With the phylogenetic study of the four higher classes of Vertebrates,
which must now engage our attention, we reach much firmer ground and
more light in the construction of our genealogy than we have, perhaps,
enjoyed up to the present. In the first place, we owe a number of very
valuable data to the very interesting class of Vertebrates that come
next to the Dipneusts and have been developed from them--the Amphibia.
To this group belong the salamander, the frog, and the toad. In
earlier days all the reptiles were, on the example of Linne, classed
with the Amphibia (lizards, serpents, crocodiles, and tortoises). But
the reptiles are much more advanced than the Amphibia, and are nearer
to the birds in the chief points of their structure. The true Amphibia
are nearer to the Dipneusta and the fishes; they are also much older
than the reptiles. There were plenty of highly-developed (and
sometimes large) Amphibia during the Carboniferous period; but the
earliest reptiles are only found in the Permian period. It is probable
that the Amphibia were evolved even earlier--during the Devonian
period--from the Dipneusta. The extinct Amphibia of which we have
fossil remains from that remote period (very numerous especially in
the Triassic strata) were distinguished for a graceful scaly coat or a
powerful bony armour on the skin (like the crocodile), whereas the
living amphibia have usually a smooth and slippery skin.
The earliest of these armoured Amphibia (Phractamphibia) form the
order of Stegocephala ("roof-headed") (Figure 2.260). It is among
these, and not among the actual Amphibia, that we must look for the
forms that are directly related to the genealogy of our race, and are
the ancestors of the three higher classes of Vertebrates. But even the
existing Amphibia have such important relations to us in their
anatomic structure, and especially their embryonic development, that
we may say: Between the Dipneusts and the Amniotes there was a series
of extinct intermediate forms which we should certainly class with the
Amphibia if we had them before us. In their whole organisation even
the actual Amphibia seem to be an instructive transitional group. In
the important respects of respiration and circulation they approach
very closely to the Dipneusta, though in other respects they are far
superior to them.
This is particularly true of the development of their limbs or
extremities. In them we find these for the first time as five-toed
feet. The thorough investigations of Gegenbaur have shown that the
fish's fins, of which very erroneous opinions were formerly held, are
many-toed feet. The various cartilaginous or bony radii that are found
in large numbers in each fin correspond to the fingers or toes of the
higher Vertebrates. The several joints of each fin-radius correspond
to the various parts of the toe. Even in the Dipneusta the fin is of
the same construction as in the fishes; it was afterwards gradually
evolved into the five-toed form, which we first encounter in the
Amphibia. This reduction of the number of the toes to six, and then to
five, probably took place in the second half of the Devonian
period--at the latest, in the subsequent Carboniferous period--in
those Dipneusta which we regard as the ancestors of the Amphibia. We
have several fossil remains of five-toed Amphibia from this period.
There are numbers of fossil impressions of them in the Triassic of
Thuringia (Chirotherium).
(FIGURE 2.260. Fossil amphibian from the Permian, found in the Plauen
terrain near Dresden (Branchiosaurus amblystomus). (From Credner.) A
skeleton of a young larva. B larva, restored, with gills. C the adult
form, natural size.)
The fact that the toes number five is of great importance, because
they have clearly been transmitted from the Amphibia to all the higher
Vertebrates. Man entirely resembles his amphibian ancestors in this
respect, and indeed in the whole structure of the bony skeleton of his
five-toed extremities. A careful comparison of the skeleton of the
frog with our own is enough to show this. It is well known that this
hereditary number of the toes has assumed a very great practical
importance from remote times; on it our whole system of enumeration
(the decimal system applied to measurement of time, mass, weight,
etc.) is based. There is absolutely no reason why there should be five
toes in the fore and hind feet in the lowest Amphibia, the reptiles,
and the higher Vertebrates, unless we ascribe it to inheritance from a
common stem-form. Heredity alone can explain it. It is true that we
find less than five toes in many of the Amphibia and of the higher
Vertebrates. But in all these cases we can prove that some of the toes
atrophied, and were in time lost altogether.
The causes of this evolution of the five-toed foot from the many-toed
fin in the amphibian ancestor must be sought in adaptation to the
entire change of function that the limbs experienced in passing from
an exclusively aquatic to a partly terrestrial life. The many-toed fin
had been used almost solely for motion in the water; it had now also
to support the body in creeping on the solid ground. This led to a
modification both of the skeleton and the muscles of the limbs. The
number of the fin-radii was gradually reduced, and sank finally to
five. But these five remaining radii became much stronger. The soft
cartilaginous radii became bony rods. The rest of the skeleton was
similarly strengthened. Thus from the one-armed lever of the many-toed
fish-fin arose the improved many-armed lever system of the five-toed
amphibian limbs. The movements of the body gained in variety as well
as in strength. The various parts of the skeletal system and
correlated muscular system began to differentiate more and more. In
view of the close correlation of the muscular and nervous systems,
this also made great advance in structure and function. Hence we find,
as a matter of fact, that the brain is much more developed in the
higher Amphibia than in the fishes, the Dipneusta, and the lower
Amphibia.
The first advance in organisation that was occasioned by the adoption
of life on land was naturally the construction of an organ for
breathing air--a lung. This was formed directly from the
floating-bladder inherited from the fishes. At first its function was
insignificant beside that of the gills, the older organ for
water-respiration. Hence we find in the lowest Amphibia, the gilled
Amphibia, that, like the Dipneusta, they pass the greater part of
their life in the water, and breathe water through gills. They only
come to the surface at brief intervals, or creep on to the land, and
then breathe air by their lungs. But some of the tailed Amphibia--the
salamanders--remain entirely in the water when they are young, and
afterwards spend most of their time on land. In the adult state they
only breathe air through lungs. The same applies to the most advanced
of the Amphibia, the Batrachia (frogs and toads); some of them have
entirely lost the gill-bearing larva form.* (* The tree-frog of
Martinique (Hylades martinicensis) loses the gills on the seventh, and
the tail and yelk-sac on the eighth, day of foetal life. On the ninth
or tenth day after fecundation the frog emerges from the egg.) This is
also the case with certain small, serpentine Amphibia, the Caecilia
(which live in the ground like earth-worms).
(FIGURE 2.261. Larva of the Spotted Salamander (Salamandra maculata),
seen from the ventral side. In the centre a yelk-sac still hangs from
the gut. The external gills are gracefully ramified. The two pairs of
legs are still very small.)
The great interest of the natural history of the Amphibia consists
especially in their intermediate position between the lower and higher
Vertebrates. The lower Amphibia approach very closely to the Dipneusta
in their whole organisation, live mainly in the water, and breathe by
gills; but the higher Amphibia are just as close to the Amniotes, live
mainly on land, and breathe by lungs. But in their younger state the
latter resemble the former, and only reach the higher stage by a
complete metamorphosis. The embryonic development of most of the
higher Amphibia still faithfully reproduces the stem-history of the
whole class, and the various stages of the advance that was made by
the lower Vertebrates in passing from aquatic to terrestrial life
during the Devonian or the Carboniferous period are repeated in the
spring by every frog that develops from an egg in our ponds.
(FIGURE 2.262. Larva of the common grass-frog (Rana temporaria), or
"tadpole." m mouth, n a pair of suckers for fastening on to stones, d
skin-fold from which the gill-cover develops; behind it the
gill-clefts, from which the branching gills (k) protrude, s
tail-muscles, f cutaneous fin-fringe of the tail.)
The common frog leaves the egg in the shape of a larva, like the
tailed salamander (Figure 2.261), and this is altogether different
from the mature frog (Figure 2.262). The short trunk ends in a long
tail, with the form and structure of a fish's tail (s). There are no
limbs at first. The respiration is exclusively branchial, first
through external (k) and then internal gills. In harmony with this the
heart has the same structure as in the fish, and consists of two
sections--an atrium that receives the venous blood from the body, and
a ventricle that forces it through the arteries into the gills.
We find the larvae of the frog (or tadpoles, Gyrini) in great numbers
in our ponds every spring in this fish-form, using their muscular
tails in swimming, just like the fishes and young Ascidia. When they
have reached a certain size, the remarkable metamorphosis from the
fish-form to the frog begins. A blind sac grows out of the gullet, and
expands into a couple of spacious sacs: these are the lungs. The
simple chamber of the heart is divided into two sections by the
development of a partition, and there are at the same time
considerable changes in the structure of the chief arteries.
Previously all the blood went from the auricle through the aortic
arches into the gills, but now only part of it goes to the gills, the
other part passing to the lungs through the new-formed pulmonary
artery. From this point arterial blood returns to the left auricle of
the heart, while the venous blood gathers in the right auricle. As
both auricles open into a single ventricle, this contains mixed blood.
The dipneust form has now succeeded to the fish-form. In the further
course of the metamorphosis the gills and the branchial vessels
entirely disappear, and the respiration becomes exclusively pulmonary.
Later, the long swimming tail is lost, and the frog now hops to the
land with the legs that have grown meantime.
This remarkable metamorphosis of the Amphibia is very instructive in
connection with our human genealogy, and is particularly interesting
from the fact that the various groups of actual Amphibia have remained
at different stages of their stem-history, in harmony with the
biogenetic law. We have first of all a very low order of Amphibia--the
Sozobranchia ("gilled-amphibia"), which retain their gills throughout
life, like the fishes. In a second order of the salamanders the gills
are lost in the metamorphosis, and when fully grown they have only
pulmonary respiration. Some of the tailed Amphibia still retain the
gill-clefts in the side of the neck, though they have lost the gills
themselves (Menopoma). If we force the larvae of our salamanders
(Figure 2.261) and tritons to remain in the water, and prevent them
from reaching the land, we can in favourable circumstances make them
retain their gills. In this fish-like condition they reach sexual
maturity, and remain throughout life at the lower stage of the gilled
Amphibia.
(FIGURE 2.263. Fossil mailed amphibian, from the Bohemian
Carboniferous (Seeleya). (From Fritsch.) The scaly coat is retained on
the left.)
We have the reverse of this experiment in a Mexican gilled salamander,
the fish-like axolotl (Siredon pisciformis). It was formerly regarded
as a permanent gilled amphibian persisting throughout life at the
fish-stage. But some of the hundreds of these animals that are kept in
the Botanical Garden at Paris got on to the land for some reason or
other, lost their gills, and changed into a form closely resembling
the salamander (Amblystoma). Other species of the genus became
sexually mature for the first time in this condition. This has been
regarded as an astounding phenomenon, although every common frog and
salamander repeats the metamorphosis in the spring. The whole change
from the aquatic and gill-breathing animal to the terrestrial
lung-breathing form may be followed step by step in this case. But
what we see here in the development of the individual has happened to
the whole class in the course of its stem-history.
The metamorphosis goes farther in a third order of Amphibia, the
Batrachia or Anura, than in the salamander. To this belong the various
kinds of toads, ringed snakes, water-frogs, tree-frogs, etc. These
lose, not only the gills, but also (sooner or later) the tail, during
metamorphosis.
The ontogenetic loss of the gills and the tail in the frog and toad
can only be explained on the assumption that they are descended from
long-tailed Amphibia of the salamander type. This is also clear from
the comparative anatomy of the two groups. This remarkable
metamorphosis is, however, also interesting because it throws a
certain light on the phylogeny of the tail-less apes and man. Their
ancestors also had long tails and gills like the gilled Amphibia, as
the tail and the gill-arches of the human embryo clearly show.
For comparative anatomical and ontogenetic reasons, we must not seek
these amphibian ancestors of ours--as one would be inclined to do,
perhaps--among the tail-less Batrachia, but among the tailed lower
Amphibia.
The vertebrate form that comes next to the Amphibia in the series of
our ancestors is a lizard-like animal, the earlier existence of which
can be confidently deduced from the facts of comparative anatomy and
ontogeny. The living Hatteria of New Zealand (Figure 2.264) and the
extinct Rhyncocephala of the Permian period (Figure 2.265) are closely
related to this important stem-form; we may call them the
Protamniotes, or Primitive Amniotes. All the Vertebrates above the
Amphibia--or the three classes of reptiles, birds, and mammals--differ
so much in their whole organisation from all the lower Vertebrates we
have yet considered, and have so great a resemblance to each other,
that we put them all together in a single group with the title of
Amniotes. In these three classes alone we find the remarkable
embryonic membrane, already mentioned, which we called the amnion; a
cenogenetic adaptation that we may regard as a result of the sinking
of the growing embryo into the yelk-sac.
All the Amniotes known to us--all reptiles, birds, and mammals
(including man)--agree in so many important points of internal
structure and development that their descent from a common ancestor
can be affirmed with tolerable certainty. If the evidence of
comparative anatomy and ontogeny is ever entirely beyond suspicion, it
is certainly the case here. All the peculiarities that accompany and
follow the formation of the amnion, and that we have learned in our
consideration of human embryology; all the peculiarities in the
development of the organs which we will presently follow in detail;
finally, all the principal special features of the internal structure
of the full-grown Amniotes--prove so clearly the common origin of all
the Amniotes from single extinct stem-form that it is difficult to
entertain the idea of their evolution from several independent stems.
This unknown common stem-form is our primitive Amniote (Protamnion).
In outward appearance it was probably something between the salamander
and the lizard.
It is very probable that some part of the Permian period was the age
of the origin of the Protamniotes. This follows from the fact that the
Amphibia are not fully developed until the Carboniferous period, and
that the first fossil reptiles (Palaehatteria, Homoeosaurus,
Proterosaurus) are found towards the close of the Permian period.
Among the important changes of the vertebrate organisation that marked
the rise of the first Amniotes from salamandrine Amphibia during this
period the following three are especially noteworthy: the entire
disappearance of the water-breathing gills and the conversion of the
gill-arches into other organs, the formation of the allantois or
primitive urinary sac, and the development of the amnion.
One of the most salient characteristics of the Amniotes is the
complete loss of the gills. All Amniotes, even if living in water
(such as sea-serpents and whales), breathe air through lungs, never
water through gills. All the Amphibia (with very rare exceptions)
retain their gills for some time when young, and have for a time (if
not permanently) branchial respiration; but after these there is no
question of branchial respiration. The Protamniote itself must have
entirely abandoned water-breathing. Nevertheless, the gill-arches are
preserved by heredity, and develop into totally different (in part
rudimentary) organs--various parts of the bone of the tongue, the
frame of the jaws, the organ of hearing, etc. But we do not find in
the embryos of the Amniotes any trace of gill-leaves, or of real
respiratory organs on the gill-arches.
With this complete abandonment of the gills is probably connected the
formation of another organ, to which we have already referred in
embryology--namely, the allantois or primitive urinary sac (cf.