Dipneusts is the first structure of the allantois. We find in these a
urinary bladder that proceeds from the lower wall of the hind end of
the gut, and serves as receptacle for the renal secretions. This organ
has been transmitted to the Amphibia, as we can see in the frog.
The formation of the amnion and the allantois and the complete
disappearance of the gills are the chief characteristics that
distinguish the Amniotes from the lower Vertebrates we have hitherto
considered. To these we may add several subordinate features that are
transmitted to all the Amniotes, and are found in these only. One
striking embryonic character of the Amniotes is the great curve of the
head and neck in the embryo. We also find an advance in the structure
of several of the internal organs of the Amniotes which raises them
above the highest of the anamnia. In particular, a partition is formed
in the simple ventricle of the heart, dividing into right and left
chambers. In connection with the complete metamorphosis of the
gill-arches we find a further development of the auscultory organs.
Also, there is a great advance in the structure of the brain,
skeleton, muscular system, and other parts. Finally, one of the most
important changes is the reconstruction of the kidneys. In all the
earlier Vertebrates we have found the primitive kidneys as excretory
organs, and these appear at an early stage in the embryos of all the
higher Vertebrates up to man. But in the Amniotes these primitive
kidneys cease to act at an early stage of embryonic life, and their
function is taken up by the permanent or secondary kidneys, which
develop from the terminal section of the prorenal ducts.
(FIGURE 2.264. The lizard (Hatteria punctata = Sphenodon punctatus) of
New Zealand. The sole surviving proreptile. (From Brehm.))
Taking all these peculiarities of the Amniotes together, it is
impossible to doubt that all the animals of this group--all reptiles,
birds, and mammals--have a common origin, and form a single
blood-related stem. Our own race belongs to this stem. Man is, in
every feature of his organisation and embryonic development, a true
Amniote, and has descended from the Protamniote with all the other
Amniotes. Though they appeared at the end (possibly even in the
middle) of the Paleozoic age, the Amniotes only reached their full
development during the Mesozoic age. The birds and mammals made their
first appearance during this period. Even the reptiles show their
greatest growth at this time, so that it is called "the reptile age."
The extinct Protamniote, the ancestor of the whole group, belongs in
its whole organisation to the reptile class.
The genealogical tree of the amniote group is clearly indicated in its
chief lines by their paleontology, comparative anatomy, and ontogeny.
The group succeeding the Protamniote divided into two branches. The
branch that will claim our whole interest is the class of the Mammals.
The other branch, which developed in a totally different direction,
and only comes in contact with the Mammals at its root, is the
combined group of the reptiles and birds; these two classes may, with
Huxley, be conveniently grouped together as the Sauropsida. Their
common stem-form is an extinct lizard-like reptile of the order of the
Rhyncocephalia. From this have been developed in various directions
the serpents, crocodiles, tortoises, etc.--in a word, all the members
of the reptile class. But the remarkable class of the birds has also
been evolved directly from a branch of the reptile group, as is now
established beyond question. The embryos of the reptiles and birds are
identical until a very late stage, and have an astonishing resemblance
even later. Their whole structure agrees so much that no anatomist now
questions the descent of the birds from the reptiles. On the other
hand, the mammal line has descended from the group of the
Sauromammalia, a different branch of the Proreptilia. It is connected
at its deepest roots with the reptile line, but it then diverges
completely from it and follows a distinctive development. Man is the
highest outcome of this class, the "crown of creation." The hypothesis
that the three higher Vertebrate classes represent a single
Amniote-stem, and that the common root of this stem is to be found in
the amphibian class, is now generally admitted.
(FIGURE 2.265. Homoeosaurus pulchellus, a Jurassic proreptile from
Kehlheim. (From Zittel.))
The instructive group of the Permian Tocosauria, the common root from
which the divergent stems of the Sauropsids and mammals have issued,
merits our particular attention as the stem-group of all the Amniotes.
Fortunately a living representative of this extinct ancestral group
has been preserved to our day; this is the remarkable lizard of New
Zealand, Hatteria punctata (Figure 2.264). Externally it differs
little from the ordinary lizard; but in many important points of
internal structure, especially in the primitive construction of the
vertebral column, the skull, and the limbs, it occupies a much lower
position, and approaches its amphibian ancestors, the Stegocephala.
Hence Hatteria is the phylogenetically oldest of all living reptiles,
an isolated survivor from the Permian period, closely resembling the
common ancestor of the Amniotes. It must differ so little from this
extinct form, our hypothetical Protamniote, that we put it next to the
Proreptilia. The remarkable Permian Palaehatteria, that Credner
discovered in the Plauen terrain at Dresden in 1888, belongs to the
same group (Figure 2.266). The Jurassic genus Homoeosaurus (Figure
2.265), of which well-preserved skeletons are found in the Solenhofen
schists, is perhaps still more closely related to them.
Unfortunately, the numerous fossil remains of Permian and Triassic
Tocosauria that we have found in the last two decades are, for the
most part, very imperfectly preserved. Very often we can make only
precarious inferences from these skeletal fragments as to the anatomic
characters of the soft parts that went with the bony skeleton of the
extinct Tocosauria. Hence it has not yet been possible to arrange
these important fossils with any confidence in the ancestral series
that descend from the Protamniotes to the Sauropsids on the one side
and the Mammals on the other. Opinions are particularly divided as to
the place in classification and the phylogenetic significance of the
remarkable Theromorpha. Cope gives this name to a very interesting and
extensive group of extinct terrestrial reptiles, of which we have only
fossil remains from the Permian and Triassic strata. Forty years ago
some of these Therosauria (fresh-water animals) were described by Owen
as Anomodontia. But during the last twenty years the distinguished
American paleontologists, Cope and Osborn, have greatly increased our
knowledge of them, and have claimed that the stem-forms of the Mammals
must be sought in this order. As a matter of fact, the Theromorpha are
nearer to the Mammals in the chief points of structure than any other
reptiles. This is especially true of the Thereodontia, to which the
Pureosauria and Pelycosauria belong (Figure 2.267). The whole
structure of their pelvis and hind-feet has attained the same form as
in the Monotremes, the lowest Mammals. The formation of the scapula
and the quadrate bone shows an approach to the Mammals such as we find
in no other group of reptiles. The teeth also are already divided into
incisors, canines, and molars. Nevertheless, it is very doubtful
whether the Theromorpha really are in the ancestral line of the
Sauromammals, or lead direct from the Tocosauria to the earliest
Mammals. Other experts on this group believe that it is an independent
legion of the reptiles, connected, perhaps, at its lowest root, with
the Sauromammals, but developed quite independently of the
Mammals--though parallel to them in many ways.
One of the most important of the zoological facts that we rely on in
our investigation of the genealogy of the human race is the position
of man in the Mammal class. However different the views of zoologists
may have been as to this position in detail, and as to his relations
to the apes, no scientist has ever doubted that man is a true mammal
in his whole organisation and development. Linne drew attention to
this fact in the first edition of his famous Systema Naturae (1735).
As will be seen in any museum of anatomy or any manual of comparative
anatomy; the human frame has all the characteristics that are common
to the Mammals and distinguish them conspicuously from all other
animals.
(FIGURE 2.266. Skull of a Permian lizard (Palaehatteria longicaudata).
(From Credner.) n nasal bone, pf frontal bone, l lachrymal bone, po
postorbital bone, sq covering bone, i cheek-bone, vo vomer, im
inter-maxillary.)
If we examine this undoubted fact from the point of view of phylogeny,
in the light of the theory of descent, it follows at once that man is
of a common stem with all the other Mammals, and comes from the same
root as they. But the various features in which the Mammals agree and
by which they are distinguished are of such a character as to make a
polyphyletic hypothesis quite inadmissible. It is impossible to
entertain the idea that all the living and extinct Mammals come from a
number of separate roots. If we accept the general theory of
evolution, we are bound to admit the monophyletic hypothesis of the
descent of all the Mammals (including man) from a single mammalian
stem-form. We may call this long-extinct root-form and its earliest
descendants (a few genera of one family) "primitive mammals" or
"stem-mammals" (Promammalia). As we have already seen, this root-form
developed from the primitive Proreptile stem in a totally different
direction from the birds, and soon separated from the main stem of the
reptiles. The differences between the Mammals and the reptiles and
birds are so important and characteristic that we can assume with
complete confidence this division of the vertebrate stem at the
commencement of the development of the Amniotes. The reptiles and
birds, which we group together as the Sauropsids, generally agree in
the characteristic structure of the skull and brain, and this is
notably different from that of the Mammals. In most of the reptiles
and birds the skull is connected with the first cervical vertebra (the
atlas) by a single, and in the Mammals (and Amphibia) by a double,
condyle at the back of the head. In the former the lower jaw is
composed of several pieces, and connected with the skull so that it
can move by a special maxillary bone (the quadratum); in the Mammals
the lower jaw consists of one pair of bony pieces, which articulate
directly with the temporal bone. Further, in the Sauropsids the skin
is clothed with scales or feathers; in the Mammals with hair. The red
blood-cells of the former have a nucleus; those of the latter have
not. In fine, two quite characteristic features of the Mammals, which
distinguish them not only from the birds and reptiles, but from all
other animals, are the possession of a complete diaphragm and of
mammary glands that produce the milk for the nutrition of the young.
It is only in the Mammals that the diaphragm forms a transverse
partition of the body-cavity, completely separating the pectoral from
the abdominal cavity. It is only in the mammals that the mother
suckles its young, and this rightly gives the name to the whole class
(mamma = breast).
(FIGURE 2.267. Skull of a Triassic theromorphum (Galesaurus
planiceps), from the Karoo formation in South Africa. (From Owen.) a
from the right, b from below, c from above, d tricuspid tooth. N
nostrils, NA nasal bone, Mx upper jaw, Prf prefrontal, Fr frontal
bone, A eye-pits, S temple-pits. Pa Parietal eye, Bo joint at back of
head, Pt pterygoid-bone, Md lower jaw.)
From these pregnant facts of comparative anatomy and ontogeny it
follows absolutely that the whole of the Mammals belong to a single
natural stem, which branched off at an early date from the
reptile-root. It follows further with the same absolute certainty that
the human race is also a branch of this stem. Man shares all the
characteristics I have described with all the Mammals, and differs in
them from all other animals. Finally, from these facts we deduce with
the same confidence those advances in the vertebrate organisation by
which one branch of the Sauromammals was converted into the stem-form
of the Mammals. Of these advances the chief were: (1) The
characteristic modification of the skull and the brain; (2) the
development of a hairy coat; (3) the complete formation of the
diaphragm; and (4) the construction of the mammary glands and
adaptation to suckling. Other important changes of structure proceeded
step by step with these.
The epoch at which these important advances were made, and the
foundation of the Mammal class was laid, may be put with great
probability in the first section of the Mesozoic or secondary age--the
Triassic period. The oldest fossil remains of mammals that we know
were found in strata that belong to the earliest Triassic period--the
upper Kueper. One of the earliest forms is the genus Dromatherium,
from the North American Triassic (Figure 2.268). Their teeth still
strikingly recall those of the Pelycosauria. Hence we may assume that
this small and probably insectivorous mammal belonged to the
stem-group of the Promammals. We do not find any positive trace of the
third and most advanced division of the Mammals--the Placentals. These
(including man) are much younger, and we do not find indisputable
fossil remains of them until the Cenozoic age, or the Tertiary period.
This paleontological fact is very important, because it fully
harmonises with the evolutionary succession of the Mammal orders that
is deduced from their comparative anatomy and ontogeny.
The latter science teaches us that the whole Mammal class divides into
three main groups or sub-classes, which correspond to three successive
phylogenetic stages. These three stages, which also represent three
important stages in our human genealogy, were first distinguished in
1816 by the eminent French zoologist, Blainville, and received the
names of Ornithodelphia, Didelphia, and Monodelphia, according to the
construction of the female organs (delphys = uterus or womb). Huxley
afterwards gave them the names of Prototheria, Metatheria, and
Epitheria. But the three sub-classes differ so widely from each other,
not only in the construction of the sexual organs, but in many other
respects also, that we may confidently draw up the following important
phylogenetic thesis: The Monodelphia or Placentals descend from the
Didelphia or Marsupials; and the latter, in turn, are descended from
the Monotremes or Ornithodelphia.
Thus we must regard as the twenty-first stage in our genealogical tree
the earliest and lowest chief group of the Mammals--the sub-class of
the Monotremes ("cloaca-animals," Ornithodelphia, or Prototheria,
Figures 2.269 and 2.270). They take their name from the cloaca which
they share with all the lower Vertebrates. This cloaca is the common
outlet for the passage of the excrements, the urine, and the sexual
products. The urinary ducts and sexual canals open into the hindmost
part of the gut, while in all the other Mammals they are separated
from the rectum and anus. The latter have a special uro-genital outlet
(porus urogenitalis). The bladder also opens into the cloaca in the
Monotremes, and, indeed, apart from the two urinary ducts; in all the
other Mammals the latter open directly into the bladder. It was proved
by Haacke and Caldwell in 1884 that the Monotremes lay large eggs like
the reptiles, while all the other Mammals are viviparous. In 1894
Richard Semon further proved that these large eggs, rich in food-yelk,
have a partial segmentation and discoid gastrulation, as I had
hypothetically assumed in 1879; here again they resemble their
reptilian ancestors. The construction of the mammary gland is also
peculiar in the Monotremes. In them the glands have no teats for the
young animal to suck, but there is a special part of the breast
pierced with holes like a sieve, from which the milk issues, and the
young Monotreme must lick it off. Further, the brain of the Monotremes
is very little advanced. It is feebler than that of any of the other
Mammals. The fore-brain or cerebrum, in particular, is so small that
it does not cover the cerebellum. In the skeleton (Figure 2.270) the
formation of the scapula among other parts is curious; it is quite
different from that of the other Mammals, and rather agrees with that
of the reptiles and Amphibia. Like these, the Monotremes have a
strongly developed caracoideum. From these and other less prominent
characteristics it follows absolutely that the Monotremes occupy the
lowest place among the Mammals, and represent a transitional group
between the Tocosauria and the rest of the Mammals. All these
remarkable reptilian characters must have been possessed by the
stem-form of the whole mammal class, the Promammal of the Triassic
period, and have been inherited from the Proreptiles.
(FIGURE 2.268. Lower jaw of a Primitive Mammal or Promammal
(Dromatherium silvestre) from the North American Triassic. i incisors,
c canine, p premolars, m molars. (From Doderlein.))
During the Triassic and Jurassic periods the sub-class of the
Monotremes was represented by a number of different stem-mammals.
Numerous fossil remains of them have lately been discovered in the
Mesozoic strata of Europe, Africa, and America. To-day there are only
two surviving specimens of the group, which we place together in the
family of the duck-bills, Ornithostoma. They are confined to Australia
and the neighbouring island of Van Diemen's Land (or Tasmania); they
become scarcer every year, and will soon, like their blood-relatives,
be counted among the extinct animals. One form lives in the rivers,
and builds subterraneous dwellings on the banks; this is the
Ornithorhyncus paradoxus, with webbed feet, a thick soft fur, and
broad flat jaws, which look very much like the bill of a duck (Figures
2.269 and 2.270). The other form, the land duck-bill, or spiny
ant-eater (Echidna hystrix), is very much like the anteaters in its
habits and the peculiar construction of its thin snout and very long
tongue; it is covered with needles, and can roll itself up like a
hedgehog. A cognate form (Parechidna Bruyni) has lately been found in
New Guinea.
These modern Ornithostoma are the scattered survivors of the vast
Mesozoic group of Monotremes; hence they have the same interest in
connection with the stem history of the Mammals as the living
stem-reptiles (Hatteria) for that of the reptiles, and the isolated
Acrania (Amphioxus) for the phylogeny of the Vertebrate stem.
The Australian duck-bills are distinguished externally by a toothless
bird-like beak or snout. This absence of real bony teeth is a late
result of adaptation, as in the toothless Placentals (Edentata,
armadillos and ant-eaters). The extinct Monotremes, to which the
Promammalia belonged, must have had developed teeth, inherited from
the reptiles. Lately small rudiments of real molars have been
discovered in the young of the Ornithorhyncus, which has horny plates
in the jaws instead of real teeth.
(FIGURE 2.269. The Ornithorhyncus or Duck-mole. (Ornithorhyncus
paradoxus).
FIGURE 2.270. Skeleton of the Ornithorhyncus.)
The living Ornithostoma and the stem-forms of the Marsupials (or
Didelphia) must be regarded as two widely diverging lines from the
Promammals. This second sub-class of the Mammals is very interesting
as a perfect intermediate stage between the other two. While the
Marsupials retain a great part of the characteristics of the
Monotremes, they have also acquired some of the chief features of the
Placentals. Some features are also peculiar to the Marsupials, such as
the construction of the male and female sexual organs and the form of
the lower jaw. The Marsupials are distinguished by a peculiar
hook-like bony process that bends from the corner of the lower jaw and
points inwards. As most of the Placentals have not this process, we
can, with some probability, recognise the Marsupial from this feature
alone. Most of the mammal remains that we have from the Jurassic and
Cretaceous deposits are merely lower jaws, and most of the jaws found
in the Jurassic deposits at Stonesfield and Purbeck have the peculiar
hook-like process that characterises the lower jaw of the Marsupial.
On the strength of this paleontological fact, we may suppose that they
belonged to Marsupials. Placentals do not seem to have existed at the
middle of the Mesozoic age--not until towards its close (in the
Cretaceous period). At all events, we have no fossil remains of
indubitable Placentals from that period.
The existing Marsupials, of which the plant-eating kangaroo and the
carnivorous opossum (Figure 2.272) are the best known, differ a good
deal in structure, shape, and size, and correspond in many respects to
the various orders of Placentals. Most of them live in Australia, and
a small part of the Australian and East Malayan islands. There is now
not a single living Marsupial on the mainland of Europe, Asia, or
Africa. It was very different during the Mesozoic and even during the
Cenozoic age. The sedimentary deposits of these periods contain a
great number and variety of marsupial remains, sometimes of a colossal
size, in various parts of the earth, and even in Europe. We may infer
from this that the existing Marsupials are the remnant of an extensive
earlier group that was distributed all over the earth. It had to give
way in the struggle for life to the more powerful Placentals during
the Tertiary period. The survivors of the group were able to keep
alive in Australia and South America because the one was completely
separated from the other parts of the earth during the whole of the
Tertiary period, and the other during the greater part of it.
(FIGURE 2.271. Lower jaw of a Promammal (Dryolestes priscus), from the
Jurassic of the Felsen strata. (From Marsh.))
From the comparative anatomy and ontogeny of the existing Marsupials
we may draw very interesting conclusions as to their intermediate
position between the earlier Monotremes and the later Placentals. The
defective development of the brain (especially the cerebrum), the
possession of marsupial bones, and the simple construction of the
allantois (without any placenta as yet) were inherited by the
Marsupials, with many other features, from the Monotremes, and
preserved. On the other hand, they have lost the independent bone
(caracoideum) at the shoulder-blade. But we have a more important
advance in the disappearance of the cloaca; the rectum and anus are
separated by a partition from the uro-genital opening (sinus
urogenitalis). Moreover, all the Marsupials have teats on the mammary
glands, at which the new-born animal sucks. The teats pass into the
cavity of a pouch or pocket on the ventral side of the mother, and
this is supported by a couple of marsupial bones. The young are born
in a very imperfect condition, and carried by the mother for some time
longer in her pouch, until they are fully developed (Figure 2.272). In
the giant kangaroo, which is as tall as a man, the embryo only
develops for a month in the uterus, is then born in a very imperfect
state, and finishes its growth in the mother's pouch (marsupium); it
remains in this about nine months, and at first hangs continually on
to the teat of the mammary gland.
(FIGURE 2.272. The crab-eating Opossum (Philander cancrivorus). The
female has three young in the pouch. (From Brehm.)
From these and other characteristics (especially the peculiar
construction of the internal and external sexual organs in male and
female) it is clear that we must conceive the whole sub-class of the
Marsupials as one stem group, which has been developed from the
Promammalia. From one branch of these Marsupials (possibly from more
than one) the stem-forms of the higher Mammals, the Placentals, were
afterwards evolved. Of the existing forms of the Marsupials, which
have undergone various modifications through adaptation to different
environments, the family of the opossums (Didelphida or Pedimana)
seems to be the oldest and nearest to the common stem-form of the
whole class. To this family belong the crab-eating opossum of Brazil
(Figure 2.272) and the opossum of Virginia, on the embryology of which
Selenka has given us a valuable work (cf. Figures 1.63 to 1.67 and
1.131 to 1.135). These Didelphida climb trees like the apes, grasping
the branches with their hand-shaped hind feet. We may conclude from
this that the stem-forms of the Primates, which we must regard as the
earliest Lemurs, were evolved directly from the opossum. We must not
forget, however, that the conversion of the five-toed foot into a
prehensile hand is polyphyletic. By the same adaptation to climbing
trees the habit of grasping their branches with the feet has in many
different cases brought about that opposition of the thumb or great
toe to the other toes which makes the hand prehensile. We see this in
the climbing lizards (chameleon), the birds, and the tree-dwelling
mammals of various orders.
Some zoologists have lately advanced the opposite opinion, that the
Marsupials represent a completely independent sub-class of the
Mammals, with no direct relation to the Placentals, and developing
independently of them from the Monotremes. But this opinion is
untenable if we examine carefully the whole organisation of the three
sub-classes, and do not lay the chief stress on incidental features
and secondary adaptations (such as the formation of the marsupium). It
is then clear that the Marsupials--viviparous Mammals without
placenta--are a necessary transition from the oviparous Monotremes to
the higher Placentals with chorion-villi. In this sense the Marsupial
class certainly contains some of man's ancestors.