher stems
also remained in the water during the primordial epoch. It was only
towards its close that some of them came to live on land. We find
isolated fossil remains of terrestrial animals first in the Upper
Silurian, and in larger numbers in the Devonian strata, which were
deposited at the beginning of the second chief section of geology (the
paleozoic age). The number increases considerably in the Carboniferous
and Permian deposits. We find many species both of the articulate and
the vertebrate stem that lived on land and breathed the atmosphere;
their aquatic ancestors of the Silurian period only breathed water.
This important change in respiration is the chief modification that
the animal organism underwent in passing from the water to the solid
land. The first consequence was the formation of lungs for breathing
air; up to that time the gills alone had served for respiration. But
there was at the same time a great change in the circulation and its
organs; these are always very closely correlated to the respiratory
organs. Moreover, the limbs and other organs were also more or less
modified, either in consequence of remote correlation to the preceding
or owing to new adaptations.
(FIGURE 2.252. Tooth of a gigantic shark (Carcharodon megalodon), from
the Pliocene at Malta. Half natural size. (From Zittel.))
In the vertebrate stem it was unquestionably a branch of the
fishes--in fact, of the Ganoids--that made the first fortunate
experiment during the Devonian period of adapting themselves to
terrestrial life and breathing the atmosphere. This led to a
modification of the heart and the nose. The true fishes have merely a
pair of blind olfactory pits on the surface of the head; but a
connection of these with the cavity of the mouth was now formed. A
canal made its appearance on each side, and led directly from the
nasal depression into the mouth-cavity, thus conveying atmospheric air
to the lungs even when the mouth was closed. Further, in all true
fishes the heart has only two sections--an atrium that receives the
venous blood from the veins, and a ventricle that propels it through a
conical artery to the gills; the atrium was now divided into two
halves, or right and left auricles, by an incomplete partition. The
right auricle alone now received the venous blood from the body, while
the left auricle received the venous blood that flowed from the lungs
and gills to the heart. Thus the double circulation of the higher
vertebrates was evolved from the simple circulation of the true
fishes, and, in accordance with the laws of correlation, this advance
led to others in the structure of other organs.
(FIGURE 2.253. A Devonian Crossopterygius (Holoptychius nobilissimus,
from the Scotch old red sandstone. (From Huxley.)
FIGURE 2.254. A Jurassic Crossopterygius (Undina penicillata), from
the upper Jurassic at Eichstatt. (From Zittel.) j jugular plates, b
three ribbed scales.
FIGURE 2.255. A living Crossopterygius, from the Upper Nile
(Polypterus bichir).
FIGURE 2.256. Fossil Dipneust (Dipterus Valenciennesi), from the old
red sandstone (Devon). (From Pander.)
FIGURE 2.257. The Australian Dipneust (Ceratodus Forsteri). B view
from the right, A lower side of the skull, C lower jaw. (From
Gunther.) Qu quadrate bone, Psph parasphenoid, PtP pterygopalatinum,
Vo vomer, d teeth, na nostrils, Br branchial cavity, C first rib. D
lower-jaw teeth of the fossil Ceratodus Kaupi (from the Triassic).)
The vertebrate class, that thus adapted itself to breathing the
atmosphere, and was developed from a branch of the Ganoids, takes the
name of the Dipneusts or Dipnoa ("double-breathers"), because they
retained the earlier gill-respiration along with the new pulmonary
(lung) respiration, like the lowest amphibia. This class was
represented during the paleozoic age (or the Devonian, Carboniferous,
and Permian periods) by a number of different genera. There are only
three genera of the class living to-day: Protopterus annectens in the
rivers of tropical Africa (the White Nile, the Niger, Quelliman,
etc.), Lepidosiren paradoxa in tropical South America (in the
tributaries of the Amazon), and Ceratodus Forsteri in the rivers of
East Australia. This wide distribution of the three isolated survivors
proves that they represent a group that was formerly very large. In
their whole structure they form a transition from the fishes to the
amphibia. The transitional formation between the two classes is so
pronounced in the whole organisation of these remarkable animals that
zoologists had a lively controversy over the question whether they
were really fishes or amphibia. Several distinguished zoologists
classed them with the amphibia, though most now associate them with
the fishes. As a matter of fact, the characters of the two classes are
so far united in the Dipneusts that the answer to the question depends
entirely on the definition we give of "fish" and "amphibian." In
habits they are true amphibia. During the tropical winter, in the
rainy season, they swim in the water like the fishes, and
breathe water by gills. During the dry season they bury themselves in
the dry mud, and breathe the atmosphere through lungs, like the
amphibia and the higher vertebrates. In this double respiration they
resemble the lower amphibia, and have the same characteristic
formation of the heart; in this they are much superior to the fishes.
But in most other features they approach nearer to the fishes, and are
inferior to the amphibia. Externally they are entirely fish-like.
(FIGURE 2.258. Young ceratodus, shortly after issuing from the egg,
magnified ten times. k gill-cover, l liver. (From Richard Semon.)
FIGURE 2.259. Young ceratodus six weeks after issuing from the egg. s
spiral fold of gut, b rudimentary belly-fin. (From Richard Semon.))
In the Dipneusts the head is not marked off from the trunk. The skin
is covered with large scales. The skeleton is soft, cartilaginous, and
at a low stage of development, as in the lower Selachii and the
earliest Ganoids. The chorda is completely retained, and surrounded by
an unsegmented sheath. The two pairs of limbs are very simple fins of
a primitive type, like those of the lowest Selachii. The formation of
the brain, the gut, and the sexual organs is also the same as in the
Selachii. Thus the Dipneusts have preserved by heredity many of the
less advanced features of our primitive fish-like ancestors, and at
the same time have made a great step forward in adaptation to
air-breathing by means of lungs and the correlative improvement of the
heart.
Ceratodus is particularly interesting on account of the primitive
build of its skeleton; the cartilaginous skeleton of its two pairs of
fins, for instance, has still the original form of a bi-serial or
feathered leaf, and was on that account described by Gegenbaur as a
"primitive fin-skeleton." On the other hand, the skeleton of the pairs
of fins is greatly reduced in the African dipneust (Protopterus) and
the American (Lepidosiren). Further, the lungs are double in these
modern dipneusts, as in all the other air-breathing vertebrates; they
have on that account been called "double-lunged" (Dipneumones) in
contrast to the Ceratodus; the latter has only a single lung
(Monopneumones). At the same time the gills also are developed as
water-breathing organs in all these lung-fishes. Protopterus has
external as well as internal gills.
The paleozoic Dipneusts that are in the direct line of our ancestry,
and form the connecting-bridge between the Ganoids and the Amphibia,
differ in many respects from their living descendants, but agree with
them in the above essential features. This is confirmed by a number of
interesting facts that have lately come to our knowledge in connection
with the embryonic development of the Ceratodus and Lepidosiren; they
give us important information as to the stem-history of the lower
Vertebrates, and therefore of our early ancestors of the paleozoic
age.