FIG. 38.--Molar teeth of A, _Phenacodus_, and B, the Creodont

_Palaeonictis_. _End_, endoconid; _hld_, hypoconulid; _hyd_, hypoconid;

_med_, metaconid; _prd_, protoconid. (After Osborn and Wortman.)

This question is, however, wrapped up in a wider one. Its solution depends

upon the ancestry of mammals. If the Mammalia are to be derived from

reptiles with simple conical teeth, then the first stage in the development

of trituberculy is proved. On the other hand, however, the evidence is

gradually growing that the Theromorpha represent more nearly than any

non-mammalian group with which we are acquainted the probable ancestral

form of the mammals. These animals offer some support to both the leading

views. _Cynognathus_ had triconodont teeth which, as will be pointed out

later, are a theoretically intermediate stage in the evolution of

tritubercular teeth; on the other hand, the teeth of _Diademodon_ and some

others are multituberculate, and have been very properly compared to the

multitubercular teeth of such primitive mammalia as the Ornithorhynchus.

Professor Osborn is no doubt correct in italicising a remark of an

anonymous writer in _Science_ to the effect that in _Diademodon_ the teeth,

though multitubercular, show the prevalence of three cusps arranged in the

tritubercular fashion. {57} But this may be only a proof that the

multitubercular antedates the tritubercular. It may be, indeed, that the

mammalian tooth was already differentiated among the mammal-like Saurians

and that from such a form as _Cynognathus_ the Eutheria and other forms in

which a tritubercular arrangement can be detected were evolved, and from

such form as _Tritylodon_ the Monotrematous branch of the mammals. This way

of looking at the matter harmonises a much-disputed question, but involves

a diphyletic origin of the mammals--an origin which for other reasons is

not without its supporters.

We shall now attempt to give a general idea of the facts and arguments

which support or tend to support "trituberculy." As a matter of fact the

name is inaccurate; for the holders of this view do not derive the

mammalian molar from a trituberculate condition, but in the first place

from a simple cone such as that of a crocodile!

To this main and at first only cusp came as a reinforcement an additional

cusp at each side, or rather at each end, having regard to their position

with reference to the long axis of the jaw. This stage is the "triconodont"

stage, and teeth exist among living as well as extinct mammals which show

this early form of tooth. We have, indeed, the genus _Triconodon_, so named

on that very account. Among living mammals the Seals and the Thylacine all

show some triconodont teeth. A Toothed Whale, it may be remarked, is a

living example of a mammal with monoconodont teeth. The three primary

cusps, as the supporters of Cope's theory of trituberculism denominate

them, are termed respectively the protocone, paracone, and metacone, or, if

they are in the teeth of the lower jaw, protoconid, paraconid, and

metaconid. At a slightly later stage, or coincidently, a rim partly

surrounded the crown of the tooth; the rim is known as the cingulum, and

from a prominent elevation of this rim a fourth cusp, the hypocone, was

developed. The three main cones then moved, or rather two of them moved, so

as to form a triangle; this is the tritubercular stage. Teeth of this

pattern are common, and occur in such ancient forms as Insectivora and

Lemurs, besides numerous extinct groups. An amendment has been suggested,

and that is to term the teeth with the simple primitive triangle

"trigonodont," and to reserve the term tritubercular for those teeth in

which the hypocone has appeared. The platform bearing the hypocone widened

into the {58} "talon"; and this ledge became produced into two additional

cusps, the hypoconule or hypoconulid, and the ectocone or ectoconid. Thus

the typical sextuberculate tooth of the primitive Ungulate, and indeed of

many primitive Eutherians, is arrived at. From this the still further

complicated teeth of modern Ungulates can be derived by further additions

or fusions, etc.[31] On the other hand, the development of the Primate

molar stops short at the stage of four cusps.

[Illustration]

FIG. 39.--Epitome of the evolution of a cusped tooth. 1, Reptile; 2,

_Dromatherium_; 3, _Microconodon_; 4, _Spalacotherium_; _me_, metaconid;

_pa_, paraconid; _pr_, protoconid; 5, _Amphitherium_. (After Osborn.)

That such a series can be traced is an undoubted fact. Every stage exists,

or has existed. But whether the stages can be connected or not is quite

another question. It is by three main lines of argument that the view here

sketched out in brief is supported. In the first place, the tracing of the

pedigrees of many groups of mammals has met with very considerable success;

and it is clear that as we pass from the living Horse and Rhinoceros, with

their complicated molars, to their forerunners, we find that both can be

referred to a primitive Ungulate molar with but six cusps. Going still

further back to the lowest Eocene and ancestral type as it appears,

_Euprotogonia_, we still find in the molar tooth the sextubercular plan of

structure. We can hardly get further back in the evolution of the

Perissodactyles with any probability of security. On the other hand, many

facts point to a fundamental relationship between the primitive Ungulates

and the early Creodonts. The latter frequently show plainly tritubercular

molars. Such Ungulates as _Euprotogonia_ and _Protogonodon_, though sex- or

quinque-tubercular as to their molars, have a distinctly prevailing

trituberculism, when the _size_ and importance of three of the cusps is

taken into account. But this {59} lacks finality as a convincing proof of

the tritubercular tooth as a primitive Ungulate tooth.

Professor Osborn has ingeniously utilised certain deviations from the

normal type of tooth structure (for the group) in favour of his

strongly-urged opinions. If the stages of development have been as he

suggests, a retrogression would naturally be in the inverse order; thus the

"apparently 'triconodont' lower molar of _Thylacinus_" may be interpreted

as a retrogression from a tritubercular tooth. In the same way may be

explained the triconodont teeth of Seals and of the Cetacean _Zeuglodon_.

Finally, the modern Toothed Whales have retrograded into "haplodonty."

Embryological evidence has also been called in, and with some success, to

contribute towards the proof of the tritubercular theory of teeth. Taeker

has shown that in the Horse and the Pig, and some other Ungulates, there is

first of all a single hillock or cusp, and that later the additional cones

arise separately. An apparent stumbling-block raised by these

investigations is that it is not always the protocone or its equivalent in

the upper jaw which arises first, as it obviously ought to do

phylogenetically. This, however, is not a final argument in either

direction. We know from plenty of examples that ontogenetic processes

sometimes do not correspond in their order with phylogenetic changes. Thus

in the mammalian heart the ventricle divides before the auricle; and of

coarse, phylogenetically, the reverse ought to occur, since a divided

auricle precedes a divided ventricle. This method of development has,

moreover, been interpreted otherwise. It has been held to signify that the

complex teeth of mammals are indeed derived from simple cones but by the

fusion of a number of those cones.

On the other hand there are the claims of the multitubercular theory of the

origin of mammalian teeth to be considered. The palaeontological evidence

has been already, to some extent, utilised. The occurrence of such teeth

among the possible forerunners of mammals, and in some of the most

primitive types of Mammalia, has been referred to. Senor Ameghino dwells

upon the sextubercular condition of many primitive mammals even belonging

to the Eutheria. In a recent communication[32] he attempts to identify six

tubercles in the molars of types belonging to a {60} variety of Orders. The

same condition, as has been noted, characterises that ancient Ungulate form

_Euprotogonia_. Even where the teeth seem at first sight to be

tritubercular a detailed study shows traces of otherwise vanished cusps.

It must be remembered in basing arguments upon the early Jurassic and

Cretaceous mammals, that our knowledge of them mainly depends upon lower

jaws, the teeth of which are usually simpler in pattern than those of the

upper jaws. Moreover, another fact, not always insisted upon, must not be

lost sight of. In many of those creatures the jaws were of small size, and

yet accommodated a large series of molar teeth. _Amphitherium_, for

example, had six molar teeth, and five is a number frequently met with. As

the teeth are so numerous and the jaws so small it seems reasonable to

connect the simplicity of the structure of the teeth with the need for

crowding a number together. The same argument may partly account for the

superabundant teeth of many Toothed Whales. It is true that the Manatee has

very numerous grinders which are yet complex; but then in this animal there

is a succession, and the jaw does not hold at a given time the entire

series, with which it is provided in relays. On the other hand, where there

are few molars they are often of the multitubercular type, or at least

approach it; of this the Multituberculate _Polymastodon_ is a good example;

so, too, the molars of _Hydrochoerus_, and of many other Rodents.

It is well known that the fourth deciduous molar of the upper jaw, which is

replaced by a permanent premolar in the fully adult animal, is of a more

complex structure than its successor. This may indeed be extended to

premolars earlier in the series. In the Dog "the second and first milk

molars closely resemble the third and second premolars"; now the milk

premolars belong evidently to the same dentition as the permanent molars,

and they are earlier teeth than the later-developed replacing teeth. It is

therefore significant that these earlier teeth should be more cuspidate

than the later teeth. It tells distinctly in favour of the simplification

as opposed to the complication of teeth in time, in the groups concerned.

These facts may possibly be applied in explanation of the simple teeth of

some of the Jurassic and Cretaceous mammals. It has been mentioned that

absolute trituberculy is exceedingly rare among those ancient creatures;

more generally there are to {61} be found at least traces of more cusps.

Now in some of them we may be dealing with instances of a complete tooth

change; the suppression, save for one tooth, which is found in Marsupials,

was probably not developed in at least some of these early mammals. The

simplicity may therefore have been preceded by complexity, and may have

been merely an adaptation to an insectivorous diet.

ALIMENTARY CANAL.--The _mouth_ of the Mammalia is remarkable for the fact

that with a few exceptions, such as the Whales, there are thick and fleshy

lips. The office of these is to seize the food. The roof of the mouth is

formed by the "hard palate" in front, which covers over the maxillary and

palatine regions. This region is often covered with raised ridges, which

have a symmetrical disposition, and are particularly strong in Ruminant

animals. They are much reduced in the Rodents, where the anterior part of

the palate is ill-defined owing to the way in which its sides fade into the

lateral surface of the face. It has been shown that these ridges, in the

Cat at least, develop as separate papilliform outgrowths, and it has been

suggested that these papillae, which later become united to form the

ridges, are the last remnant of palatine teeth such as occur in lower

vertebrates.

[Illustration]

FIG. 40.--Palatal folds of the Raccoon (_Procyon lotor_). _p.p_, Papilla

palatina; _r.p_, palatal folds. (From Wiedersheim's _Structure of Man_.)

The _tongue_ is a well-developed organ, usually playing a double part. It

acts as an organ of prehension, especially in such animals as the Giraffe

and the Anteater, where it is long and protrusible beyond the mouth for a

considerable distance. It also carries gustatory organs, which serve for

the discrimination of the nature of the food. Beneath the tongue there may

be a hardish plate, known as the sublingua. This is especially prominent in

the Lemurs, where it projects as a horny structure below the tongue, and

has an independent and free tip. It is supported in some of these animals

by a cartilaginous {62} structure. It is held by Gegenbaur that this organ

is the equivalent of the reptilian tongue, and that in the skeletal

vestiges which it contains are to be found the equivalents of the hyoid

skeletal cartilages which support the tongue in lizards. In this case the

tongue of mammals is a subsequently added structure.

The _oesophagus_ leads from the mouth cavity to the _stomach_. The latter

organ has commonly a distinctive shape in mammals. This is well shown in

Man. The orifices of the oesophagus and intestine are somewhat

approximated; and this causes a bulging of the lower border of the organ,

usually spoken of as the greater curvature. A stomach of this typical form

is found in many orders of mammals, and is unlike the stomach in any of the

groups of lower vertebrates in shape. Sometimes the shape of the organ is

greatly altered: it may be drawn out, sacculated, or divided, as in the

Ruminants and Whales, into a series of differentiated chambers, each of

which plays some special part in the phenomena of digestion.

The _intestine_ of mammals is always long and much coiled, though the

length and consequent degree of coiling naturally varies. On the whole it

is perhaps safe to say that it is shorter in carnivorous than in

vegetable-feeding beasts. Thus the Paca has an intestine of 39 inches total

length, while the Cat, an animal of about the same size, has an intestine

which is only 36 inches long. A fish diet, however, to judge from the

Seals, is associated with a long intestinal tract. The intestine is

divisible in the vast majority of mammals into a small and a large

intestine. The two are separated by a valvular constriction save in certain

Carnivores; and in the majority of cases the distinction is also emphasised

by the presence at the junction of a blindly-ending diverticulum, the

_caecum_. This latter organ varies greatly in length, being very short in

the Cat-tribe and exceedingly long in Rodents. Its size is, to some extent,

dependent upon the flesh-eating or grass-eating propensities of the animal

in which it occurs. One of the longest caeca is possessed by the Vulpine

Phalanger, in which the organ is one-fifth of the length of the small

intestine; while the opposite extremity is reached by _Felis macroscelis_,

which has a small intestine one hundred times the length of the caecum.

{63} [Illustration]

FIG. 41.--Different forms of the stomach in Mammals. A, Dog; B, _Mus

decumanus_; C, _Mus musculus_; D, Weasel; E, scheme of the Ruminant

stomach, the arrow with the dotted line showing the course taken by the

food; F, Human stomach. _a_, Minor curvature; _b_, major curvature; _c_,

cardiac end. G, Camel; H, _Echidna aculeata_. _Cma_, Major curvature;

_Cmi_, minor curvature. I, _Bradypus tridactylus_. _Du_, Duodenum; _MB_,

coecal diverticulum; **, outgrowths of duodenum; +, reticulum; ++, rumen.

_A_ (in E and G), Abomasum; _Ca_, cardiac division; _O_, psalterium; _Oe_,

oesophagus; _P_, pylorus; _R_ (to the right in E and to the left in G),

rumen; _R_ (to the left in E and to the right in G), reticulum; _Sc_,

cardiac division; _Sp_, pyloric division; _WZ_, water-cells. (From

Wiedersheim's _Comparative Anatomy_.)

An interesting point in connexion with the gut of mammals is the varying

proportion of the small to the large intestine. As a general rule the

former is very considerably longer than the latter; in _Paradoxurus_, for

instance, the small intestine may be fifteen times the length of the large.

The excess of length of one section over the other is not generally so

marked {64} as this. In _Phalanger maculatus_ the two sections of the gut

are as nearly as possible equal in length, while in _Phaseolarctos_ the

large intestine is considerably longer than the small, the lengths being

respectively 160 inches and 111 inches. It is common among the Marsupials

and also among the Rodents for these proportions to exist, _i.e._ for the

large intestine to be as long as, or longer than, the small. But there are

so many exceptions that no general statements can be extracted from the

facts.

Some few details will be found in the systematic part of this book. Mr.

Chalmers Mitchell has brought forward some reasons for associating a great

length of large intestine with an archaic systematic position, in the birds

at any rate. The facts here briefly touched upon are not at variance with

the extension of such a view to the mammals.

[Illustration]

FIG. 42.--Diagrammatic plan of the liver of a Mammal (posterior surface).

_c_, Caudate lobe; _cf_, cystic fissure; _dv_, ductus venosus; _g_,

gall-bladder; _lc_, left central lobe; _ll_, left lateral lobe; _llf_, left

lateral fissure; _p_, portal vein entering transverse fissure; _rc_, right

central lobe; _rl_, right lateral lobe; _rlf_, right lateral fissure; _s_,

Spigelian lobe; _u_, umbilical vein; _vc_, post-caval vein. (After Flower

and Lydekker.)

Appended to the alimentary tract are three glands or sets of glands.

Opening into the mouth cavity are the _salivary glands_, which are of

enormous size in Anteaters, and small or absent in Whales. In their number

and position these glands are characteristic of mammals. Into the intestine

open the ducts of the pancreas and liver, two glands which the mammals

share with lower vertebrates. The form of the _liver_ is, however,

generally characteristic of mammals. It is divided as a rule into a right

and a left half, the line of division being marked by the insertion of the

umbilical ligament, a vestige of the primitive ventral mesentery. Each half

is again commonly subdivided into central and lateral lobes. In addition to

these, two other divisions are often to be seen--the Spigelian and the

caudate lobe. The liver is less divided in Cetacea and {65} some others,

very much subdivided in Rodents and other groups. The degree of subdivision

and the proportions of the several lobes frequently offer valuable

systematic characters. The gall-bladder may be present or absent; it is

always a diverticulum of the hepatic duct. The two are never separate, as

in birds, for instance.

ORGANS OF CIRCULATION.--The heart of all mammals is a completely

four-chambered organ. In the adult heart there is no communication between

the right and left halves. The auricles are comparatively thin-walled, the

ventricles thick-walled, in relation to the amount of work that they have

severally to perform. The right ventricle, moreover, which has only to

drive the blood into the lungs, is much thinner-walled than the left

ventricle, which is concerned with the entire systemic circulation. The

exits of the arteries and the auriculo-ventricular orifices are guarded by

valves, which are so arranged as only to permit the blood to flow in the

proper direction. But these valves have a morphological as well as a

physiological interest. At the origin of each artery, the aorta and the

pulmonary, there is a row of three watch-pocket valves, as they have been

generally termed on account of their form. These three valves meet

accurately in the middle of the lumen of the arterial tube when liquid is

poured into them from above, and thus completely occlude the orifice. The

auriculo-ventricular valves differ in structure in the two ventricles. That

of the left ventricle has only two flaps, and is therefore often spoken of

as the bicuspid or mitral valve. Both these flaps are membranous, and

together they completely surround the exit from the auricle into the

ventricle. The edges of the valve are bound down to the parietes of the

heart by numerous branching tendinous threads, the chordae tendineae, which

often take their origin from pillar-like muscles arising from the walls of

the heart, the so-called musculi papillares. The valve of the right

ventricle is composed of three flaps, and is therefore often spoken of as

the tricuspid valve; it is in the same way membranous, and has chordae

tendineae and musculi papillares connected with it. The disposition of the

musculi papillares and their number differ in different mammals, but no

exhaustive study has as yet been made of the arrangements in different

groups; the amount of individual variation even is not known, though it is

certainly considerable in some cases, for {66} instance in the heart of the

Rabbit. The heart of the Monotremata presents differences of some

importance from those of other Mammalia; the modern knowledge of the

Monotrematous heart is mainly due to Gegenbaur[33] and Lankester,[34] in

whose memoirs references to the older literature will be found. The

principal features of interest in which the heart of the Monotremata

differs from that of the higher Mammalia are these. When the two ventricles

are cut across transversely, the cavity of the right is seen to be wrapped

round that of the left in a fashion precisely like that of the bird's

heart; on the other hand in the higher mammal the two cavities lie side by

side. The main difference between Monotremes and other Mammals concerns the

right auriculo-ventricular valve. The differences which it presents from

the corresponding structure of the rest of the Mammalia are two: in the

first place, the valve itself does not completely surround the ostium; it

is only developed on one side; the septal half (_i.e._ that turned towards

the interventricular septum) is either entirely absent or more generally

represented by a small bit of membrane; nevertheless I found[35] recently

in an _Ornithorhynchus_ heart a complete septal half to the right

auriculo-ventricular valve. The second point of interest in connexion with

this valve is, that the musculi papillares instead of ending in chordae

tendineae attached to the free edge of the valve are directly attached to

the valve, and in some cases pass through its membranous flap, to be

attached to its origin at the boundary of the auricle and of the ventricle.

The invading of the valve-flap by muscle in this way is highly interesting,

as it recalls the heart of the bird and of the crocodile. The imperfect

condition of the valve (from which, as has already been stated, the septal

half is as a rule nearly absent) is a point of resemblance to the heart of

the bird; the corresponding valve of the crocodile's heart being complete.

{67}

[Illustration]

FIG. 43.--_Lepus cuniculus._ Ventral view of the vascular system. The heart

is somewhat displaced towards the left of the subject; the arteries of the

right and the veins of the left side are in great measure removed. _a.epg_,

internal mammary artery; _a.f_, anterior facial vein; _a.m_, anterior

mesenteric artery; _a.ph_, anterior phrenic vein; _az.v_, azygos vein;

_br_, brachial artery; _c.il.a_, common iliac artery; _c.il.v_, common

iliac vein; _coe_, coeliac artery; _d.ao_, dorsal aorta: _e.c_, external

carotid artery; _e.il.a_, external iliac artery; _e.il.v_, external iliac

vein; _e.ju_, external jugular vein; _fm.a_, femoral artery; _fm.v_,

femoral vein; _h.v_, hepatic veins; _i.c_, internal carotid artery; _i.cs_,

intercostal vessels; _i.il.a_, internal iliac artery; _i.il.v_, internal

iliac vein; _i.ju_, internal jugular vein; _i.l_, iliolumbar artery and

vein; _in_, innominate artery; _l.au_, left auricle; _l.c.c_; left common

carotid artery; _l.pr.c_, left pre-caval vein; _l.v_, left ventricle;

_m.sc_, median sacral artery; _p.a_, pulmonary artery; _p.epg_, epigastric

artery and vein; _p.f_, posterior facial vein; _p.m_, posterior mesenteric

artery; _p.ph_, posterior phrenic veins; _pt.c_, post-caval vein; _p.v_,

pulmonary vein; _r_, renal artery and vein; _r.au_, right auricle; _r.c.c_,

right common carotid artery; _r.prc_, right pre-caval vein; _r.v_, right

ventricle; _s.cl.a_, right subclavian artery; _s.cl.v_, subclavian vein;

_spm_, spermatic artery; _s.vs_, vesical artery; _ut_, uterine artery and

vein; _vr_, vertebral artery. (From Parker's _Zootomy_.)

There are also features in the system of arteries and veins which are

eminently distinctive of mammals. In the first place, the aorta leaving the

heart and conveying blood to the body is only a half arch, and bends to the

left side as seen in Fig. 43. The right and left halves are present in

reptiles, and meet behind the heart. In the bird the right half alone has

remained. This fact, therefore, shows that the mammal cannot have been

derived from a bird-like ancestor, but that both must have independently

come from an ancestor with both halves of the aortic arch present, of which

one half has disappeared in one group, and the other half in the other. It

is an interesting fact, too, to notice that the four {68} cavities of the

mammal's heart, which fourfold division it shares with birds alone, do not

exactly correspond compartment for compartment with those of the bird's

heart, at least in so far as concerns the ventricles. For the reptilian

heart is provided with only one ventricle, and therefore the division of

that cavity must have been independently accomplished in mammals and in

birds.

There are two features in the venous system which distinguish all the

Mammalia (with the exception of _Echidna_ in one of these points) from

vertebrates standing lower in the series. The hepatic portal system is

limited to a vein which conveys to the liver blood derived from the

alimentary tract; in no mammal except in _Echidna_ is there any

representative of the anterior abdominal vein of lower vertebrates. In that

animal there is such a vein, which apparently arises from a capillary

network upon the bladder and passes up, supported by a membrane, along the

ventral wall of the abdomen to the liver, thus emptying blood into that

organ exactly as does the anterior abdominal vein of the frog. In no mammal

is there any trace of a renal portal system. The kidneys derive their blood

from the renal arteries only.

Many mammals have two superior venae cavae; this is the case, for instance,

in the Elephant and the Rodents and other types lying comparatively far

down in the series. In most if not in all mammals there are considerable

remains of one of the posterior cardinals, in the form of the azygos vein,

which opens into the vena cava superior or pre-caval vein, _i.e._ the

superior cardinal just before the latter debouches into the heart. This one

posterior cardinal is usually on the right side; but it may be on the left

side, for instance in _Trichosurus vulpecula_. In _Halmaturus bennettii_

there are two azygos veins, one left and one right, of which the left is

rather the larger.[36]

URINARY ORGANS.--The kidneys in the Mammalia have a compact form, which

contrasts with the somewhat diffuse and vaguely-outlined kidneys of the

Sauropsida. In mammals the organ is as a rule of that peculiar shape which

is called "kidney-shaped"; a depression termed the hilum, which receives

the ducts of the glands, indenting the border of an otherwise oval-shaped

gland. In some few mammals the kidney is broken up {69} into lobules; this

is the case with the Whales, the Bears, the Oxen, and a few other forms. A

curious fact about the kidneys of the Mammalia is their very general

asymmetry of position. One of them usually lies in a more advanced position

than the other. The ureters lead from the kidneys to the urinary bladder,

which in its form and relations is quite distinctive of the Mammalia. The

bladder is formed out of the remains of the allantois, and is therefore not

the exact homologue of the bladder of the frog, which is the equivalent of

the entire sac which grows out of the cloaca in the mammal, and is the

foetal allantois. The ureters open into the bladder in the higher Mammalia,

but lower down in the urino-genital passage in the more primitive mammals.

THE BODY CAVITY.--The Mammalia differ from all other living vertebrates by

the arrangement of the body cavity in which lie the viscera. That cavity is

divided into two by a partly muscular and partly tendinous partition, the

diaphragm. No other vertebrate has this precise disposition of the coelom.

The diaphragm lies usually transversely to the longitudinal axis of the

body, but gets a much more oblique arrangement in the Cetacea and the

Sirenia, whose needs demand a more expanded chamber for the lungs. For in

front of the diaphragm lie the lungs and heart; behind it the stomach,

liver, intestines, and the organs of reproduction and excretion. The

diaphragm is used in respiration; when its muscles contract, the surface

directed toward the pleural cavity becomes less convex, and the cavity of

the lungs is thus increased, allowing them to expand under the pressure of

the entering air.

THE LUNGS.--The lungs of the Mammalia differ from those of animals lying

lower in the series by the fact, just referred to, that they occupy a

pleural cavity completely shut off from the abdomen by the diaphragm. As a

rule the lungs of the Mammalia are to be distinguished by their more or

less extensive lobation. In the Whales, however, and in the Sirenia, they

are not much divided, but present the appearance of the simple sac-like

lungs of the reptiles. In some mammals there is a median and posterior

unpaired lobe of the lung, which lies in the post-pericardial cavity behind

the pericardium. This is not universally present. The lungs are very

frequently not symmetrical in their lobation, the number of separate lobes

on the right side {70} and on the left being different. The lungs of

mammals agree with those of the lower reptiles in being freely suspended

within their coelomic cavity, and in not being, as in birds, crocodiles,

and the Varanidae among lizards, tied down to the dorsal surface of that

cavity by a sheet of peritoneum covering them.

[Illustration]

FIG. 44.--Part of a sagittal section of an ovary of a child just born.

_bl.v_, Blood-vessels; _foll_, strings and groups of cells derived from the

germinal epithelium becoming developed into follicles; _g.ep_, germinal

epithelium; _in_, ingrowing cord of cells from the germinal epithelium;

_pr.ov_, primitive ova. (From Hertwig, after Waldeyer.)

THE GONADS (OVARIES AND TESTES).--The ovary in the Mammalia is always

paired; there is never a partial or complete abortion of one gonad as in

birds--except of course in pathological cases. The ovaries are small, and

lie in the abdominal cavity behind the kidneys. In the immense majority of

the Mammalia the ova which are produced within the ovaries are of minute

size; those of even the colossal Rorqual are, so far as we know, not

markedly larger than the ova of a Mouse. The smallness of size of these

reproductive elements implies necessarily an absence of much nutritive

yolk; and as a consequence the developing embryo, since it is not hatched

in an early stage as a free living larva, has to be nourished by the

mother, to whose tissues it is attached through the intermediary of the

placenta, a structure partly composed of foetal structures derived from the

embryo, and partly of portions of the lining membranes of the uterus of the

mother. The ova of the {71} Eutherian mammals, including the Marsupials,

are very small as compared with those of any other vertebrates, excepting

only _Amphioxus_, where the young are hatched early as free swimming

larvae. They also differ in a highly characteristic way in the mode of

their development within the ovary. These processes are to some extent

illustrated in Fig. 44. The main framework of the ovary is formed of the

so-called "stroma," which is a mass of tissue formed of more or less

connective-tissue-like cells. Within this are numerous cavities, the

Graafian follicles. The very young follicles consist of but a single layer

of follicular cells surrounding the ovum, which lies centrally. The

follicular cells gradually increase in number until the ovum lies in the

midst of several layers of cells. At this period a vacuity is formed

between some of these cells, and grows into a large cell-free cavity; the

ovum does not lie loosely in this space, but is connected at one side with

the follicular cells, which still line the interior of the Graafian

follicle by the so-called discus or cumulus proligerus. The egg or ovum

has, moreover, a layer of cells immediately surrounding itself. All these

facts can be gathered by an inspection of Fig. 45. It has been shown that,

as in lower vertebrates, the cells immediately surrounding the ovum are

connected with it directly by delicate processes which penetrate the actual

membrane of the egg.

[Illustration]

FIG. 45.--Two stages in the development of the Graafian follicle. A, With

the follicular fluid beginning to appear; B, after the space has largely

increased. _caps_, Capsule; _disc_, cumulus proligerus; _memb_, membrana

granulosa; _ov_, ovum; _sp_, space containing fluid. (After Hertwig.)

[Illustration]

FIG. 46.--Ovarian egg of _Echidna_. _b_, Basilar membrane; _fe_, follicular

epithelium; _o_, oil globules; _vm_, vitelline membrane; _y_^1, _y_^2,

yolk. (Partly after Caldwell.)

The only ova which depart at all in structure from that above described are

those of the Monotremata. The credit of this {72} discovery rests with Owen

and with Professor Poulton, who pointed out in 1884,[37] that the ovum of

_Ornithorhynchus_ is very large as compared with those of other Mammalia (6

mm. as against .2 mm.), that it is filled with yolk, and that it completely

fills the follicle, being surrounded by two layers of follicular cells

only. This latter fact was proved by Caldwell. Subsequently Gyldberg[38]

and I[39] described the ovarian ovum of _Echidna_, showing it to be

identical with that of _Ornithorhynchus_. Later still a more elaborate and

beautifully illustrated paper was published by Caldwell[40] upon the early

stages of development in the Monotremata and Marsupials, in which the ovum

of the former was accurately described (see Fig. 46). In the particulars

mentioned above, the ovum of the Monotremata is practically identical with

that of the large-yolked ova of the Sauropsida. {73}

[Illustration]

FIG. 47.--_Lepus cuniculus._ The anterior end of the vagina, with the right

uterus, Fallopian tube, and ovary. (Nat. size.) Part of the ventral wall of

the vagina is removed, and the proximal end of the left uterus is shown in

longitudinal section, _fl.t_, Fallopian tube; _fl.t_', its peritoneal

aperture; _l.ut_, left uterus; _l.ut_', left os uteri; _ov_, ovary; _r.ut_,

right uterus; _r.ut_', right os uteri; _s_, vaginal septum; _va_, vagina.

(From Parker's _Zootomy_.)

It is the general rule among vertebrate animals that the ovaries are

completely independent of the ducts which convey their products to the

exterior. In certain fishes, however, there is an absolute continuity

between the two structures, which is believed to be due to a simple

concrescence between the originally distinct ovary and oviduct. The latter

has grown round the former, an obvious advantage in preventing the eggs

from wandering into the abdominal cavity and becoming lost. In the Mammalia

we find discontinuity as a general rule. But in quite a number of forms

folds of the lining membrane of the abdominal cavity are developed, which

practically ensure the passage of the ova into the oviduct when they are

extruded from the ovaries. The oviduct, moreover, has a large and

fimbriated mouth, called in human anatomy--which is provided with a number

of fanciful names--the morsus diaboli. This almost wraps round the ovary,

and thus prevents the ova from straying in the wrong direction. Moreover,

the ovary itself is often so arranged that it can easily be withdrawn into

a pocket of the peritoneum, from which the obvious exit is by the gaping

mouth of the oviduct. This disposition of the generative parts is still

further modified in a few animals, such as the Rat[41] and the

Kinkajou.[42] In these animals the mouth of the oviduct actually opens into

the interior of a closed chamber which contains the ovary. In this case

there is but one route for the {74} extruded ova to follow. This series of

steps in the perfecting of the mode of safe extrusion of the ova is highly

interesting, and is a piece of evidence in favour of the high position of

the mammals.

[Illustration]

FIG. 48.--Female urino-genital apparatus of various Marsupials. A,

_Didelphys dorsigera_ (young); B, _Trichosurus_; C, _Phascolomys wombat_.

_B_, Urinary bladder; _Cl_, "cloaca"; _Fim_, fimbriae; _g_, clitoris; _N_,

kidney; _Od_, Fallopian tube; _Ot_, aperture of Fallopian tube; _Ov_,

ovary; _r_, rectum; _Sp_, septum dividing vagina; _Sug_, urino-genital

sinus; _Ur_, ureter; _Ut_, uterus; _Ut_', opening of the uterus into the

median vagina (_VgB_); _Vg_, lateral vagina; _Vg_', its opening into the

urino-genital sinus; + (in B), point of approximation of uteri; + (in C)

and *, rectal glands. (From Wiedersheim's _Comparative Anatomy_.)

{75}

The oviducal apparatus of the mammal is more specialised than that of lower

vertebrates. It is most simple, as might be imagined, in the egg-laying

Monotremes, where, indeed, it is on the same level as that of reptiles. But

in the Eutheria the fimbriated mouth of the oviduct passes into a narrow

and winding tube, the Fallopian tube; this widens into a uterus, and the

two uteri combine into a single tube in the higher forms. They are called

the Monodelphia on this account. In the Marsupials the uteri are distinct

though they often join above, and from this junction depends a median

"uterus." After the uterus or the uteri follows in every case a single

vagina.

The testes of the Mammalia, like those of other vertebrates, occupy

primitively a position within the body cavity precisely corresponding to

that of the ovaries. And in the lowly-organised Monotremata, and some other

forms, such as the Whales, they retain that primitive position within the

body. It is, however, distinctive of the Mammalia as opposed to lower

vertebrates that the testes descend later into a scrotum, which is simply a

protrusion of the skin of the body surrounded by muscles, and, of course,

containing a section of the body cavity in which lie the testes. The penis

of the Mammalia, represented by the clitoris and associated structures in

the female, is of a structure entirely peculiar to this group.

[Illustration]

FIG. 49.--Brain of Dog. A, ventral; B, dorsal; C, lateral aspect. _B.ol_,

Olfactory lobe; _Cr.ce_, crura cerebri; _Fi.p_, great longitudinal fissure;

_HH_, _HH_^1, lateral lobes of cerebellum; _Hyp_, hypophysis; _Med_, spinal

cord; _NH_, medulla oblongata; _Po_, pons Varolii; _VH_, cerebral

hemispheres; _Wu_, middle lobe (vermis) of cerebellum; _I-XII_, cerebral

nerves. (From Wiedersheim's _Comparative Anatomy_.)

THE BRAIN.--Inasmuch as Professor Wiedersheim has said with perfect truth

that "the brain of the extinct Ungulate _Dinoceras_ shows so striking a

likeness to that of a lizard that one would be compelled to explain it as

that of a lizard without a knowledge of the skeleton," it is clear that to

define the mammalian brain is a difficult matter. The existing Mammalia,

however, all possess brains which can be readily distinguished from those

of vertebrates lying lower in the scale. They are of relatively large size,

brought about mainly by the dimensions of the cerebral hemispheres, which

have an importance in this class of vertebrates that they have not

elsewhere. Coupled with this large size of the hemispheres is a more

elaborate system of transverse commissures uniting the two; and this

culminates in the higher Mammalia, where the corpus callosum attains a

large size and great physiological importance. A {76} very marked feature,

moreover, of the mammal's brain is the development of regular fissures upon

its surface, which fissures are only absent from _Ornithorhynchus_, various

small Rodents, Bats, and Insectivores, among living mammals. It is

sometimes, but erroneously, said that the more complicated the fissures of

the brain are, the higher in intelligence and "zoological position" is the

possessor of that brain. Instances can undoubtedly be quoted to support

such a view; but they are {77} merely selected cases, which do not indicate

a wide applicability of such a generalisation. Thus it is true that the

brain of a Man is more elaborate in its furrows and convolutions than is

that of a Cat. The real fact of the matter is, that the complexity of the

brain from this point of view increases with the size of the animal within

the group.

[Illustration]

FIG. 50.--_Lepus cuniculus._ Longitudinal vertical section of the brain.

(Nat. size.) _a.co_, Anterior commissure; _b.fo_, body of the fornix; _cb_,

cerebellum, showing arbor vitae; _c.c_, crus cerebri; _c.h_^1,

parencephalon or cerebral hemisphere; _c.h_^2, temporal lobe; _c.ma_,

corpus mammillare; _cp.cl_, corpus callosum; _f.m_, foramen of Monro;

_inf_, infundibulum; _l.t_, lamina terminalis; _ly_, lyra; _m.co_, middle

commissure; _m.o_, medulla oblongata; _o.ch_, optic chiasma; _o.l_^1,

_o.l_^2, corpora quadrigemina or optic lobes; _olf_, olfactory lobe;

_p.co_, posterior commissure; _pd.pn_, peduncle of the pineal "gland,"

_pn_; _p.fo_, anterior pillar of the fornix; _pty_, pituitary body; _pv.a_,

pons Varolii; _sp.lu_, septum lucidum; _v_^4, fourth ventricle; _vl.ip_,

velum interpositum; _v.vn_, valve of Vicussens; _II_, optic nerve. (From

Parker's _Zootomy_.)

The Gorilla and the Chimpanzee have a more furrowed brain than has the

little Marmoset; the Bear a more complicated brain than the Weasel, etc.

The most highly-convoluted brains of all mammals are those of the

Elephants, and there does not seem in the Ungulates to be so marked a

relation between size and abundance of fissures as there is among other

mammals. A regular plan of the fissures can be detected with certainty for

each group considered by itself; but it is not so easy to homologise the

details of arrangement from group to group. This is so far in accord with

the view that the existing groups of mammals have diverged from each other

_ab initio_.

Another marked characteristic of the mammalian as opposed to other brains

is the relatively small importance in size and yet the fourfold nature of

the optic lobes. What was the case with the optic lobes of the early

Ungulates is difficult to understand, on account of the fact that the casts

are necessarily imperfect. {78} Altogether the enormous progress in the

complexity of the brain from the early Tertiary mammals down to the

present, is one of the most remarkable revelations of palaeontology. It

goes perhaps some way in explaining the remarkable diversity in mode of

life exhibited by the mammals as compared, for example, with the birds,

whose brains have not diverged so much or in so many directions from the

primitive form.

THE PRESENT DISTRIBUTION OF THE MAMMALIA.--In the following pages some of

the principal facts in the geographical range of the orders, families, and

many of the genera of Mammalia will be given. It has been justly observed

by Mr. Sclater that the habitat of an animal is as much a part of its

definition as is its structure or external form. No systematic account of

the Mammalia would therefore be complete without such geographical facts.

But that branch of zoology which is concerned with the past and present

distribution of animals is wider in scope than this. Zoogeography deals not

only with the actual facts in the range of animals, but with the inferences

as to past changes in the relations of land and sea which the facts seem to

indicate, and with speculations as to the place of origin of the different

groups, of which more than hints are sometimes given by their past and

present distribution. In addition to this, the earth can be mapped out into

provinces and regions which are definable by their animal inhabitants. In

the present volume, dealing only with the Mammalia, it will be obviously

impossible to enter fully into the entire subject of zoogeography. All that

will be attempted is a brief general survey of the science so far as it can

be illustrated by the Mammalia. For fuller knowledge the reader is referred

to the treatises mentioned below.[43]

There are certain facts in the distribution of animals which are

commonplaces of knowledge, but which may be set forth with definiteness.

Everybody knows that an animal has a given range: Elephants, for example,

are found in India and certain adjacent parts of Asia, and again in Africa;

the Rhinoceroses have roughly the same range; the Tiger is limited to Asia;

the {79} Jaguar to America, and so forth. The entire expanse of country

which is inhabited by an animal is called its area of distribution. Such

areas are larger or smaller. The Lion ranges over the whole of Africa, a

small part of India, and some neighbouring countries; on the other hand,

the Insectivore _Solenodon_ is limited to Cuba and Hayti, a separate

species to each. Among other groups of animals are instances of an even

more restricted range. There are humming-birds confined to the slopes of a

single mountain, and fishes limited in their range to a single small lake.

A species may be found everywhere within the area of its distribution, or

it may be confined to a number of limited tracts within that area. In this

case it is usual to speak of "stations." In such cases the species in

question is generally suited to some particular kind of environment. Thus

the Otter and other aquatic mammals will only be found where there is

water; and intervening tracts of waterless country will contain no Otters.

Goats and Chamois live only upon mountains; the intervening plains are

destitute of them. This discontinuity of distribution within the area is

very general. But a discontinuity of area is also seen--not so commonly

however; and, indeed, when it does occur, it is a matter of a genus and not

of a species. Thus the Tapir is found in the East Indies on the one hand

and in South and Central America on the other, being absent in the

intermediate tracts.

It is clear that tracts of country eminently suitable for the housing of a

particular mammal do not always possess that kind, or even an allied form.

Africa, for example, possesses no arboreal Anteaters; there are no

Anteaters at all (of the order Edentata) in Australia, though there are

plenty of ants for them to feed upon, and tropical conditions of climate

prevail. But as in these cases the inference may be denied on the grounds

that no experiments exist to prove or to disprove the assertion, the matter

may be better emphasised by such cases as the introduction of the Rabbit

into Australia, and various mammals, such as Goats, into oceanic islands.

The plague caused by the former is a matter of notoriety. But although

climate and conditions and animal inhabitants do not march accurately

together, there is certainly some connexion between temperature and the

range of animals. Mr. Lydekker writes on this point as follows: "The

llama-like animals, respectively known as vicunas and guanacos, are met

with in {80} company on the highlands of the Cordillera in Peru and

Ecuador, but as we go farther south the latter are found on the plains of

southern Argentina and Patagonia, as well as on the island of Tierra del

Fuego at the sea level. Here then is a clear proof of the intimate

connexion existing between temperature and station; the guanaco being an

animal which can only live in cold or temperate climates, finds suitable

conditions for its existence in tropical latitudes solely at a height of so

many thousands of feet, although farther south it is able to thrive at the

sea level." This, however, cannot be pushed too far--the world cannot be

mapped out into areas bounded by parallels of temperature as was once

attempted--since there are plenty of cases like that of the Tiger, which is

as much at home in a tropical jungle as on the icy plains of Northern Asia.

Seeing that there are in many cases no climatic barriers to the spreading

of a given race of animals over a larger area of distribution than it

actually occupies, it becomes important to inquire why there are so many

cases of restriction in range.

It is possible to see, at any rate, three causes which are responsible for

a large number of such cases. In the first place, a given species of animal

must have originated at a certain spot; its multiplication in individuals

must always be a slow matter, since enemies, and untoward events generally,

would conspire to check the natural multiplication by geometrical

progression. A long time might therefore elapse before the species greatly

extended its range. A restricted distribution may therefore, in some cases,

mean a modern race. In the second place, there are definite physical

barriers which check the migration of species. The terrestrial Mammalia

cannot cross wide arms of the sea; that they can and do swim for

considerable distances has been proved in several instances; but, as has

been pointed out, it is unlikely that a purely terrestrial mammal would

voluntarily swim out into an unknown sea. And then if it did, and

successfully reached the opposite side, nothing would happen unless it were

a pregnant female; or, if not pregnant, till a male swam very soon

afterwards in exactly the same direction. Many travellers have told of

floating islands, formed of torn-up trees and brushwood, which have been

seen at the mouths of large rivers, with animal passengers upon them. These

are, however, so much at the mercy of currents and storms, {81} that but

little reliance can be placed on them as a means of transit; besides, here

again, two individuals, or a pregnant female, would be required to effect a

settlement on a foreign shore. The existence of oceanic islands is often

urged as a proof of this inability to cross tracts of sea; even those which

are comparatively near an extensive continent, such as, for example,

Fernando Noronha in the Atlantic, are destitute of mammals (except, indeed,

the ubiquitous Mouse, which is believed to have been carried there, often

in company with the equally widely-spread Rat, in ships). This argument,

however, is not so conclusive as might appear; it doubtless is in the case

of far-distant islands. But the size of the islands has to be taken into

account. For there are islands, such as the Galapagos, or, to take a less

contested instance, some of the islands of the Malagasy Archipelago,

undoubtedly continental, which have an exceedingly reduced number of

mammals. An area of a certain size seems to be a necessity.

The converse of this is in many cases easy to show, that is, the wide range

of animals when there are no marine barriers to stop their spreading. John

Hunter, the celebrated anatomist and surgeon (not often quoted, however, as

an authority upon geographical distribution), observes: "It is a curious

circumstance in the natural history of animals to find most of the northern

animals the same both on the continent of America and what is called the

Old World, while those of the warmer parts of both continents are not so.

Thus we find the bear, fox, wolf, elk, reindeer, ptarmigan, etc., in the

northern parts of both.... The reason why the same animals are to be found

in the northern parts is the nearness of the two continents. They are so

near as to be within the power of accident to bring the animals, especially

the large ones, from one continent to the other either on the ice or even

by water. But the continents diverging from each other southward, so as to

be at a very considerable distance from each other even beyond the flight

of birds, is the reason why the quadrupeds are not the same."

There is no doubt, in fact, that the ocean is the most insuperable of all

barriers to the dispersal of mammals. In a less degree mountain ranges and

deserts are also barriers. The Desert of Sahara is a striking instance to

the point; it separates two exceedingly different faunas. {82}

A third cause of more or less limited range is the barrier due to

competition. If the ground is already taken up, there is no room for new

immigrants. There is obviously a limit to the number of Antelopes or Deer

that can graze upon a given tract of grassy plain. These two groups of

Ungulates illustrate the matter well: the Antelopes are African and Indian,

especially the former, while Africa has no Deer at all; America, on the

other hand, has plenty of Deer but no Antelopes, save the Prong-horn. The

more nearly akin the two species or groups of species are, the fiercer will

be the competition; for a near kinship will at least often imply similar

habits, the need for similar food, and other likenesses which will prevent

both from successfully occupying the same tract of country. The remarkable

fauna of Australia is believed to afford an example of this. In that

country the prevalent inhabitants are the Marsupials. The Monotremes are

found there also, and nowhere else save in New Guinea and Tasmania. The

remaining mammals are inconspicuous; they embrace a few Rodents and Bats,

and the doubtfully indigenous Dingo-dog. Now the Marsupials are fitted to

every variety of life. We have the grazing Kangaroos and Wallabies, the

burrowing Wombats, the arboreal Phalangers, and the carnivorous Dasyures.

In the second place, it is an unquestioned fact that the Marsupials are an

older race than are the existing Eutherian mammals; they were the dominant

mammals during the Secondary epoch. At that time they were more widely

distributed than at present. In most parts of the world they are now

absent, since they have been successfully ousted by the more highly

organised groups of Eutheria. But at that period, when the higher Eutheria

were in the ascendant, Australia and the islands to the north became cut

off from Asia, and thus became freed from inroads of Eutheria, which were

partly prevented by the physical barrier of the sea from effecting a

settlement, and partly perhaps prevented owing to the ground being already

taken up by the Marsupials. Likeness of habit gave the older inhabitants

victory in the struggle for existence.

The general statements that have been here made are in accord with current

opinion upon the factors of geographical distribution. But the past range

of animals appears to be less consonant with the received views. In the

Tertiary {83} period, groups of animals had often a far wider range than at

present. To-day the Rhinoceroses are limited to Asia and Africa, and to

quite limited parts of the former continent. In the past, these animals

were abundant in Europe and North America. Wild Horses now have a range

which is not widely different from that of the Rhinoceroses, save that they

extend into the more northern regions of Asia. Their remains are abundant

both in North and South America. The Hippopotamus, now confined to Africa,

once ranged over Europe, Madagascar, and India. There were plenty of

American and European Lemurs. Elephants were nearly world-wide in their

range; and, in short, restricted distribution seems to be on the whole a

characteristic of animals of the present day.

These statements, however, though perfectly true, must not lead to

erroneous inferences. It is rather impressed upon the reader, in books

which contain sections dealing with geographical distribution, that animals

on the whole occupy more restricted areas at present than in the past.

There are, however, plenty of examples of groups of extinct creatures which

had, so far as we know, quite a restricted range. Thus the Toxodonts were

purely South American, as were the Glyptodonts and some other forms. And,

on the other hand, the Cervidae of to-day are as widely, if not more

widely, distributed than at any other time. The Hares and Rabbits are now

nearly universal in range; the Cats almost so. We meet with Bovidae, even

excluding the Sheep and Goats, in all the four quarters of the globe,

excluding only South America and, of course, Australia. The Camelidae are

still common to both the Old and the New Worlds.

During certain periods of the Tertiary epoch it is true that there was more

similarity between Europe and North America than there is at present. It

would have been quite necessary to unite both into a Holarctic area, such

as is now insisted upon by many; but the reasons for this union would then

have been stronger. The fact is, however, that the closer resemblances were

due to the larger number of families of animals which existed then than

now; these have decayed away from both continents, and allowed the

unlikenesses between the mammalian fauna of both to become evident. But the

likenesses which still survive have led many to associate the two regions

closely together.

So far as the history of a genus or family or larger division {84} can be

traced, it results as a conclusion that from a given area of origin the

group in question migrated in all directions where possible to a varying

degree; it then died out in intervening tracts, or was left only in a

certain part of its former and more extensive area of range.

ZOOLOGICAL REGIONS.--Seeing that each species of animal has its own

definite range, it is clear that the earth's surface can be apportioned

into divisions which are characterised by their animal inhabitants. We

shall divide the earth into realms, which are the largest divisions; then

into regions; and finally into subregions. It must be borne in mind that

the various groups of the animal kingdom are of different ages,

geologically speaking, and have therefore had less or more time, as the

case may be, to settle down into their present distribution, and that

different animals differ greatly in their rate of multiplication, their

power of migration, and their susceptibility to the effectiveness of

various natural and other barriers to distribution. It is not, therefore,

possible to divide the world into realms and regions which shall express

the facts of distribution of the entire animal kingdom. Such divisions,

which are common in text-books of zoology having but a small section

devoted to zoogeography, are at best mere approximations and averages; no

good is gained by taking such a comprehensive view of the matter, as the

essential object of subdividing the earth's surface is thereby lost sight

of. The zoogeographical division of the earth which will be adopted here is

that originally recommended by Dr. Blanford, and now accepted by a number

of authorities. There are three "realms," to which a fourth may perhaps be

added--though on negative grounds, and merely for the purpose of

emphasising the parts of the world to which mammals have not gained access.

The realms are again divisible into regions, at least in the case of one of

them, and the regions may be again separated into more or less distinct

subregions or provinces. The three primary divisions or realms which

contain mammals are the Notogaean, including Australia and certain islands

to the north of it; the Neogaean, or the South American continent and

Central America; the Arctogaean, including the continents of North America,

Europe, Asia, and Africa, together with the adjacent islands, such as the

West Indies, East Indies (exclusive of those which fall within {85} the

realm of Notogaea), and Madagascar; and finally, the realm of Antarctogaea

or Atheriogaea, which embraces New Zealand, the Antarctic continent, and a

series of islands such as South Georgia and Kerguelen, and possibly even

the extreme south of Patagonia. This latter quarter of the globe will need

no further reference, as it has no truly indigenous terrestrial mammalian

inhabitants. We cannot include the Bats in this statement, as their

distribution is due to different powers of extending their range, and to

different barriers from those which govern the range of other groups of

mammals.

(1) Notogaea.[44] This realm is characterised by the exclusive possession

of the Monotremes:--that is to say, one of the two primary divisions of the

Mammalia is absolutely restricted to this area. It contains, moreover, the

vast majority of the Marsupials. Further, the realm of Notogaea is to be

distinguished by the entire absence of the higher mammals, with the

exception of a few small Rodents. (The Bats are ignored for the reasons

stated, and the Dingo is believed to have been an importation.) It cannot

be disputed that this is a very distinctly-marked area of the earth's

surface.

(2) Neogaea. The continent of South America has no Monotremes and only a

few Marsupials, all of which, with the exception of _Caenolestes_, belong

to the Polyprotodont division of that order, and to a peculiar family,

Didelphyidae. The recent discovery of other fossil Marsupials, however, to

some extent favours Huxley's view that Neogaea and Notogaea form one realm

as opposed to the rest of the world. Besides this, Neogaea possesses the

Edentata, which are found nowhere else;--that is, the division of the

Edentata to which the name is now restricted by some authorities. It is

also characterised by the nearly entire absence of the important order of

Insectivora; and, as minor marks of distinction, by the absence of

Antelopes, Oxen and Sheep, of the Ichneumon tribe, of Horses, and of

Lemurs. It has the exclusive possession of the Hapalidae and Cebidae, and

of several families of Rodents.

(3) Arctogaea. This vast realm is clearly capable of subdivision into four

regions, which will be considered in detail later. In the meantime the

points of likeness between these subdivisions is more marked than are

either the resemblances or the {86} differences of any one of them to

either of the two realms which have just been defined. The two realms that

have been discussed retain their distinctness from each other and from

Arctogaea for a considerable way back into the Tertiary period. It is not

until we reach very early Tertiary times that Edentates are met with in

North America; and then it cannot be regarded as absolutely settled that

the Ganodonta are really the forerunners of the Armadillos, Sloths, etc.

Nor do we find Marsupials in Europe until far back in time, and at a

corresponding period in North America. Indeed the fauna of South America in

late Tertiary times was even more distinct than it is now; for then we had

confined to that region the Toxodonts, Glyptodonts, _Macrauchenia_, and

other forms, while in Australia there were still Marsupials. In late

Tertiary times Europe and India were by no means so distinct from Africa as

they are to-day. North America does not resemble the Old World quite so

much as the subdivisions of the Old World resemble each other; but, as will

be pointed out later, there are and were very substantial agreements. The

Elephants, Rhinoceroses, Giraffe, Hippopotamus, _Orycteropus_, are now

distinctively African or Indian animals; but all these genera, or at least

families (in the case of the Giraffe), have occurred in Europe during quite

recent times. _Lycaon_ indeed, now confined to Africa, is thought to have

had a European origin from its occurrence in caves there. The Hyaena and

the Lion, certain members of the Horse tribe, Apes, and other animals, were

also but are not now European.

India again, and the Oriental region generally, once possessed the

Hippopotamus, the Chimpanzee, Giraffidae, the Antelopes, _Cobus_,

_Hippotragus_, _Strepsiceros_, and _Orias_, which are now purely African

animals. It shares at present with the Ethiopian region the Catarhines,

including the Anthropoid Apes, the Lemurs, Tragulina (the genus

_Dorcatherium_ is also known from fossils in India), _Manis_, _Hyaena_, the

Cheetah, Elephant, Rhinoceros, and the Ratel. There is, in fact, no order

of mammals which is now absent from one of these three regions though

present in the others, save the Lemurs, and they occurred in past times in

Europe. The Tapir of India is known fossil in Europe, and the latter

continent had its Monkeys and even Anthropoids. On the other hand, North

America is more distinct. It has no Lemurs, Apes, Elephants, Rhinoceroses,

Tapirs, Old World Edentates {87} (Effodientia), Viverridae, Horses, or

Antelopes, excepting _Antilocapra_, a type of a separate division of

Bovidae. But since several of these groups have been represented in recent

times, no primary line of division can be profitably drawn.

Arctogaea as a whole may be characterised by both negative and positive

characters. As negative features may be mentioned;--the entire absence of

Edentates (_Necrodasypus_ of Filhol is rather doubtful, see p. 164, n.),

though a few crept up into the Nearctic region from Neogaea during past

times; and of Hapalidae, Cebidae, and Marsupials, except an Opossum in

North America. This realm has, on the other hand, all the Lemurs, all the

Insectivores with the exception of the West Indian _Solenodon_, all the

Proboscidea, Rhinoceroses, Horses, Deer, Antelopes, the last group

including the Oxen and a variety of other important families. It is in fact

the headquarters of all the Eutheria with the exception of the Edentata and

Marsupials.

The subdivisions of this realm have been variously effected. The classical

subdivisions are of course those of Mr. Sclater, who would recognise (1)

the Nearctic, North America; (2) the Palaearctic, including Europe,

Northern Asia, and Japan; (3) the Oriental, including Asia south of the

Himalayas and the islands of the Malay Archipelago as far east as the

Australian region; and (4) the Ethiopian, _i.e._ tropical Africa and

Madagascar. Some would alter this by uniting America and the north of the

Old World into a Holarctic region, separating off the southern parts of the

North American continent into a Sonoran region. To some, the claims of

Madagascar to form a separate region are convincing. To distinguish the

boundaries of the several regions is a difficult task; they dovetail into

each other on the frontiers with the complex curves of a puzzle-map. The

difficulty has been grappled with by the suggestion of intermediate

transitional areas; but this proceeding really doubles the difficulty, for

there are then two frontiers to delimit in each case instead of only one.

The animal inhabitants must be expected to mingle somewhat at the lines of

junction of one region with another.

The Sonoran region does not appear to us to have great claims to

recognition. It shows a mingling of southern with northern forms exactly as

might be expected. An Armadillo and _Didelphys_ have, as it is believed,

invaded it from the Neogaeic realm; it possesses also the South American

genera, _Dicotyles_, _Nasua_, {88} _Conepatus_, _Sigmodon_. On the other

hand, the Sonoran genera _Antilocapra_, _Cynomys_, _Procyon_, and the

Insectivora _Blarina_ and _Scapanus_, extend further north. Peculiar to

this region are only six genera of Rodents, which seems an insufficient

reason for raising the Sonoran province to the dignity of a region.

Considered from the point of view of numbers of peculiar forms, the

Thibetan subregion has more claims to distinction as a region; for confined

to that area we have the genera _Nectogale_, _Aeluropus_, _Eupetaurus_,

_Pantholops_, _Budorcas_; while by slightly extending its limits, a number

of other peculiar forms might be added. Madagascar has distinctly more

claims to regional division. Absolutely confined to it are eleven of the

seventeen existing genera of Lemurs, the family Centetidae among the

Insectivora, which contains seven genera, and another recently discovered

and peculiar genus, _Geogale_; it has six peculiar genera of Viverridae; it

has five peculiar genera of Rodents. In addition to this it is negatively

characterised by the absence of the following typical African animals,

Felidae, Proboscidea, Rhinocerotidae, Equidae, Monkeys, etc. It seems to be

impossible to avoid allowing the rank of a region to this part of the

world.

In separating the Nearctic from the Palaearctic region, stress must be laid

rather upon the absence of Asiatic and European forms from North America

than upon the existence in the northern half of the New World of many

peculiar forms. Peculiar to the Nearctic are the Goat genus _Haploceros_,

the Rodents _Erethizon_, _Zapus_, and the family Haplodontidae. The Mole

genus _Condylura_ is also restricted to this part of the New World. Even so

it has more peculiar forms than the Sonoran. If we add to this the absence

of Horses, Antelopes except _Antilocapra_, Pigs, Hyaenas, etc., there are

strong grounds for retaining this division. It must be agreed, however,

that it comes rather nearer to the Eurasian district than the latter does

to the Oriental.

The Oriental region has many characteristic animals. It has among the

Anthropoid Apes the Orangs and Gibbons; of Old World Apes it has confined

to its own area the genera _Semnopithecus_ and _Nasalis_. Of Lemurs there

are _Loris_ and _Nycticebus_, and _Tarsius_, representing a family of that

order, or even a sub-order. The Galeopithecidae are entirely Malayan. There

are many Rodent, Carnivorous, and Insectivorous genera; the Rhinoceroses

and the Elephant of this region differ from those of Africa. {89}

_Tragulus_ concludes a sample from a very rich list of peculiar forms.

The Ethiopian region has also its Anthropoids, the Gorilla and the

Chimpanzee, but they belong to genera or a genus different from those which

include the Oriental forms. There are five peculiar genera of

Cercopithecidae. The Lemurs restricted to this region are _Galago_,

_Perodicticus_ and _Arctocebus_. The peculiar Insectivorous families

Macroscelidae and Chrysochloridae are only found here, besides many other

peculiar genera. Africa is especially the home of Antelopes, and the

Giraffe is not found now outside its borders. The Elephant and the

Rhinoceroses are of different species from those of India. There are many

peculiar Rodents and Ungulates.

* * * * *

{90}

CHAPTER III

THE POSSIBLE FORERUNNERS OF THE MAMMALIA

The relationship of Mammals to Vertebrates lying below them in the scale,

their origin in fact, is a much-debated question, with many attempted

solutions. To enter into this large question in detail would involve a

great deal of useless statement of arguments founded upon misleading or

upon quite inaccurate "facts." It will perhaps be sufficient if we reflect

here the current view most in vogue at the present, _i.e._ that which would

refer the Mammalia to reptiles belonging to the extinct Permian and

Triassic group of the Theromorpha (also called Anomodontia). These have

been explored lately to a very large extent, and chiefly by Professor

Seeley.[45] The very fact that a genus _Tritylodon_, only known by the

forepart of the skull, has been called Mammalian and Anomodont by various

authors, shows at least the difficulty of differentiating the two groups

when the material for study is imperfect. As a matter of fact these

Theromorpha are without doubt reptiles; they show, for example, a lower jaw

formed out of several distinct pieces, of which the articular articulates

with a fixed quadrate on the skull. They possess the characteristic

reptilian bones, the "transverse," the pre- and post-frontals, and there

are various other points of structure which leave no room for doubt as to

their truly reptilian nature. There are, however, numerous indications of

an evolution in the mammalian direction in all parts of the skeleton, to

the more important of which some reference will be made here. It may be as

well to clear the {91} ground by mentioning the fact that among the

Theromorpha four distinct types of reptiles are included, which are

considered to form four orders, _i.e._ the Pareiasauri, the Theriodontia,

the Anomodontia (Dicynodontia), and the Placodontia.

The first of these divisions includes what seem to be basal forms. These

reptiles show numerous points of likeness to the Amphibian

Labyrinthodonts.[46] On the other hand the third division, that of the

Dicynodontia, are highly-specialised Theromorpha, from which no further

evolution would appear to have been possible. Thus the dentition was either

completely lost, or reduced to tusks as in _Dicynodon_. We need not

therefore concern ourselves in the present volume with these Anomodonts. It

is with the Theriodonts that our business lies. The very name, be it

observed, is aptly chosen on the hypothesis to be explained here; but it is

not only in the teeth that these reptiles show likenesses to the Theria or

Mammals, but in almost every feature of their organisation. Unlike other

reptiles, the Theromorpha in general were lifted comparatively high above

the ground on legs of fair length and of mammalian relationship in the

position of the segments of the limbs. The typical reptile grovels upon the

earth with legs sprawling out, as indeed the very name suggests. One bar to

the Theriodonts being on the direct line of mammalian ancestry has been

urged as a preliminary difficulty, and that is their large size. The

earliest undoubted mammals were small creatures, comparable to a Rat or a

Mouse in size; whereas a good-sized Bear or a Wolf is a better standard of

size for some of the best-known genera of Theriodonts. It has, however,

been quite permissibly suggested that living in company with these large

Theriodonts were less obtrusive genera, from which the mammals might have

sprung. It is so familiar a fact that a given group of animals generally

contains giants, dwarfs, and members of intermediate size, that this

suggestion may almost be accepted as a fact. It need at least present no

difficulties to us in our comparisons.

The most salient "mammalian" feature of the Theriodonts is the heterodonty

of the teeth, the pattern of the "molars," and the limited number which

constitute the series. The fact, too, that they are limited to the dentary

bones below and to the {92} maxillae and the premaxillae above, is a _sine

qua non_ for mammalian comparison. In the more basal Theromorpha the teeth

are not so limited in position. Finally, to complete the remarkable

mammalian resemblance of the teeth of these reptiles, it must be mentioned

that in _Tritylodon_ and _Diademodon_ the roots of the molars, as we may

fairly term them, though not actually divided after the mammalian fashion,

were deeply marked by a groove, which suggests an incipient division or a

fusion of two distinct roots. Some of these facts of structure may now be

considered in further detail. As to the incisors and canines, it is

sufficient to say that the numbers of the former, and the shape of the

latter, are in perfect consonance with a derivation of the Mammalia from

this group. The molar series can be divided into premolars and molars, at

least in so far as regards their shape; for the anterior teeth are often

smaller and less complicated than those which follow, as is often the case

with the two series in mammals. The molar series also consist of teeth in

close apposition to each other and separated from the canines by a

diastema, which is a character of mammalian teeth. The fact that in the

reptile _Cynognathus_ and the mammal _Myrmecobius_ there are nine of these

molar teeth in each half of each jaw is perhaps not a point upon which it

is desirable to dwell with too much weight; but the general fact that the

molars are further reduced in some genera of Theriodontia than in that

which has been mentioned, is clearly a matter of significance when the

ancestry of the mammals is under consideration.

The most interesting fact about the molar series in the Theriodontia is

that we meet with the two types of molars that occur in the mammals.

_Cynognathus_ and other genera have molars which consist of a main cusp,

and of one cusp before and one after the main cusp; in fact these teeth are

triconodont as in certain early mammals, a state of affairs which is

believed by the "trituberculists" (see p. 56) to have preceded the

tritubercular tooth. There are also "multitubercular" teeth, especially

well developed in _Tritylodon_, where they exactly resemble those of

certain Multituberculata, and whose structure originally led to the placing

of _Tritylodon_ among the mammals of that group. If there is any question

about the mammalian nature of this fossil, there remain several other

Theriodontia in which the multituberculism is well marked. It is so in

_Trirhackodon_ {93} and in _Diademodon_ for instance. This incidentally

lends some support to the idea that the Mammalia have been evolved from two

sources, a way of looking at the origin of the group that will coincide

with the views of some authors like the late Dr. Mivart, and will at the

same time reconcile the trituberculists and the multituberculists. For we

should then assume that the Eutheria and Triconodontia had originated from

some such form as _Cynognathus_; and the Multituberculata and the existing

Monotremes from some form like _Diademodon_. It is not of great use to

point out that _Diademodon_ is really of the trituberculate pattern,

because in its molars, though multituberculate, the trituberculate main

cones can be recognised; for that state of affairs could just as well have

been brought about by a reduction from the multituberculate type. The skull

of these Theriodonts shows some well-marked approximations to the mammalian

type. There is in the first place a commencing consolidation and reduction

of the individual bones, which is so distinguishing a feature of the

mammalian skull as opposed to the skull of lower vertebrates. In

_Cynognathus_ the postorbital is fused with the jugal, and the

supratemporal with the squamosal, forming apparently one bone. In the lower

jaw the splenial is often reduced to the thinness of paper, thus indicating

a commencing disappearance. In many Theromorpha the squamosal shares

largely in the formation of the articular facet for the lower jaw,

obviously an important mammalian characteristic; this is brought about by

the reduction of the quadrate, which latter bone, moreover, acquires in

certain particulars the appearance of the mammalian malleus, with which it

is, according to many, homologous. But this subject has been already dealt

with on page 26. A very pronounced likeness to the mammalian skull is that

there are two occipital condyles. That this has been brought about by the

further development of a tripartite condyle such as occurs in tortoises,

and that by the suppression of the basi-occipital part, does not affect the

resemblance to the mammalian skull; in fact it explains the origin of two

condyles from the typical reptilian single condyle, and disposes of the

necessity for believing, with Huxley and others, the Amphibia to be on the

main line of mammalian evolution on account of their two condyles. The

general aspect of the skull in _Cynognathus_ has been {94} compared to that

"of _Thylacinus_ or _Dissacus_." No one can examine the actual sketches of

the skull of that Theriodont without endorsing that opinion. As a curious

detailed point of likeness to certain Mammalia may be mentioned "a small

descending process of the malar bone, which may be a diminutive

representative of the descending element of the malar seen in _Elotherium_,

_Nototherium_, _Diprotodon_, _Macropus_, certain Edentata, such as

_Glyptodon_, _Megatherium_, _Mylodon_, _Bradypus_, but unparalleled so far

as I am aware in fossil reptiles." (Osborn.) The zoologist cannot help

being impressed with the significance of small details of similarity, which

do not seem to be due in any way to surrounding conditions of life, and

thus referable to mere convergence, like the fish-like form of Whales and

Seals.

The rest of the skeleton of the Theriodontia is by no means so well known

as the skull and teeth. But from what is known, other mammalian characters

can be pointed out. Perhaps the most striking mammalian feature is to be

found in the scapula of _Cynognathus_. It is in this creature somewhat

narrow and elongated; but it has a well-marked spine, ending in a hooked

acromion. Now it is to be noted in support, so far, of the diphyletic

origin of mammals, that in the Monotreme, as in Whales indeed, the spine

forms the anterior border of the scapula, and is coincident with it, there

being thus no prescapula at all in the Monotreme, and only a trace of it in

certain Whales.[47] Whether the multituberculate _Tritylodon_ or

_Diademodon_ had a scapula after the Monotreme pattern is not known; but it

is clear that the scapula of the triconodont _Cynognathus_ is quite after

the pattern of the Eutherian scapula. Furthermore, Professor Seeley is of

opinion that the coracoid was relatively small, and indeed smaller than the

same bone in Edentates, and _a fortiori_ than in Monotremes. Another fact

of structure which points also, possibly, in the direction of a diphyletic

origin for the Mammalia, is the double-headed ribs of _Cynognathus_. As is

well known, the ribs of the Monotremata have only the central head, the

capitulum.

As a general mark of affinity with mammals the reduction of the intercentra

in _Cynognathus_ may be noted, and also the existence of a small though

perfectly obvious obturator-foramen, separating the pubis from the ischium.

There are further details {95} which tend in the same direction. And we

shall probably not go far wrong in the present state of our knowledge if we

assign the origin of the mammals to some type which would be included in

the order Theriodontia or at least in the sub-class Theromorpha.

* * * * *

{96}

CHAPTER IV

THE DAWN OF MAMMALIAN LIFE

The animals that we considered in the last chapter, though showing certain

unmistakable likenesses to the mammals, are nevertheless unquestionably not

mammals but reptiles. In the Triassic strata, however, we first meet with

the remains of undoubted mammals. The Mammalia first appeared upon the

earth in a tentative and hesitating way: they had not cast off many of the

characters of their supposed reptilian forefathers; they shrank from

observation and destruction by their small size, and apparently, so far at

any rate as their teeth afford a clue, by an omnivorous diet. The world

abounded at that period in large and carnivorous reptiles, which may indeed

have been the principal enemies with which the first mammals had to cope.

These early mammals lingered on to so late a period as the Eocene; but the

majority of the genera were Triassic, Jurassic, and Cretaceous. Certain of

the primitive mammalian forms have been referred to the Marsupials, and

their resemblances to the Monotremata have also been pointed out. The

current view of the present time, however, is that they form a special

order, which may possibly have embraced the ancestors of both Marsupials

and Monotremes; for it is reasonable to explain in this way the combination

of characters of these two orders which they present. For this group the

name Allotheria has been proposed by Marsh, and Multituberculata by Cope;

the latter term is the less suitable, in that the Monotremata

(_Ornithorhynchus_) are also "multituberculate." The group is known in a

very imperfect fashion. The remains are but few and fragmentary; and for

the most part we have only a few teeth to speculate upon. This is natural

enough, for the harder teeth might easily be supposed to {97} have resisted

the decay which would more readily affect the softer bones. Where there are

bones it is frequently the lower jaw alone which has been preserved for

us--a bone which has also been preserved in the case of some of the

contemporary Marsupials.

It has been pointed out (from the observation of dead dogs floating in

canals) that the lower jaw is occasionally detached from the carcase. It is

the most readily separable part which contains a skeleton. It may be,

therefore, that the remains of these early mammals, floating down some

river to the sea, may have lost their jaws while in the river, or at

furthest in the shallow waters of the sea, the rest of the carcase floating

out to a greater distance, and being finally entombed in the stomach of

some carnivorous fish, or in the mud at the bottom of a deep ocean, which

has never since seen the light.

The characters of this group are really more those of the Monotremata than

of the Marsupials. The undoubted likeness which their molar teeth show to

the temporary teeth of the Platypus have already been commented upon. Like

the Monotremes the Allotheria appear to have possessed a large and

independent coracoid; the evidence for this rests upon the discovery of the

lower end of a scapula of _Camptomus_, a Cretaceous genus from North

America upon which there is a distinct facet for the articulation of what

can have been nothing else than a coracoid. On the other hand they differ

from the Monotremata by the presence of incisor teeth which were

Rodent-like in form, and not very different from those of certain

Marsupials. This point of difference cannot be regarded as of very

first-rate importance; no one would relegate the Sloth and the Armadillo to

different orders on account of their tooth differences, which are about on

a par with those to which we have just referred. It seems indeed likely

that it will be ultimately necessary to rub out the boundary line which now

divides the Allotheria and the Monotremata.

The Plagiaulacidae are unquestionably mammals, and they are placed by most

naturalists in this at present uncertain group of Multituberculata, which

will be retained here in deference to the distinguished authorities who

have instituted the group, though there are but few characters by which it

can be defined. This family though appearing in the Trias, extends down in

time to the Eocene. The type-genus, that which has given its name to {98}

the family, is _Plagiaulax_. As it is not Triassic, the consideration of

its characters will be deferred until later. _Microlestes_ is a Rhaetic

genus, known from rocks in Germany and England; but it is entirely based

upon molar teeth. _M. antiquus_ has a two-rooted molar of an elongated form

with a row of tubercles on either side of a median groove, which traverses

the long axis of the tooth. To some extent the teeth of the ancient form

resemble those of _Ornithorhynchus_. _Microlestes_ has been sometimes

spoken of as a Marsupial, but Mr. Tomes[48] has found that it does not show

one very universal character of the Marsupial teeth: it has not those

continuations of the dentinal tubes which traverse the enamel in all

Marsupials that have been examined with the sole exception of the Wombat.

The rarity of the remains of mammals in these earliest rocks of the

Secondary epoch has been accounted for in another way from that which has

been suggested above. It may be that the group Mammalia was not evolved in

Europe at all, and that the stray remains which have been found in that

continent represent the fragmentary remnants of a few scattered immigrants

which heralded the later invasion of more numerous genera during the

Jurassic period.

THE MAMMALS OF THE JURASSIC PERIOD.--Some of the Allotheria or

Multituberculata described in the last section occur in the rocks of this

early part of the Secondary epoch. They are doubtful in position, as

already stated; some of them indeed, as for instance _Tritylodon_ and

_Dromatherium_, are possibly not mammals at all, while the remainder

probably belong to a non-existent order of mammals. Along with these

dubious creatures are the fragmentary remains of small animals which are

not merely mammals, but in all probability definitely Marsupials. It is

true that here again we have little beyond lower jaws and teeth to deal

with; so that there may be less certainty in referring them to the

Marsupials than appears to be the opinion of the majority of

Palaeontologists.

Professor Osborn in fact considers that the Mesozoic mammals consist of

three groups: (1) The Multituberculata, including the Bolodontidae,

Stereognathidae, Plagiaulacidae, Polymastodontidae, and possibly the

Tritylodontidae (which, however, are regarded by him and by others as more

probably reptiles of the {99} Theromorphous group). (2) The Triconodonta,

which were Marsupials, though in all probability with a complete succession

of teeth and with an allantoic placentation. This group will include the

genera _Phascolotherium_ and _Amphilestes_, as well as _Triconodon_ and

_Spalacotherium_. Finally we have (3) the Trituberculata (or Insectivora

Primitiva) with the genera _Amphitherium_, _Peramus_, _Amblotherium_,

_Stylacodon_, and _Dryolestes_.

We shall take these three groups in order. The Multituberculata have

already been to some extent defined, if such a word can be used to express

the summation of the very scanty information at our disposal. Of this

group, _Plagiaulax_ is a genus which occurs in the Purbeck beds; it is only

known by lower jaws implying an animal of the size of a Rat or rather

smaller. The jaws have in front a large incisor which looks Rodent-like,

and also like those of the Diprotodont Marsupials; but it is held that

these teeth did not grow from persistent pulps, and there is in any case no

anterior thickened coating of enamel. Canines are absent; the diastema is

followed by four premolars increasing progressively in size and possessing

somewhat complicated grinding surfaces. These surfaces are formed by

several obliquely-set ridges. The succeeding teeth are termed molars on

account of their difference in structure, and there are but two of them on

each side. The molars are of a pattern common in the Multituberculata; the

centre is hollowed, and the raised rim is beset by tubercles. Other

Jurassic genera of Multituberculates are _Bolodon_, _Allodon_, and

_Stereognathus_. All of these possess the same multituberculate molars.

Of the Triconodonta the type-genus is _Triconodon_. This genus is better

known than most Jurassic mammals, since both the upper and the lower

dentition have been described. It appears to have possessed the typical

Eutherian dentition of forty-four teeth, to which a fourth molar is added

in some species. The great difference between the molars and premolars

argues a complete tooth-change. The genus is American as well as European.

_Spalacotherium_ has more molars, five or six.

_Phascolotherium bucklandi_, on the other hand, is a much older type in the

form of its teeth. There are, however, not so many of them as in

_Amphitherium_; _Phascolotherium_ has but two premolars and five molars,

making a total of forty-eight teeth. The teeth are of the triconodont form,

the three cusps being in line, and the middle one the largest. {100}

_Amphilestes_ has teeth of the same pattern but has more of them, the

premolars and molars being respectively four and five. All these animals

had the lower jaw inflected. Whether they are all Marsupials or not, it is

clear that _Phascolotherium_ and _Amphilestes_ should be united and placed

away from _Amphitherium_ on account of the more primitive form of their

teeth.

We next come to the Trituberculata.

Among the most celebrated of these remains are a few jaws discovered in the

Stonesfield slates near Oxford, and examined by Buckland, Cuvier, and some

of the most eminent naturalists of the beginning of the last century. These

jaws have been lately submitted to a careful re-examination by Mr.

Goodrich,[49] who has increased our knowledge of the subject by exposing

from the rocky matrix in which the jaws lie fresh details of their

structure; it is probable therefore that now all that there is to be learnt

from these specimens has been recorded.

_Amphitherium prevostii_ was a creature about the size of a Rat. Its jaw

was first brought to Dean Buckland about the year 1814, and described six

years later. Buckland thought the jaw to be that of an Opossum, an opinion

in which Cuvier concurred. The jaw, however, is marked by a groove running

along its length, and this groove was regarded by de Blainville as evidence

of the composition of the jaw out of more than one element, which would

naturally lead to its being regarded as the jaw of a reptile.[50] This

species and another named after Sir Richard Owen have a dental formula

which, like that of the Marsupials, is large as compared with that of the

Placental mammals; it runs: I 4, C 1, Pm 5, M 6--_i.e._ 64 teeth

altogether. This is a larger number than we find in any existing Marsupial.

But as in Marsupials, and in certain Insectivora also, the angle of the jaw

is inflected. These teeth are of the tritubercular pattern with a "heel."

They are in fact closely like those of the living _Myrmecobius_; but not,

it should be remarked, unlike those of certain Insectivora.

THE MAMMALS OF THE CRETACEOUS PERIOD.--At one time there was a totally

inexplicable gap between the Jurassic and the basal Eocene, a series of

strata which occupy an enormous expanse of time in the history of the earth

having appeared to {101} be devoid of mammalian remains. This gap, however,

has been filled up by the discovery of mammalian remains in the North

American Laramic formation, which seems to be clearly of Cretaceous age.

Furthermore, it is held by some that the Purbeck beds are more properly to

be placed with the Cretaceous, which would then necessitate the

consideration under the present heading of some of the types already dealt

with; and if, as is suggested in the following section, the lowest

so-called Eocene beds are really referable to the Cretaceous, there is no

lack of mammalian remains in that period. And, moreover, it was in that

case the Cretaceous period which witnessed the evolution of the existing

orders of Placental mammals. Otherwise the mammalian remains of the

Cretaceous agree with those of the Jurassic. We find remains of the

Multituberculata in fragments of Plagiaulacidae and Polymastodontidae.

_Ptilodus_ is a genus which has two premolars; and _Meniscoessus_ is

another multituberculate from the same Laramic formation. The other

detached fragments of mammals are thought by Osborn to represent both

Placentals and Marsupials.

THE MAMMALS OF THE TERTIARY PERIOD.--Unless the lowest beds of the earliest

Tertiary period, the Eocene, such as the Torrejon of North America, should

be in reality referred to the Cretaceous, there is no evidence that the

modern groups of Mammalia existed before the present epoch of the earth's

history. It is probable, however, that the Eutheria as a group were

Mesozoic. The fossil jaws that have been considered in the last chapter may

quite probably be primitive Eutherians, or even divisible, as believed by

Professor Osborn, into Marsupials and Insectivores. In the Tertiary,

however, apart from the question as to the nature of the Puerco and

Torrejon formations, and as to certain South American strata whose fossil

contents have been investigated by Professor Ameghino, we find the first

traces of mammals definitely referable to existing orders, or to be

distinctly compared with existing orders. Since, however, representatives

of types which have obvious relationships to modern types appear in

considerable profusion in the very earliest strata of the Eocene, it seems

clear that much remains to be discovered in beds earlier than these.

Confining ourselves, however, to facts and to comparisons which can be made

on more than a few lower jaws and scattered teeth, which is practically all

that we {102} possess of earlier mammals, we must arrive at the general

conclusion that two of the existing larger groups of the Eutherian,

non-Marsupial, mammals were differentiated at quite the beginning of the

Eocene, and were represented by forms from which it is possible to derive

at least the existing Carnivora, Insectivora, Artiodactyla, and

Perissodactyla. These were the Creodonta and the Ungulate Condylarthra. In

addition to these we may enumerate as very early types the Lemuroidea,

represented by such forms as _Indrodon_ in the New World, and (though

later) by _Necrolemur_, etc., in the Old World, and the Edentata, if we are

to allow as their ancestors the Ganodonta.

The early Eocene strata also contain representatives of at least one order,

the Amblypoda, which increased subsequently, but has died out without

descendants, unless we are to believe with some that the Elephants are to

be derived from these Eocene "pachyderms." In later Eocene times the great

majority of the existing orders, and even subdivisions of orders, are to be

met with; and there are in addition such totally extinct orders as the

Typotheria, Ancylopoda, and Tillodontia. Coupled with this gradual

specialisation in the orders of Eutherian mammals, there is naturally a

vast increase in the number of generic and family types. This culminates

perhaps in the Miocene, from which time there has been a gradual decline in

mammalian variety, so that it is justly said that we live now in an epoch

which is impoverished of mammals. This gradual decay has persisted until

to-day, as is witnessed by the extinction of the Rhytina and the Quagga,

and the growing rarity of the White Rhinoceros and the American Bison.

The early Eutherian stock consisted of small mammals with small heads and

slender, long tails. The limbs were pentadactyle, ensheathed in claws or

broader hoofs. The fore-limbs may have been partly prehensile. The teeth

were forty-four, completely differentiated into incisors, canines, molars,

and premolars; and there appears to have been a complete diphyodontism. The

canines were not greatly enlarged, and no diastema separated any of the

teeth. The molars were bunodont or of a more cutting pattern, with some

five or six tubercles. These animals were, moreover, very small-brained.

This early stock is represented by Creodont and Condylarthrous animals, the

exact boundaries between which are hardly marked in the {103} very early

types. Professor Osborn has argued that from this early Eutherian stock

there were two waves of progress, or, as he expresses it, "two great

centres of functional radiation."[51]

The first was largely ineffective, the second has produced all the

Eutherian orders of to-day. These two divisions are termed by him

"Mesoplacentalia" and "Cenoplacentalia." The first division embraces the

Amblypoda and their descendants the Coryphodonts and Dinocerata, many of

the Condylarthra, the bulk of the Creodonts and the Tillodonts. These

creatures persisted for a time, but died out in the Miocene. They were

mainly distinguished by the smallness of their brain; the great

specialisation of structure which they exhibit having left that organ

unaffected, and therefore tending in the long run to render them unable to

cope with changes in the inorganic and organic world. The successful

division of the primitive Eutheria comprises the groups which exist at the

present day, and is not connected directly with those small-brained

Mesoplacentals; it has apparently originated, however, from the least

specialised of their ancestors. Professor Osborn thinks, moreover, that the

Lemurs and the Insectivores are persistent descendants of the earlier wave

of Eutherian life. It appears in fact as if Nature had created the existing

Ungulate, Unguiculate, and other types on a defective plan, and, instead of

mending them to suit more modern requirements, had evolved an entirely new

set of similarly-organised types from some of the more ancient and plastic

forms remaining over. The Marsupials may be the only group of the early

wave remaining, and they have been able to hold their own for the

geological reason that Australia was early cut off from communication with

the rest of the world. That they are disappearing seems to be shown by

their gradual diminution as we pass from Australia towards the continent of

Asia, through the islands of the Malay Archipelago. Competition has here

decimated them, as it may do in the remote future in Australia.

It is often said, but with some looseness of statement, that ancient

quadrupeds are huger than their modern representatives. This statement is

partly true in fact, but largely wrong in implication. For it suggests

that--and the suggestion is often expressed in books that are not

authoritative--huge animals {104} have left a dwarfish offspring; that

there were giants of old, and that there is a puny race to-day. As a matter

of fact, the study of the gradual evolution of the early Tertiary Mammalia

into their descendants of later times shows very plainly the truth of this

interesting generalisation: That the primitive types were all small

creatures, and that in those instances where we can trace a pedigree, there

was a gradual increase in size up to a point where greater increase led to

extinction. We point out later on a number of facts illustrating this

matter in detail. It has been ascertained, for instance, that the pedigree

of the Horses, the Camels, the Rhinoceroses, and many other groups,

commences with small forms and culminates in large ones. It may be urged

that such animals as the Tapir are to-day smallish forms, and that related

to them in the past were the gigantic Titanotheres; but in this and similar

cases it will be found that the extinct giants were not in the direct line

of pedigree, but represented side-branches which waxed huge on their own

account and then disappeared.

* * * * *

{105}

CHAPTER V

THE EXISTING ORDERS OF MAMMALS

PROTOTHERIA--MONOTREMATA

Apart from those creatures whose fragmentary remains have been considered

in the last chapter, and which belong to the earliest of mammaliferous

strata, the remains of Mammalia are all referable to existing orders. In

the pages which follow we shall therefore deal with the actual

representatives of living families side by side with their extinct

relatives. The existing orders of Mammalia, together with those of their

fossil allies, can be plainly divided into two great subdivisions, or, as

we shall term them, sub-classes; the Mammalia as a whole being termed a

class of the Vertebrata comparable with the class Reptilia, etc. It has

been usual, owing to the initiative of Professor Huxley, to divide the

Mammalia into three divisions of primary importance. We shall adduce

reasons later for not accepting this mode of division, but that which

allows of only two primary divisions. These two divisions are (1)

Prototheria and (2) Eutheria. Whether the Multituberculata, Trituberculata,

and Triconodonta, considered in the last chapter, are really to be

distributed among these two sub-classes is a matter upon which it is

possible to form an opinion, but not to dogmatise. The Prototheria stand at

the base of the mammalian series, and present many likenesses to the

Sauropsida; the Eutheria are the animals which are most fully

differentiated as mammals. We shall commence with

SUB-CLASS I.--_PROTOTHERIA._

To this group belongs the order Monotremata, and possibly also the

so-called Allotheria or Multituberculata. As, however, {106} the latter are

only known from very fragmentary remains, which are not sufficient to

determine the systematic position of the animals of which they are

fragments, I have not thought it worth while to attempt a serious

definition of the order Multituberculata. I have introduced a short account

of the principal facts which are known concerning the creatures grouped

together under this name into the historical sketch of the progress of

mammalian life in Chapter IV. As to the Monotremata, there is no question

that they are entitled to rank in a group equivalent to that including all

other mammals of which we have sufficient knowledge to construct a

classificatory scheme. There have been, indeed, naturalists, such as

Meckel, who would altogether deny the mammalian rank of these creatures.

The Monotremata or Ornithodelphia may be thus defined:--

Mammalia with no teats, but with a temporary pouch in which the young are

hatched, or to which they are transferred after hatching, and into which

open the ducts of the mammary glands. An anterior abdominal vein, or at

least the membrane supporting it, persists throughout the abdominal cavity.

Heart with an incomplete and largely fleshy right auriculo-ventricular

valve. Brain without a corpus callosum. Shoulder girdle with a large

coracoid reaching the sternum; clavicles and an interclavicle present.

There are "marsupial" or epipubic bones attached to the pelvis. Vertebrae

with no epiphyses for the most part. Ribs with only capitulum and no

tuberculum. Mammary glands of the sudoriparous and not the sebaceous type

of epidermic gland.[52] Oviparous, with a large-yolked and meroblastic

ovum, enclosed within a follicle of two rows of cells.

To call these animals Mammalia is of course an abuse of the meaning of that

word in one sense, but it is not in another; since the pouch of these

Monotremes is, as has been explained elsewhere (p. 16), the real equivalent

of a teat, and not of the pouch of the Marsupials.

The most salient characteristic of this group of mammals in the estimation

of their position in the vertebrate series is not so much the fact that

they are oviparous as that the eggs are large-yolked, and develop

therefore, so far as regards their early stages, after the fashion of the

egg of a reptile. The laying {107} of eggs, or at least ovoviviparity,

would follow from the structure of the egg, since the abundance of yolk

would do away with the necessity for a placenta. That the eggs had this

Saurian characteristic was first definitely made known by Professor

Poulton[53] for _Ornithorhynchus_, and his results were confirmed later for

_Echidna_.[54] The structure of the eggs has, however, already been dealt

with on p. 72. The fact that these animals lay eggs appears to have been

known for a very long time, though rediscovered so lately as 1884 by Mr.

Caldwell.[55] In connexion with the structure of the ova, the ovaries

themselves and the oviducts are built upon the Sauropsidan plan. In the

male the testes retain the primitive abdominal position. The fact that the

urinary and genital products escape by means of their ducts into a chamber

which also receives the end of the alimentary tract is not a distinctive

feature of this group, inasmuch as it is seen in the Marsupials, and also

in certain low Eutheria, such as the Beaver and other Rodents, and a few

Insectivores. As to external features, the Monotremata show certain archaic

characters. The unspecialised arrangement of the mammary glands has already

been described. These animals are plantigrade, if the term may be used also

to describe the aquatic _Ornithorhynchus_. The ears are absolutely

destitute of a conch. The remarkable spur upon the hind-legs furnished with

a gland, which is more marked in the male, and indeed disappears in the

female of _Ornithorhynchus_, is a structure which argues the specialised

condition of these two modern representatives of what must have been a

large order in the past.

{108} [Illustration]

FIG. 51.--Ventral view of skull of _Echidna aculeata_, and right half of

mandible, _ang_, Angle of mandible; _aud.oss_, auditory ossicles; _cond_,

condyle of mandible; _cor_, coronoid process; _max_, maxilla; _oc.cond_,

occipital condyle; _pal_, palatine; _p.max_, premaxilla; _pt_, pterygoid;

_sq_, squamosal; _ty_, tympanic ring. (After Parker and Haswell.)

The skeleton shows numerous ancient characteristics. In the skull there is

no demarcation of the orbit from the temporal fossa, a feature widely found

in archaic mammals. The tympanic remains as a slender ring, there being no

auditory bulla formed either from this or from any other bone. The malleus

and incus are large, and thus reminiscent of the quadrate and articular

bone of reptiles. In the lower jaw the absence of a marked coronoid

process, and the absence of a firm ossification at the meeting of the two

rami, may be a primitive state of affairs. It must be remembered, however,

that the Cetacea show the same characters, though it is possible that they

too are developed from a low mammalian stock. In the vertebral column we

find the typical mammalian seven cervicals; but those characteristically

mammalian structures the epiphyses are totally absent in _Echidna_, and

only to be seen in the tail-region in _Ornithorhynchus_. In having only the

capitular head to the ribs, these mammals are evidently far removed from

all other mammals, and are even more reptilian than the Theromorphous

reptiles. The large clavicles and the interclavicle (Fig. 52, {109} p. 109)

are characteristic of the group, and the latter bone is peculiar to the

Monotremata among mammals. So, too, is the large coracoid. In the scapula

there is a spine which coincides with the anterior border of that bone. The

arrangement of the muscles in this region proves conclusively that this

projection is the homologue of the spine and the acromion of other mammals.

Here, again, we have a point of likeness to the Cetacea.[56] In the pelvis

the acetabulum is perforate (in _Echidna_), as in Sauropsida.

[Illustration]

FIG. 52.--Side view of right half of the shoulder girdle of a young Echidna

(_Echidna aculeata_). x 1. _a_, Acromion; _c_, coracoid; _cb_, coracoid

border; _cl_, clavicle; _css_, coraco-scapular suture; _ec_, epicoracoid;

_gb_, glenoid border; _gc_, glenoid cavity; _ic_, interclavicle; _pf_,

postscapular fossa; _ps_, presternum; _s_, spine; _ss_, suprascapular

epiphysis; _ssf_, subscapular fossa. (From Flower's _Osteology_.)

Considering the numerous very archaic features which the general structure

of this group displays, it is surprising to find how typically mammalian

they are in certain other peculiarities. The mammalian diaphragm, one of

the distinguishing features of the class, is perfectly normal in the

Monotremata. The alimentary canal shows no great divergences from the

normal structure. The stomach is almost globular, with a projecting pyloric

region in _Ornithorhynchus_; the intestine is divided into a "small" and

"large" intestine by a slender caecum. The liver has the subdivisions that

this organ usually shows in the Mammalia. However, the presence of the

ventral mesentery and of the abdominal vein in _Echidna_ and

_Ornithorhynchus_ has already been mentioned as a distinctive character.

The peculiar and apparently partly primitive valve of the right ventricle

has been described above (see p. 66). The brain is in most respects

mammalian in its characters, but naturally shows some important

differences. Dr. Elliot Smith, who has most recently studied this

question,[57] is of opinion that the size of the cerebral hemispheres is

not at all reptilian; indeed, it "greatly exceeds that of {110} many other

mammals." In _Echidna_, too, but not in _Ornithorhynchus_, the hemispheres

are well convoluted, though the arrangement of these convolutions cannot be

brought into line with what is known concerning the convolutions upon the

hemispheres of other mammals. It had been stated that in these animals, at

least in _Echidna_, there were only two optic lobes, as in lower

vertebrates, instead of the mammalian four. The late Sir W. H. Flower set

this matter at rest,[58] and showed that _Echidna_ was in this respect

typically mammalian. The absence of the corpus callosum is one of the

principal features separating the Monotremes from other mammals.

The Monotremata are represented to-day by two types, _Ornithorhynchus_ and

_Echidna_, which are no doubt worthy of being placed in separate families.

Fossil remains of the group (apart from the problematical Multituberculata)

are only known from Pleistocene times in Australia, and consist of the

bones of a large species of _Echidna_, and some fragments of

_Ornithorhynchus_, indicating a smaller animal than the living Platypus.

[Illustration]

FIG. 53.--Brain of _Echidna aculeata_, dorsal view. (Nat. size.) (From

Parker and Haswell's _Zoology_.)

FAM. 1. ECHIDNIDAE.--This family contains two genera, of which _Echidna_ is

the older and much the better known. The skin is abundantly covered with

spines, with which are mingled hairs. The snout is tapering, the tail

rudimentary, and the fingers and toes five in number. The spur and gland

upon the calcaneum are smaller than in _Ornithorhynchus_. The claws are

very strong, serving to tear open the ants' nests, upon the inhabitants of

which the Echidna feeds, licking them up with a long extensile tongue like

that of _Myrmecophaga_. In relation to this habit the salivary glands are

enormously developed, and indeed the animal has been confounded with

_Myrmecophaga_,[59] as the vernacular name "Australian Anteater"

exemplifies. {111}

In the skull the Echidna differs from _Ornithorhynchus_ in the greater

extension backwards of the palatines, and the larger size of the

pterygoids. The extent and relations of these bones to each other is not at

all unlike that which obtains in many Whales. The premaxillae show traces

of the same divergence followed by convergence of their ends that is seen

in the Platypus. There are only sixteen pairs of ribs, and either three or

four lumbar vertebrae. _Echidna_ has no trace of teeth, and there are no

horny pads which take their place; the mouth is as edentulous as in the

true American Anteaters. The brain (Fig. 53) is marked by sulci, contrary

to what we find in _Ornithorhynchus_. The genus has been divided into three

species, but it is doubtful whether more than one can be allowed, which

ranges from Australia through the Papuan region. While there is but one

species of true _Echidna_, a New Guinea species must clearly be referred to

a distinct genus _Proechidna_.[60] This animal is to be distinguished by

the fact that there are usually but three toes on each foot. But there are

copious rudiments of the other phalanges, upon which claws are sometimes

developed. The beak is curved downwards, and the back is rather arched; the

whole animal has the most singular likeness to an Elephant! The ribs are

increased by one pair, and there are four lumbar vertebrae. The one species

is named _P. bruijnii_. The Hon. W. Rothschild[61] distinguishes a form _P.

nigroaculeata_, which is allowed by Mr. Lydekker.

[Illustration]

FIG. 54.--Australian Anteater. _Echidna aculeata._ x 1/6.

{112}

The Echidna feeds like anteaters, by thrusting its tongue into an ant-hill,

and waiting until it is covered with indignant and marauding ants, which

are then swallowed. But this animal also devours worms and insects, which

are extracted from their hiding-places by the tongue. It is mainly

nocturnal, and prefers the seclusion of the densest scrubs of the bush, or

rocky spots where it is free from intrusion. Dr. Semon did not find that

the spur of this animal was used at all in self-defence; but he thinks that

possibly the weapon may be used, in the breeding season only, in the

combats of the males for the females, when perhaps, as has been shown to be

the case in _Ornithorhynchus_, the gland attached to it produces a

poisonous secretion.

The egg, as it appears, is transferred to the pouch by the mouth of the

mother; the shell is broken by the emerging young one, which has an

egg-breaking tubercle on its snout for this purpose; the mother removes the

shell. When the young has attained a certain size, the mother removes it

from the pouch, but takes it in from time to time to suckle it. When on her

nightly rambles the young one is left in a burrow dug for the purpose. Dr.

Semon was able, from his own observations, to substantiate this act of

intelligence on the part of the Echidna. It is well known that the

temperature of the Monotremes is less than that of higher mammals; in

addition to this fact Dr. Semon found that the range of variation of

temperature in the Echidna was as much as 13 degrees or more. It is thus

intermediate between the "poikilothermal" reptiles and the "homoeothermal"

mammals.

FAM. 2. ORNITHORHYNCHIDAE.--There is no need to attempt to define this

family, since it contains but one genus _Ornithorhynchus_, with but one

species, _O. anatinus_. The general aspect of the animal is well known. It

is covered with dense fur of a blackish brown colour; the limbs are short

and five-toed, the toes being webbed. The tail is longish and broad, being

flattened from above downwards. The webbing on the anterior toes

considerably outdistances the tips of the claws, as in the Seals. But this

is not the case with the hind-feet. The "beak," which is broad and flat,

and does actually suggest that of a duck, is not covered with horn, as is

often stated, but with a fine, soft, sensitive, naked skin, which abounds

in sense-organs of a tactile nature. As to characters derived from the

skeleton, _Ornithorhynchus_ has {113} seventeen pairs of ribs and only two

lumbar vertebrae. The skull is expanded in front, and the bill is supported

by two, at first diverging, and then converging, premaxillae. Between them

is the famous "dumb-bell shaped bone," which is believed to be the

representative of the reptilian prevomer. The pterygoids are smaller than

in _Echidna_, and the hard palate does not extend so far back as in that

genus. The brain of this genus is smooth.

[Illustration]

FIG. 55.--Duck-billed Platypus. _Ornithorhynchus anatinus._ x 1/6.

The discovery of the real teeth of _Ornithorhynchus_ only dates from the

year 1888, when they were found by Professor Poulton[62] in an embryo.

Later Mr. Thomas found[63] that the teeth persist for a considerable

portion of the animal's life, and are only shed, like milk teeth, "after

being worn down by friction with food and sand." We have already (p. 98)

called attention to the general similarity of these teeth to those of

certain of the earliest Mammalia and of mammal-like reptiles. The teeth are

all molars, and they are either eight or ten in number. They are replaced

by the horny plates of the adult animal; but the mode of replacement is

curious. The plates are developed from the epithelium of the mouth, but

round and under the true teeth; the epithelium of the mouth grows gradually

under the calcified teeth, a method of growth which has possibly something

to do with the shedding of the latter. The hollows and {114} grooves in the

plates are the remains of the original alveoli of the teeth.

[Illustration]

FIG. 56.--Skeleton of male _Ornithorhynchus_. Ventral view. The right

fore-limb has been separated and turned round so as to bring into view the

dorsal surface of the manus. The lower jaw is removed. _acc.tars_,

Accessory tarsal bone supporting the spur; _ant.pal.for_, anterior palatine

foramen; _ast_, astragalus; _atl_, atlas; _ax_, axis; _bs.oc_,

basi-occipital; _bs.sph_, basi-sphenoid; _calc_, calcaneum; _cbd_, cuboid;

_cerv.rb_, cervical rib; _clac_, clavicle; _cond.for_, foramen above inner

condyle of humerus; _cor_, coracoid; _cun_, cuneiform of carpus; _dent_,

horny dental plate; _ect.cun_, ecto-cuneiform; _ent.cun_, ento-cuneiform;

_ep.co_, epicoracoid; _epist_, episternum; _ep.pb_, epipubis; _fb_, fibula;

_fem_, femur; _for.mag_, foramen magnum; _glen_, glenoid cavity of

shoulder-joint; _glen_, glenoid cavity for mandible; _hum_, humerus;

_in.cond_, inner condyle of humerus; _inf.orb.for_, points to position of

infra-orbital foramen; _infr.proc_, inferior processes of caudal vertebrae;

_int.rbs_, intermediate ribs; _isch_, ischium; _mag_, magnum of carpus;

_max_, maxilla; _max.for_, maxillary foramen; _metat.I_, first metatarsal;

_metat.V_, fifth metatarsal; _nas.cart_, nasal cartilage; _obt_, obturator

foramen; _ol_, olecranon; _out.cond_, outer condyle of humerus; _pal_,

palatine; _pat_, patella; _post.pal.for_, posterior palatine foramen;

_pr.max_, premaxilla; _pr.st_, presternum; _pter_, pterygoid; _pub_, pubis;

_rad_, radius; _scap_, scapula; _scaph_, scaphoid of tarsus; _scaph.lun_,

scapho-lunar; _ses_, sesamoid bones of wrist and ankle; _sp_, tarsal horny

spur; _sq_, squamosal; _tib_, tibia; _trd_, trapezoid; _trm_, trapezium;

_tym.c_, tympanic cavity; _uln_, ulna; _unc_, unciform; _vom_, vomer; _x_,

dumb-bell shaped bone; _zyg_, zygomatic arch; _I-V_, digits of manus; _V_,

foramen for fifth nerve. (From Parker's _Zoology_.)

{115}

The Duck-billed Platypus is, as every one knows, an aquatic animal. It is

not found all over Australia, but is limited to the southern and eastern

parts of that continent, and to Tasmania. The animal excavates a burrow for

itself in the bank of the slow streams which it frequents. The burrow has

one opening below the water and one above; and it is of some length, twenty

to fifty feet. The Platypus feeds upon animal food, chiefly "grubs, worms,

snails, and, most of all, mussels." These it stows away when captured into

its capacious cheek-pouches. The food is then chewed and swallowed above

the surface as the animal drifts slowly along. Dr. Semon, from whose work,

_In the Australian Bush_, this account of the animal's habits is quoted,

thinks that in the nature of the food of the creature the explanation of

the loss of the teeth is to be found. He is of opinion that for cracking

the hard shells of the mollusc _Corbicula nepeanensis_, upon which

_Ornithorhynchus_ mainly feeds, the horny plates are preferable to brittle

teeth. _Ornithorhynchus_ is apparently not eaten by the natives by reason

of its ancient and fish-like smell. Besides, it is hard to catch on account

of its diving capacities, which are aided by an acute sense of sight and of

hearing. When the Duck-bill was first brought to this country it was

believed to be a deliberate fraud, analogous to the mermaids produced by

neatly stitching together the forepart of a monkey and the tail of a

salmon.

* * * * *

{116}

CHAPTER VI

INTRODUCTION TO THE SUB-CLASS EUTHERIA

SUB-CLASS II.--_EUTHERIA_

DEFINITION.--Mammalia with teats. Mammary glands of sebaceous type. Heart

with entirely membranous and complete right auriculo-ventricular valve.

Brain generally with a corpus callosum. Coracoid much reduced and not

reaching sternum. No interclavicle. Vertebrae with epiphyses. Ribs

double-headed. Viviparous, with a small ovum.

In this group are included not only the Eutheria in the sense of Huxley,

but also his Metatheria. Though the Metatheria, or Marsupials as we shall

term them, undoubtedly form a most distinct order of mammals, perhaps even

a trifle more distinct than most others, their differences from the

remaining tribes are not by any means so great as those which separate

_Ornithorhynchus_ and _Echidna_ from all other mammals. In his well-known

memoir upon the arrangement of the Mammalia,[64] Professor Huxley

enumerated eleven characters as distinguishing the Metatheria either from

the Prototheria or from the Eutheria. Of these only three were characters

in which they approach the lower mammals. According to his showing,

therefore, the preponderance of marsupial features are Eutherian. The three

characters of Prototherian type are (1) the presence of epipubes; (2) the

small corpus callosum; (3) the absence of an allantoic placenta.

The last of these can be dismissed, in consequence of the recent discovery

of an allantoic placenta in _Perameles_. The first character is apparently

a valid distinction between the Marsupials {117} and their mammalian

relatives higher in the series; but it is not a character that should have

been made use of by Huxley, since he believed in the existence of a

corresponding element in the Dog. As to the corpus callosum (Fig. 50, p.

77) being small, that seems to be not more than a slight difference of

degree.[65] A number of other characters of secondary importance were added

by Huxley to the weight of evidence which led him to form a group

Metatheria for the Marsupials. Some of these, however, are now known to be

not evidence in that direction. For instance he observed that no Marsupial

had more than a single successional tooth. It seems at the present moment

to be fairly clear that Marsupials have a milk dentition like other

Eutherians, but that only one of these teeth, the fourth premolar, comes to

functional maturity. That it is really one of a complete milk series is

evidenced by the fact that this tooth is differentiated contemporaneously

with another series formerly held to belong to the so-called prelacteal

dentition.[66] There still remains, of course, the actual fact that the

milk dentition is not for the most part functional, but its significance

breaks down with these fresh discoveries. Of this Professor Osborn has

remarked: "The discovery of the complete double series seems to have

removed the last straw from the theory of the marsupial ancestry of the

Placentals." But Huxley did not lay much stress upon this matter of the

teeth, since he observed that similar suppressions of the milk dentition

were to be found in many other mammals admittedly Eutherian.

[Illustration]

FIG. 57.--Brain of _Echidna aculeata_; sagittal section. _ant.com_,

Anterior commissure; _cbl_, cerebellum; _c.mam_, corpus mammillare;

_col.forn_, column of the fornix; _c.qu_, corpora quadrigemina; _gang.hab_,

ganglion habenulare; _hip.com_, hippocampal commissure; _med_, medulla

oblongata; _mid.com_, middle commissure; _olf_, olfactory lobe; _opt_,

optic chiasma; _tub.olf_, tuberculum olfactorium; _vent. 3_, third

ventricle. (From Parker and Haswell's _Zoology_.)

Huxley regarded the peculiarities in the reproductive organs {118} of the

Marsupials as "singularly specialised characters," in no way intermediate

in character. This view applies also to the pouch, which, as already

stated, distinguishes the adults of that group. But the impossibility of

using this last character as one of any importance has been shown by the

discovery of rudiments of it in embryos of undoubtedly Eutherian mammals

(see p. 18).

[Illustration]

FIG. 58.--Sagittal section of brain of Rock Wallaby (_Petrogale

penicillata_). _ant.com_, Anterior commissure; _cbl_, cerebellum; _c.mam_,

corpus mammillare; _c.qu_, corpora quadrigemina; _crur_, crura cerebri;

_epi_, epiphysis, with the posterior commissure immediately behind it;

_f.mon_, position of foramen of Monro; _hip.com_, hippocampal commissure,

consisting here of two layers continuous behind at the spleneium, somewhat

divergent in front where the septum lucidum extends between them; _hypo_,

hypophysis; _med_, medulla oblongata; _mid.com_, middle commissure; _olf_,

olfactory lobe; _opt_, optic chiasma; _vent. 3_, third ventricle. (From

Parker and Haswell's _Zoology_.)

Less stress is laid now upon the existence of four molars in the Marsupials

as dividing them from the higher mammals than was formerly the case. The

total dentition of the group is on the whole composed of more numerous

individual teeth than in the typical Eutheria; but we have exceptions like

the Whales, the Armadillo _Priodontes_, and the Manatee; or better, because

free from the suspicion of secondary multiplication, _Otocyon_ and

occasionally (according to Mr. Thomas) _Centetes_. In the last two there

are at least sometimes four molars.