_Abridged from 'Systemat. Phylogenie,' § 15._
Names underlined refer to hypothetical groups.
_Mammalia_
_Aves_ |
| _Reptilia_ |
| | |
+----------------+ |
| |
+--------------+
|
_Proreptilia_
| _Amphibia_
_Pisces_ | |
| | +----------+
| | |
| | | _Dipnoi_
| _Stegocephali_ |
| | |
| +---------------+
| |
+---------------+
| _Cyclostomata_
_Proselachii_ |
| |
_Tunicata_ | +--------+
| | |
| *_Archicrania_*
| | _Acrania_
| | |
| *_Prospondylia_*------+
| |
+----------+ |
| |
*_Prochordonia_*
11. Stage of _Cyclostomata_. This now small group of Lampreys and
Hagfishes represents the lowest Craniota; and although much specialized
as a side-branch of the main-stem from which the other Craniota have
sprung, they give us an idea of what the direct ancestors of the latter
must have been like:--still without visceral arches, without jaws and
without paired limbs; with a persistent pronephros; the ear with one
semicircular canal only; mouth suctorial; cranium very primitive;
and the metamerism of the vertebral column indicated only by little
blocks of cartilage in the perichordal sheath. Such creatures must
have existed at least as early as the Lower Silurian epoch; but until
1890 fossil Cyclostomes were unknown. Their life in the mud, or as
endoparasites of fishes, coupled with their soft structure, makes them
very unfit for preservation. This gives all the greater importance to
Traquair's discovery, in 1890, of many little creatures, called by him
_Palæospondylus gunni_, in the Old Red Sandstone of Caithness, which
seem to be very closely allied to Cyclostomata.
12. The _Elasmobranchi_ (sharks and skates), with their immediate
forerunners, the Acanthodi of the Devonian and Carboniferous age,
are the first typical fishes. That they existed as far back as the
Silurian age is proved by many enamelled spines of the dermal armour,
chiefly from the dorsal fins. This higher stage is characterized by the
possession of typical jaws, by visceral or gill-bearing arches, and by
two pairs of limbs. None of the Elasmobranchs, fossil or recent, stands
in the direct ancestral line; but they are the lowest Gnathostomata,
jaw-and-limb-possessing creatures, known.
13. Closely connected with the Elasmobranchs in a wider sense are the
_Crossopterygii_, which begin in the Devonian age as a large group, but
have left only two survivals, the African Polypterus and Calamoichthys.
They are possessed of dermal bones and other ossifications, and are
characterized by their lobate paired fins, which have a thick axis
beset with biserial fin rays. Their gill-clefts are covered by an
operculum, and they have a well-developed air-bladder. Whilst they
are in many respects more highly developed than the Elasmobranchs,
and are intimately connected with the typical Ganoids and other
bony fishes (all of which form a great, manifold side-branch of the
general vertebrate stem), they stand in many other respects (notably,
the structure of the paired fins, the vertebral column, and the
air-bladder) nearer the main-stem of our own ancestral line.
14. This is shown by their intimate relation to the _Dipnoi_, which
are still represented by the Australian, African, and South American
mud-fishes: Ceratodus, Protopterus, and Lepidosiren. The genus
Ceratodus existed in the Upper Trias, whence various other unmistakably
dipnoous forms lead down through the Carboniferous (_e.g._, Ctenodus)
to the Devonian strata--_e.g._, Dipterus. They are characterized as
follows: The paired fins still retain the archipterygial form (namely,
one axis with biserial rays); the heart is already trilocular, and
receives blood which is mixed arterial and venous, owing to the gills
being retained, while the air-bladder has been modified into a lung. In
fact, the generalized Dipnoi form the actual link between fishes and
_Amphibia_.
15. _Amphibia._ The earliest amphibian fossils occur in
the Carboniferous strata. They alone--the Stegocephali or
Phractamphibia--stand in the ancestral line, while the Lissamphibia, to
which all the recent forms belong, are side-branches. The Stegocephali
are the earliest Tetrapoda, the archipterygial paired fins having been
transformed into the pentadactyle fore and hind limbs, which are so
characteristic of all the higher Vertebrata. The cranium is roofed
over by dermal bones, of which, besides others, supra-occipitals,
supra-orbitals, and supra-temporals are always present. The lowest
members (Branchiosauri) still retained gills besides the lungs, while
others (Microsauri) have lost the gills. Be it remembered that all
the recent Amphibia still undergo the same metamorphosis during their
ontogenetic development.
In the very important Temnospondyli, a subgroup of the
Stegocephali--_e.g._, Trimerorhachis of the Lower Red Sandstone or
Lower Permian--the component cartilaginous or bony units which compose
the vertebræ still remained in a separate, unfused state, showing at
the same time an arrangement whence has arisen that which is typical
of the Amniota. The same applies to the limbs and their girdles. In
fact, the Stegocephali, taken as a whole, lead imperceptibly to the
_Proreptilia_.
16. _Proreptilia_ are represented by the Permian genera Eryops and
Cricotus. Until quite recently these and many other fossils from
the Carboniferous strata were looked upon as Amphibia, while many
undoubted fossil Amphibia were mistaken for reptiles, as indicated by
the frequent termination '-saurus' in their names.
The nearest living representative of these extinct Proreptilia is
the New Zealand reptile Hatteria, or Sphenodon, close relations
of which are known from the Upper Trias; while others--_e.g._,
Palæohatteria--have been discovered in the Permian. Anyhow, Sphenodon
is the reptile which stands nearest to the main stem of our ancestry.
The most important characteristics of the Reptilia, which mark a higher
stage or level, are (1) The entire suppression of the gills--although
during the embryonic development the gill-clefts still appear in all
reptiles, birds, and mammals; (2) The development of an amnion and an
allantois, both for the embryonic life only, but so characteristic
that all these animals are comprised under the name of Amniota;
(3) The articulation of the skull with the first neck vertebræ by
well-developed condyles, either single (really triple) or double (such
a condylar arrangement begins with the Amphibia, but only the two
lateral condyles are developed, while the middle portion, belonging to
the basi-occipital element, remains rudimentary[21]); (4) The formation
of centra, or bodies of the vertebræ, mainly by a ventral pair of the
original quadruple constituents, or arcualia.
[21] Similar conditions seem to have prevailed among the Proreptilia;
but in those of their descendants which have specialized into Reptiles
and Birds the basi-occipital element becomes more and more predominant
in that formation which ultimately leads to the apparently single
condyle. Hence it is misleading to divide the Tetrapoda into the two
main groups of Amphi-and Mono-condylia, and therefrom to conclude that
the two-condyled Mammalia are more closely related to the likewise
amphicondylous Amphibia than to the so-called monocondylous Reptiles.
17. Between the Proreptilia and the Mammalia, which latter occur in
the Upper Triassic epoch, we have necessarily to intercalate a group
of very low reptiles, which are still so generalized that their
descendants could branch off either into the Reptilia proper or into
the Mammalia. The changes concerned chiefly the brain and the heart;
of the skeleton, the skull and the pelvis; and, of the tegumentary
structures, the formation of a hairy covering. Many such creatures
existed in the Triassic epoch--namely, the _Theromorpha_--some of which
indeed possess so many characteristics which otherwise occur in the
Mammalia only, that these creatures have been termed _Sauro-Mammalia_.
However, it has to be emphasized that none of the Theromorpha hitherto
discovered fulfils all the requirements which would entitle them to
this important linking position. They only give us an approximate idea
of what this link was like.
18. Stage of the _Promammalia_, or _Prototheria_. The only surviving
members are the famous duck-bill, Ornithorhynchus, and the spiny
ant-eaters, Echidna and Proechidna, of the Australian region. These
few genera, however, differ so much from one another in various
important respects that they cannot but be remnants of an originally
much larger group. Indeed, many fossils from the Upper Triassic and
from the Jurassic strata have without much doubt to be referred to the
Prototheria. The Prototheria are typical mammals, because they possess
the following characteristics: The heart is completely quadrilocular;
the blood is warm, and its red corpuscles have, owing to the loss
of their nucleus, been modified from biconvex into biconcave discs;
they have a hairy coat and sweat glands, and two occipital condyles;
the ilio-sacral connection is preacetabular; the ankle-joint is
cruro-tarsal; the quadrate bone of the Reptilia has ceased to carry the
under jaw, which now articulates directly with the squamosal portion
of the skull. Their low position is shown by the retention of the
following reptilian features: Complete coracoid bones and a T-shaped
interclavicle; a cloaca, or common chamber for the passage of the
fæces, the genital and the urinary products; they are still oviparous;
the embryo develops without a chorion, and is therefore not nourished
through a placenta. Even the milk glands, which are absolutely
peculiar to the Mammalia, are still in a very primitive stage, and do
not yet produce milk proper; and there is only a temporary shallow
marsupium.
19. Stage of _Metatheria_, or _Marsupialia_, are direct descendants of
Prototheria; but they show higher development by the reduction of the
coracoid bones and the interclavicle. The original cloaca is divided
into a rectal chamber and a uro-genital sinus, completely separated,
at least in the males; they are viviparous; the young are received
into a permanent marsupium, in the walls of which are formed typical
milk glands and nipples, but the embryo is still devoid of a placenta,
although some recent marsupials show indications of such an organ. The
corpus callosum in the brain is still very weak.
Most of the marsupials are extinct. They occur from the Upper Trias
onwards, and had in the Jurassic epoch attained a wide distribution
both in Europe and in America. Since the Tertiary epoch they have
been restricted to America and to the Australian region, and are now
represented by about 150 species.
20. Stage of _Prochoriata_, or early _Placentalia_: a further
development of the Metatheria by the development of a placenta, loss
of the marsupium and the marsupial bones, complete division by the
perineum of the anal and uro-genital chambers, stronger development of
the corpus callosum, or chief commissure of the two hemispheres of the
brain.
Placentalia must have come into existence during the Cretaceous
epoch. Up to that time all the Mammalia seem to have belonged to
either Prototheria or to Metatheria; but in the early Eocene we can
distinguish the main groups of Placentalia--namely, (1) Trogontia, now
represented by the rodents; (2) Edentata, or sloths, armadilloes, etc.;
(3) Carnassia, or Insectivora and Carnivora; (4) Chiroptera, or bats;
(5) Cetomorpha, or whales and dugongs; (6) Ungulata; (7) Primates.
Of these groups, the first and second, third and fourth, fifth and
sixth, can perhaps, to judge from palæontological evidence, be combined
into three greater groups, as indicated by the fossil Esthonychida,
Ictopsida, and Condylarthra, in addition to the ancestral Primates,
or Lemuravida, as the fourth large branch of the ancestral-tree where
this has reached the placental level. Among none of the first three
branches can we look for the ancestors of the Primates. The Lemuravida,
therefore, represent a branch equivalent to the three other branches.
21. Stage of _Lemures_, or _Prosimiæ_, comprising the older members of
the Primates, consequently approaching most nearly to the Lemuravida.
The limbs are modified into pentadactyle hands and feet of the arboreal
type, and are protected by nails. The dentition is of the frugivorous
or omnivorous type, with an originally complete series of teeth, with
milk teeth and with permanent. The orbit is surrounded by a complete
bony ring, posteriorly by a fronto-jugal arch, but still widely
communicating with the temporal fossa. The placenta is diffuse and
non-deciduous.