femur, corresponding with the humerus of the anterior limb. The second
segment has two bones, the tibia and fibula, corresponding with the
radius and ulna. These bones always lie in their primitive unmodified
position, parallel to each other, the tibia on the preaxial and the
fibula on the postaxial side, and are never either permanently crossed or
capable of any considerable amount of rotation, as in the corresponding
bones of the fore limb. In the ordinary walking position the tibia is
internal, and the fibula external. In many mammals the fibula is in a
more or less rudimentary condition, and it often ankyloses with the tibia
at one or both extremities. The patella or “knee-cap,” which is found
in an ossified condition in all mammals, with the exception of some of
the Marsupialia, is a large sesamoid bone developed in the tendon of the
extensor muscles of the thigh, where the tendon passes over the front of
the knee-joint, to which it serves as a protection. There are frequently
smaller ossicles, one or two in number, situated behind the femoral
condyles, called “fabellæ.” The processes for the attachment of muscles
near the upper end of the femur are termed trochanters; and the third
trochanter, found on the hinder aspect of the shaft of this bone in many
forms is of considerable taxonomic importance.
_Pes._—The terminal segment of the hind limb is the foot or pes. Its
skeleton presents in many particulars a close resemblance to that of
the manus, being divisible into three parts: (1) a group of short, more
or less rounded or square bones, constituting the tarsus; (2) a series
of long bones placed side by side, forming the metatarsus; and (3) the
phalanges of the digits or toes.
The bones of the tarsus of many of the lower Vertebrata closely resemble
both in number and arrangement those of the carpus, as shown in Fig. 17.
They have been described in their most generalised condition by Gegenbaur
under the names expressed in the first column of the following table. The
names in the second column are those by which they are generally known to
English anatomists, while in the third column some synonyms occasionally
employed are added.
_Tibiale (?)_ } = Astragalus[13] = _Talus_.
_Intermedium_ }
_Fibulare_ = Calcaneum = _Os calcis_.
_Centrale_ = Navicular = _Scaphoideum_.
_Tarsale_ 1 = Internal cuneiform = _Entocuneiforme_.
_Tarsale_ 2 = Middle cuneiform = _Mesacuneiforme_.
_Tarsale_ 3 = External cuneiform = _Ectocuneiforme_.
_Tarsale_ 4 } = Cuboid.
_Tarsale_ 5 }
The bones of the tarsus of mammals present fewer diversities of number
and arrangement than those of the carpus. The proximal row (see Fig.
18) always consists of two bones, namely the astragalus (_a_), which
probably represents the coalesced scaphoid and lunar of the hand, and the
calcaneum (_c_). The former is placed more to the dorsal side of the foot
than the latter, and almost exclusively furnishes the tarsal part of the
tibio-tarsal or ankle-joint. The calcaneum, placed more to the ventral
or “plantar” side of the foot, is elongated backwards to form a more or
less prominent tuberosity, the “tuber calcis,” to which the tendon of
the great extensor muscles of the foot is attached. The navicular bone
(_n_) is interposed between the proximal and distal row on the inner
or tibial side of the foot, but on the outer side the bones of the two
rows come into contact. The distal row, when complete, consists of four
bones, which, beginning on the inner side, are the three cuneiform bones,
internal (_c¹_), middle (_c²_), and external (_c³_), articulated to the
distal surface of the navicular, and the cuboid (_cb_), articulated with
the calcaneum. Of these the middle cuneiform is usually the smallest in
animals in which all five digits are developed; but when the hallux is
wanting the internal cuneiform may be rudimentary or altogether absent.
The three cuneiform bones support respectively the first, second, and
third metatarsals, and the cuboid supports the fourth and fifth; they
thus exactly correspond with the four bones of the distal row of the
carpus.
[Illustration: FIG. 18.—Bones of the right Human foot. _T_, Tarsus; _M_,
metatarsus; _Ph_, phalanges, _c_, calcaneum; _a_, astragalus; _cb_,
cuboid; _n_, navicular; _c¹_, internal cuneiform; _c²_, middle cuneiform;
_c³_, external cuneiform. The digits are indicated by Roman numerals,
counting from the tibial to the fibular side.]
In addition to these constant tarsal bones, there may be supplemental
or sesamoid bones: one situated near the middle of the tibial side of
the tarsus, largely developed in many Carnivora and Rodentia; another,
less frequent, on the fibular side; and a third, often developed in the
tendons of the plantar surface of the tarsus, is especially large in
Armadillos. There is also usually a pair of sesamoid bones on the plantar
aspect of each metatarso-phalangeal articulation. In the young of the
carnivorous genus _Crytoprocta_ there may be a second centrale, which
usually coalesces with the ectocuneiform.
The metatarsal bones never exceed five in number, and the phalanges
follow the same numerical rule as in the manus, never exceeding three in
each digit. Moreover, the first digit, counting from the tibial side, or
hallux, resembles the pollex of the hand in always having one segment
less than the other digits. As the function of the hind foot is more
restricted than that of the hand the modifications of its structure are
less striking. In the Cetacea and the Sirenia it is entirely wanting,
though in some existing members of the first-named order rudiments of
the bones of both the first and second segments of the limb have been
detected, and a femur is present in the Miocene Sirenian _Halitherium_.
IV. THE DIGESTIVE SYSTEM.
_General Considerations._—The search after the purpose which every
modification of structure subserves in the economy is always full of
interest, and, if conducted with due caution and sufficient knowledge of
all the attendant circumstances, may lead to important generalisations.
It must always be borne in mind, however, that adaptation to its special
function is not the only cause of the particular form or structure of an
organ, but that this form, having in all probability been arrived at by
the successive and gradual modification of some other different form from
which it is now to a greater or less degree removed, has other factors
besides use to be taken into account. In no case is this principle so
well seen as in that of the organs of digestion. These may be considered
as machines which have to operate upon alimentary substances in very
different conditions of mechanical and chemical combination, and to
reduce them in every case to the same or precisely similar materials;
and we might well imagine that the apparatus required to produce flesh
and blood out of coarse fibrous vegetable substances would be different
from that which had to produce exactly the same results out of ready-made
flesh or blood; and in a very broad sense we find that this is so. Thus,
if we take a large number of carnivorous animals, belonging to different
fundamental types, and a large number of herbivorous animals, and strike
a kind of average of each, we shall find that there is, pervading the
first group, a general style, if we may use the expression, of the
alimentary organs, different from that of the others. That is to say,
there is a specially carnivorous and a specially herbivorous modification
of these parts. But, if function were the only element which has guided
such modification, it might be inferred that, as one form must be
supposed to be best adapted in its relation to a particular kind of diet,
that form would be found in all the animals consuming such diet. But this
is far from being the case. Thus the Horse and the Ox, for instance—two
animals whose food in the natural state is precisely similar—are most
different as regards the structure of their alimentary canal, and the
processes involved in the preparation of that food. Again, the Seal and
the Porpoise, both purely fish-eaters, which seize, swallow, and digest
precisely the same kind of prey, in precisely the same manner, have a
totally different arrangement of the alimentary canal. If the Seal’s
stomach is adapted in the best conceivable manner for the purpose it has
to fulfil, why is not the Porpoise’s stomach an exact facsimile of it,
and _vice versâ_? We can only answer that the Seal and Porpoise belong to
different natural groups of animals, formed either on different primitive
types, or descended from differently constructed ancestors. On this
principle only can we account for the fact that, whereas, owing to the
comparatively small variety of the different alimentary substances met
with in nature, few modifications would appear necessary in the organs of
digestion, there is really endless variety in the parts devoted to this
purpose.
_Mouth._—The digestive apparatus of mammals, as in other vertebrates,
consists mainly of a tube with an aperture placed at or near either
extremity of the body,—the oral and the anal orifice,—and furnished with
muscular walls, the fibres of which are so arranged as by their regular
alternate contraction and relaxation to drive onwards the contents of
the tube from the first to the second of these apertures. The anterior
or commencing portion of this tube and the parts around it are greatly
and variously modified in relation to the functions assigned to them of
selecting and seizing the food, and preparing it by various mechanical
and chemical processes for the true digestion which it has afterwards
to undergo before it can be assimilated into the system. For this end
the tube is dilated into a chamber or cavity called the mouth, bordered
externally by the lips, which are usually muscular and prehensile, and
supported by a movable framework carrying the teeth; the structure and
modifications of which have been already described. The roof of the mouth
is formed by the palate, terminating behind by a muscular, contractile
arch, having in Man and some few other species a median projection
called the uvula, beneath which the mouth communicates with the pharynx.
The anterior part of the palate is composed of mucous membrane tightly
stretched over the flat or slightly concave bony lamina separating the
mouth from the nasal passages, and is generally raised into a series
of transverse ridges, which sometimes, as in Ruminants, attain a
considerable development. In the floor of the mouth, between the rami of
the mandible, and supported behind by the hyoidean apparatus, lies the
tongue; an organ the free surface of which, especially in its posterior
part, is devoted to the sense of taste, but which also, by its great
mobility (being composed almost entirely of muscular fibres), performs
important mechanical functions connected with masticating and procuring
food. Its modifications of form in different mammals are very numerous.
Between the long, extensile, vermiform tongue of the Anteaters, which
is essential to the peculiar mode of feeding of those animals, and the
short, sessile, and almost functionless tongue of the Porpoise, every
intermediate condition is found. Whatever the form, the upper surface
is always covered with numerous fine papillæ, in which the terminal
filaments of the gustatory nerve are distributed.
_Salivary Glands._—The fluid known as the saliva is secreted by an
extensive and complex system of glands discharging into the cavity of the
mouth (buccal cavity), the position and relation of some of which are
exhibited in the woodcut on the next page (Fig. 19).
[Illustration: FIG. 19.—Salivary Glands of the Genet. _A_, Right side
of the head dissected; _p_, parotid gland; _d_, Steno’s duct; _sm_,
submaxillary gland, traversed by the jugular veins (_jv_); _o_, aperture
of Steno’s duct. _B_, Part of the head with the lip drawn up to show
(_st.d_) aperture of Steno’s duct; _z.gl_, zygomatic gland; _o_, aperture
of do.; _z_, zygomatic arch (Mivart, _Proc. Zool. Soc._ 1882, p. 504.)]
This apparatus consists of small glands embedded in the mucous membrane
or submucous tissue lining the cavity of the mouth, which are of two
kinds (the follicular and the racemose), and of others in which the
secreting structure is aggregated in distinct masses removed some
distance from the cavity; other tissues besides the lining membrane being
usually interposed, and pouring their secretion into the cavity by a
distinct tube or duct, which traverses the mucous membrane. To the latter
alone the name of “salivary glands” is ordinarily appropriated, although
the distinction between them and the smaller racemose glands is only one
of convenience for descriptive purposes, their structure being more or
less nearly identical; and, since the fluids secreted by all become mixed
in the month, their functions are, at all events in great part, common.
Under the name of salivary glands are commonly included—(1) the “parotid”
(_p_), situated very superficially on the side of the head, below or
around the cartilaginous external auditory meatus, and the secretion of
which enters the mouth by a duct (often called Steno’s or Stenson’s)
which crosses the masseter muscle and opens into the upper and back part
of the cheek (Fig. 19); and (2) the “submaxillary” (_sm_), situated in
the neck, near or below the angle of the mandible, and sending a long
duct (Wharton’s) forwards to open on the forepart of the floor of the
cavity of the mouth, below the apex of the tongue. These are the most
largely developed and constant of the salivary glands, being met with
in various degrees of development in almost all animals of the class.
Next in constancy are (3) “the sublingual,” closely associated with the
last-named, at all events in the locality in which the secretion is
poured out; and (4) the “zygomatic” (_z.gl_), found only in some animals
in the cheek, just under cover of the anterior part of the zygomatic
arch, its duct entering the buccal cavity near that of the parotid.
The most obvious function common to the secretion of these various
glands, and to that of the smaller ones placed in the mucous membrane
of the lips, the cheeks, the tongue, the palate, and fauces, is the
mechanical one of moistening and softening the food, to enable it the
more readily to be tasted, masticated, and swallowed, though each kind of
gland may contribute in different manner and different degree to perform
this function. The saliva is, moreover, of the greatest importance in the
first stage or introduction to the digestive process, as it dissolves
or makes a watery extract of all soluble substances in the food, and
so prepares them to be further acted on by the more potent digestive
fluids met with subsequently in their progress through the alimentary
canal. In addition to these functions it seems now well established by
experiment that saliva serves in Man and many animals to aid directly
in the digestive process, particularly by its power of inducing the
saccharine transformation of amylaceous substances. As a general rule,
in mammals the parotid saliva is more watery in its composition, while
that of the submaxillaries, and still more the sublingual, contains
more solid elements and is more viscid;—so much so that some anatomists
consider the latter, together with the small racemose glands of the
cheeks, lips, and tongue, as mucous glands, retaining the name of
salivary only for the parotid. These peculiar properties are sometimes
illustrated in a remarkable degree, as, for example, the great secretion
of excessively viscid saliva which lubricates the tongue of the Anteaters
and Armadillos, associated with enormously developed submaxillary glands;
while, on the other hand, the parotids are of great size in those animals
which habitually masticate dry and fibrous food.
_Stomach._—After the preparation which the aliment has undergone in the
mouth,—the extent of which varies immensely in different forms, being
reduced almost to nothing in such animals as the Seals and Cetaceans,
which, to use the familiar expression, “bolt” their food entire, and
most fully carried out in the Ruminants, which “chew the cud,”—it is
swallowed, and carried along the œsophagus by the action of its muscular
coats into the stomach. In the greater number of mammals this organ is a
simple saccular dilatation of the alimentary canal, as in Figs. 20, 21,
but in others it undergoes remarkable modifications and complexities.
The lining of the stomach is thickly beset with tubular glands, which
are generally considered to belong to two different forms, recognisable
by their structure, and different in their function—the most numerous
and important secreting the gastric juice (the active agent in stomachic
digestion), and hence called “peptic” glands, while the others are
concerned only in the elaboration of mucus. The relative distribution
of these glands in different regions of the walls of the stomach varies
greatly in different animals, and in many species there are large tracts
of the mucous membrane which do not secrete a fluid having the properties
of gastric juice, but often constitute more or less distinct cavities
devoted to storing and perhaps softening or otherwise preparing the
food for digestion. Sometimes there is a great aggregation of glands
forming distinct thickened patches of the stomach wall, as in the Beaver
and Koala, or even collected in pyriform pouches with a common narrow
opening into the cavity, as in the Manatee and the curious African Rodent
_Lophiomys_. The action of the gastric fluid is mainly exerted upon the
nitrogenous elements of the food, which it dissolves and modifies so as
to render them capable of undergoing absorption, effected partly by the
blood-vessels of the stomach, although the greater part, passes through
the pylorus, an aperture surrounded by a circular muscular valve, into
the intestinal canal. Here it comes in contact with the secretion of a
vast number of small glands called the crypts of Lieberkuhn, somewhat
similar to those of the stomach; and also of several special glands of a
different character, namely, the small racemose, duodenal, or Brunner’s
glands, the pancreas, and the liver; the position of the ducts of the two
latter organs being indicated in Fig. 20.
[Illustration: FIG. 20.—Stomach and pancreas of the Genet. Posterior or
dorsal surface, _œ_, Œsophagus; _s_, pancreas; _pd_, pancreatic duct;
_bd_, biliary duct from the liver. (From Mivart, _Proc. Zool. Soc._ 1882,
p. 305.)]
_Intestinal Canal._—The intestinal canal varies greatly in relative
length and capacity in different animals, and it also offers manifold
peculiarities of form, being sometimes a simple cylindrical tube of
nearly uniform calibre throughout, but more often subject to alterations
of form and capacity in different portions of its course,—the most
characteristic and constant being the division into an upper and
narrower, and lower and wider portion, called respectively the small
and the large intestine, the former being divided quite arbitrarily and
artificially into duodenum, jejunum, and ileum, and the latter into
colon and rectum. One of the most striking peculiarities of this part
of the alimentary canal is the frequent presence of a diverticulum or
blind pouch, the _caput cæcum coli_, as it was first called, a name
generally abbreviated into “cæcum,” situated at the junction of the large
and the small intestine, a structure presenting an immense variety of
development, from the smallest bulging of a portion of the side wall of
the tube to a huge and complex sac, greatly exceeding in capacity the
whole of the remainder of the alimentary canal. It is only in herbivorous
animals that the cæcum is developed to this great extent, and among
these there is a curious complementary relationship between the size
and complexity of this organ and that of the stomach. Where the latter
is simple the cæcum is generally the largest, and _vice versâ_. Both
the cæcum and colon are often sacculated, a disposition caused by the
arrangement of the longitudinal bands of muscular tissue in their walls;
but the small intestine is always smooth and simple-walled externally,
though its lining membrane often exhibits various contrivances for
increasing the absorbing surface without adding to the general bulk of
the organ, such as the numerous small villi by which it is everywhere
beset, and the more obvious transverse, longitudinal, or reticulating
folds projecting into the interior, met with in many animals, of which
the “valvulæ conniventes” of Man form well-known examples.
[Illustration: FIG. 21.—Diagrammatic plan of the general arrangement of
the alimentary canal in a typical Mammal. _o_, Œsophagus; _st_, stomach;
_p_, pylorus; _s_, _s_, small intestine; (abbreviated); _c_, cæcum; _l_,
_l_, large intestine or colon, ending in _r_, the rectum.]
Besides the crypts of Lieberkuhn found throughout the intestinal canal,
and the glands of Brunner confined to the duodenum, there are other
structures in the mucous membrane, about the nature of which there is
still much uncertainty, called “solitary” and “agminated” glands; the
latter being more commonly known by the name of “Peyer’s patches.” These
were formerly supposed to be secretory organs, which discharged some
kind of fluid into the intestine, but are now more generally considered
to belong to that group of structures of somewhat mysterious function
of which the lymphatic and lacteal glands are members. The solitary
glands are found scattered irregularly throughout the whole intestinal
tract; the agminated, on the other hand, are always confined to the small
intestine, and are most abundant in its lower part. They are subject to
great variation in number and in size, and even in different individuals
of the same species, and also differ in character at different periods of
life, becoming atrophied in old age.
_Liver._—The distinct glands situated outside the walls of the intestinal
canal, but which pour their secretion into it, are the pancreas and the
liver. The latter is the more important on account of its size, if not on
account of the direct action of its secretion in the digestive process.
This large gland, so complex in structure and function, is well developed
in all mammals, and its secreting tube, the bile-duct, always opens into
the duodenum, or that portion of the canal which immediately succeeds
the stomach. It is situated on the right side of the abdomen in contact
with the diaphragm and the stomach, but varies greatly in relative size,
and also in form, in different groups of mammals. In most mammals a
gall-bladder, consisting of a pyriform diverticulum from the bile-duct,
is present, but in many this appendage is wanting, and it is difficult to
find the rationale of its presence or absence in relation to use or any
other circumstance in the animal economy.
The descriptions of the livers of various animals to be met with in
treatises or memoirs on comparative anatomy are very difficult to
understand for want of a uniform system of nomenclature. The difficulty
usually met with arises from the circumstance that this organ is divided
sometimes, as in Man, Ruminants, and the Cetacea, into two main lobes,
which have been always called respectively right and left, and in other
cases, as in the lower Monkeys, Carnivora, Insectivora, and several other
orders, into a larger number of lobes. Among the latter the primary
division usually appears at first sight tripartite, the whole organ
consisting of a middle, called “cystic” or “suspensory” lobe, and two
lateral lobes, called respectively right and left lobes. This introduces
confusion in describing livers by the same terms throughout the whole
series of mammals, since the right and left lobes of the Monkey or Dog,
for instance, do not correspond with parts designated by the same names
in Man and the Sheep. There are, moreover, conditions where neither the
bipartite nor the tripartite system of nomenclature will answer, so that
we should have considerable difficulty in describing them without some
more general system. In order to arrive at such a system it appears
desirable to consider the liver in all cases as primarily divided by the
umbilical vein (see Fig. 22, _u_) into two segments, right and left. This
corresponds with its development and with the condition characteristic
of the organ in the inferior classes of vertebrates. The situation of
this division can almost always be recognised in adult animals by the
persistence of some traces of the umbilical vein in the form of the round
ligament, and by the position of the suspensory ligament.
[Illustration: FIG. 22.—Diagrammatic plan of the inferior surface of
a multilobed liver of a Mammal. The posterior or attached border is
uppermost. _u_, Umbilical vein of the fœtus, represented by the round
ligament in the adult, lying in the umbilical fissure; _dv_, the ductus
venosus; _vc_, the inferior vena cava; _p_, the vena portæ entering the
transverse fissure; _llf_, the left lateral fissure; _rlf_, the right
lateral fissure; _cf_, the cystic fissure; _ll_, the left lateral lobe;
_lc_, the left central lobe; _rc_, the right central lobe; _rl_, the
right lateral lobe; _s_, the Spigelian lobe; _c_, the caudate lobe; _g_,
the gall-bladder.]
When the two main parts into which the liver is thus divided are entire,
as in Man, the Ruminants, and Cetacea, they may be spoken of as the
right and left lobes; when fissured, as the right and left segments
of the liver, reserving the term lobe for the subdivisions. This will
involve no ambiguity, for the terms right and left lobe will no longer
be used for divisions of the more complex form of liver. In the large
majority of mammals each segment is further divided by a fissure, more
or less deep, extending from the free towards the attached border, which
are called right and left lateral fissures (Fig. 22, _rlf_ and _llf_).
When these are more deeply cut than the umbilical fissure (_u_), the
organ has that tripartite or trefoil-like form just spoken of, but it
is easily seen that it is really divided into four regions or lobes,
those included between the lateral fissures being the right and left
central (_rc_ and _lc_) separated by the umbilical fissure, and those
beyond the lateral fissures on each side being the right and left lateral
lobes (_rl_ and _ll_). The essentially bipartite character of the organ
and its uniformity of construction throughout the class are thus not
lost sight of, even in the most complex forms. The left segment of the
liver is rarely complicated to any further extent, except in some cases
by minor or secondary fissures marking off small lobules, generally
inconstant and irregular, and never worthy of any special designation.
On the other hand, the right segment is usually more complex. The
gall-bladder, when present, is always attached to the under surface of
the right central lobe, sometimes merely applied to it, in other cases
deeply embedded in its substance. In many instances the fossa in which
it is sunk is continued to the free margin of the liver as an indent, or
even a tolerably deep fissure (_cf_). The portal fissure (_p_), through
which the portal vein and hepatic artery enter and the bile-duct emerges
from the liver, crosses the right central lobe transversely, near the
attached border of the liver. The right lateral lobe always has the
great vena cava (_vc_) either grooving its surface or tunnelling through
its substance near the inner or left end of its attached border; and a
prolongation of this lobe to the left, between the vein and the portal
fissure, sometimes forming a mere flat track of hepatic substance, but
more often a prominent tongue-shaped process, is the so-called “Spigelian
lobe” (_s_). From the under surface, of the right lateral lobe a portion
is generally partially detached by a fissure, and called the “caudate
lobe” (_c_). In Man this lobe is almost obsolete, but in most mammals it
is of considerable magnitude, and has very constant and characteristic
relations. It is connected by an isthmus at the left (narrowest or
attached) end to the Spigelian lobe, behind which isthmus the vena cava
is always in relation to it, channelling through or grooving its surface.
It generally has a pointed apex, and is deeply hollowed to receive the
right kidney, to the upper and inner side of which it is applied.
Considerations derived from the comparatively small and simple condition
of the liver of the Ungulata, compared with its large size and complex
form in the Carnivora, have led to the perhaps too hasty generalisation
that the first type is related to a herbivorous and the latter to a
carnivorous diet. The exceptions to such a proposition are very numerous.
The fact of the great difference between the liver of the Cetacea and
that of the Seals cannot be accounted for by difference of habits of
life, though it perhaps may be by difference of origin.[14]
V. CIRCULATORY, ABSORBENT, RESPIRATORY, AND URINARY SYSTEMS.
_Blood._—The blood of mammals is always red, and during the life of the
animal hot, having a nearly uniform temperature, varying within a few
degrees on each side of 100° Fahr. The corpuscles are, as usual in the
vertebrates, of two kinds: (1) colourless, spheroidal, nucleated, and
exhibiting amœboid movements; while (2) the more numerous, on which
depends the characteristic hue of the fluid in which they are suspended,
are coloured, non-nucleated, flattened, slightly biconcave discs, with
circular outline in all known species except the Camels and Llamas,
where they have the elliptical form characteristic of the red corpuscles
of nearly all the other vertebrates, though adhering to the mammalian
type in the absence of nucleus and relatively small size. As a rule they
are smaller as well as more numerous than in other classes, but vary
considerably in size in different species, and not always in relation
to the magnitude of the animal; a Mouse, for instance, having as large
corpuscles as a Horse. Within the limits of any natural group there is,
however, very often some such relation, the largest corpuscles being
found among the large species and the smallest corpuscles among the small
species of the group, but even to this generalisation there are many
exceptions. The transverse diameter of the red corpuscles in Man averages
¹⁄₃₂₀₀ of an inch, which is exceptionally large, and only exceeded by
the Elephant (¹⁄₂₇₄₅), and by some Cetacea and Edentata. They are also
generally large in Apes, Rodents, and the Monotremata, and small in the
Artiodactyles, least of all in the Chevrotains (_Tragulus_), being in _T.
javanicus_ and _meminna_ not more than ¹⁄₁₂₃₂₅.[15]
_Heart._—The heart of mammals consists of four distinct cavities,
two auricles and two ventricles. Usually the ventricular portion is
externally of conical form, with a simple apex, but in the Sirenia it is
broad and flattened, and a deep notch separates the apical portion of
each ventricle. A tendency to this form is seen in the Cetacea and the
Seals. It is characteristic of mammals alone among vertebrates that the
right auriculo-ventricular valve is tendinous like the left, consisting
of flaps held in their place by fibrous ends (_chordæ tendiniæ_) and
arising from projections of the muscular walls of the ventricular cavity
(_musculi papillares_). In the Monotremata a transition between this
condition and the simple muscular flap of the Sauropsida is observed. In
most of the larger Ungulates a distinct but rather irregular ossification
(_os cordis_) is developed in the central tendinous portion of the base
of the heart.
_Blood-vessels._—The orifices of the aorta and pulmonary artery are each
guarded by three semilunar valves. The aorta is single, and arches over
the left bronchial tube. After supplying the tissues of the heart itself
with blood by means of the coronary arteries, it gives off large vessels
(“carotid”) to the head and (“brachial”) to the anterior extremities. The
mode in which these vessels arise from the aorta varies much in different
mammals, and the study of their disposition affords some guide to
classification. In nearly all cases the right brachial and carotid have a
common origin (called the “innominate artery” in anthropotomy). The other
two vessels may come off from this, as is the rule in Ungulates, the
common trunk constituting the “anterior aorta” of veterinary anatomy; or
they may be detached in various degrees, both arising separately from the
aorta, as in Man, or the left carotid from the innominate and the left
brachial from the aorta, a very common arrangement; or the last two from
a common second or left innominate, as in some Bats and Insectivores.
The aorta, after giving off the intercostal arteries, passes through the
diaphragm into the abdomen, and, after supplying the viscera of that
cavity by means of the gastric, hepatic, splenic, mesenteric, renal,
and spermatic vessels, gives off in the lumbar region a large branch
(iliac) to each of the hinder extremities, which also supplies the
pelvic viscera, and is continued onwards in the middle line, greatly
diminished in size, along the under surface of the tail as the caudal
artery. In certain mammals, arterial plexuses, called _retia mirabilia_,
formed by the breaking up of the vessel into an immense number of small
trunks, which may run in a straight course parallel to one another
(as in the limbs of Sloths and Slow Lemurs), or form a closely packed
network, as in the intracranial plexuses of Ruminants, or a sponge-like
mass of convoluted vessels, as in the intercostals of Cetaceans, are
peculiarities of the vascular system the meaning of which is not in all
cases clearly understood. In the Cetacea they are obviously receptacles
for containing a large quantity of oxygenated blood available during the
prolonged immersion, with consequent absence of respiration, to which
these animals are subject.
The vessels returning the blood to the heart from the head and upper
extremities usually unite, as in Man, to form the single _vena cava
superior_ or precaval vein, but in some Insectivores, Chiroptera, and
Rodents, in the Elephant, and all Marsupials and Monotremes, the two
superior caval veins enter the right auricle without uniting, as in
birds. In Seals and some other diving mammals there is a large venous
sinus or dilatation of the inferior vena cava immediately below the
diaphragm. In the Cetacea the purpose of this is supplied by the immense
abdominal venous plexuses. As a rule the veins of mammals are furnished
with valves, but these are said to be altogether wanting in the Cetacea,
and in the superior and inferior cava, subclavian and iliac veins, the
veins of the liver (both portal and hepatic), heart, lungs, kidneys,
brain, and spinal cord of other mammals. Many of the veins within the
cranium are included in spaces formed by the separation of the laminæ of
the dura mater, and do not admit of being dilated beyond a certain size;
these are termed sinuses. The portal circulation in mammals is limited
to the liver, the portal vein being formed by the superior and inferior
mesenteric, the splenic, the gastro-epiploic, and the pancreatic veins.
The kidney is supplied solely by arterial blood, and its veins empty
their contents only into the inferior cava.
_Lymphatic Vessels._—The _absorbent_ or _lymphatic_ system of vessels is
very fully developed in the Mammalia. Its ramifications extend through
all the soft tissues of the body, and convey a colourless fluid called
lymph, containing nucleated corpuscles, and also, during the process of
digestion, the chyle, a milky fluid taken up by the lymphatics (here
called lacteals) of the small intestine, and pour them into the general
vascular system, where they mix with the venous blood. The lymphatic
vessels of the hinder extremities, as well as those from the intestinal
canal, unite in the abdomen to form the “thoracic duct,” the hinder
end or commencement of which has a dilatation called the _receptaculum
chyli_. This duct, which is of irregular size and sometimes double, often
dividing and uniting again in its course, or even becoming plexiform,
passes forwards close to the bodies of the thoracic vertebræ, and
empties itself, by an orifice guarded by a valve, into the great left
brachio-cephalic vein, having previously received the lymphatics from
the thorax and the left side of the head and left anterior extremity.
The lymphatics from the right side of the head and right anterior limb
usually enter by a small distinct trunk into the corresponding part
of the right brachio-cephalic vein. The duct, and also the principal
lymphatic vessels, are provided with valves.
Lymphatic glands, rarely met with in the Sauropsida, are usually
present in mammals, both in the general and in the lacteal system; the
latter being called “mesenteric glands.” They are round or oval masses,
situated upon the course of the vessels, which break up in them and
assume a plexiform arrangement, and then reunite as they emerge. No
structures corresponding to the pulsating “lymphatic hearts” of the lower
vertebrates have been met with in mammals.
_Ductless Glands._—Associated with the vascular and lymphatic systems
are certain bodies (the functions of which are not properly understood),
usually, on account of their general appearance, grouped together under
the name of “ductless glands.” The largest of these is the “spleen,”
which is single, and always placed in mammals in relation to the
fundus or left end of the stomach, to which it is attached by a fold
of peritoneum. It is dark-coloured and spongy in substance, and has a
depression or “hilus” on one side, into which the splenic artery, a
branch of the cœliac axis of the abdominal aorta, enters, and from which
the vein joining the portal system emerges. The spleen varies much in
size and form in different mammals, being relatively very small in the
Cetacea. It is sometimes almost spherical, but more often flattened,
oval, triangular, or elongated, and occasionally, as in Monotremes and
most Marsupials, triradiate. The “suprarenal bodies” or “adrenals” are
two in number, each situated either in contact with, or at a short
distance in front of the anterior extremity of the kidney. They are
abundantly supplied with nerves, and are much larger relatively in
early than in adult life. The “thyroid bodies,” of which there are
generally two, though in Man and some other species they are connected
by an isthmus passing across the middle line, are constant in mammals,
though only met with in a rudimentary condition, if at all, in other
vertebrates. They are situated in the neck, in contact with the sides of
the anterior extremity of the trachea. The “thymus” lies in the anterior
part of the thorax, between the sternum and the great vessels at the
base of the heart, and differs from the thyroid in being median and
single, and having a central cavity. It attains its greatest development
during the period in which the animal is nourished by its mother’s milk,
and then it diminishes, and generally disappears before full growth is
attained.
_Nostrils._—Mammals breathe occasionally through the mouth, but usually,
and in many cases exclusively, through the nostrils or _nares_. Those are
apertures, always paired (except in the toothed Cetacea, where they unite
to form a single external opening), and situated at the fore part of the
face, generally at or beneath the end of the muzzle, a median prominence
above the mouth. This is sometimes elongated to form a proboscis, to
the extremity of which the nostrils are carried, and which attains its
maximum of development in the Elephant. In the Cetacea the nostrils
are situated at a considerable distance behind the anterior end of the
face, upon the highest part of the head, and are called “blowholes,”
from the peculiar mode of respiration of those animals. The nostrils
are kept open by means of cartilages surrounding their aperture, which
many animals have the power of moving so as to cause partial dilatation
or contraction. In diving animals, as Seals and Cetacea, they can be
completely closed at will so as to prevent the entrance of water when
beneath the surface. The passage to which the nostrils lead is in most
mammals filled by a more or less complex sieve-like apparatus, formed
of the convoluted turbinal bones and cartilages, over which a moist,
vascular, ciliated mucous membrane is spread, which intercepts particles
of dust, and also aids in warming the inspired air before it reaches the
lungs. In the Proboscidea, in which these functions are performed by the
walls of the long tubular proboscis, this apparatus is entirely wanting.
_Trachea._—The narial passages have the organ of smell situated in
their upper part, and communicate posteriorly with the pharynx, and
through the glottis with the “trachea” or windpipe, a tube by which
the air is conveyed to and from the lungs. The permanent patency of
the trachea during the varied movements of the neck is provided for by
its walls being stiffened by a series of cartilaginous rings or hoops,
which in most mammals are incomplete behind. Having entered the thorax,
the trachea bifurcates into the two bronchi, one of which enters, and,
dividing dichotomously, ramifies through each lung. In some of the
Cetacea and Artiodactyla a third bronchus is given off from the lower
part of the trachea, above its bifurcation and enters the right lung.
_Larynx._—The upper end of the trachea is modified into the organ of
voice or “larynx,” the air passing through which to and from the lungs
is made use of to set the edges of the “vocal cords,” or fibrous bands
stretched one on each side of the tube, into vibration. The larynx is
composed of several cartilages, such as the “thyroid,” the “cricoid,”
and the “arytenoid” which are moved upon one another by muscles, and
suspended from the hyoidean arch. By alteration of the relative position
of these cartilages the cords can be tightened or relaxed, approximated
or divaricated, as required to modulate the tone and volume of the voice.
A median tongue-shaped fibro-cartilage at the top of the larynx, the
“epiglottis,” protects the “glottis,” or aperture by which the larynx
communicates with the pharynx, from the entry of particles of food during
deglutition. The form of the larynx and development of the vocal cords
present many variations in different members of the class, the greatest
modification from the ordinary type being met with in the Cetacea,
where the arytenoid cartilages and epiglottis are united in a tubular
manner, so as to project into the nasal passage, and, being grasped by
the muscular posterior margin of the palate, provide a direct channel
of communication from the lungs to the external surface. An approach to
this condition is met with in the Hippopotamus and some other Ungulates;
it is indeed so general as an abnormality, that Howes suggests that
an internarial epiglottis may have been a primitive feature common
throughout the class. Nearly all mammals have a voice, although sometimes
it is only exercised at seasons of sexual excitement. Some Marsupials
and Edentates appear to be quite mute. In no mammal is there an inferior
larynx, or “syrinx,” as in birds.
_Diaphragm._—The thoracic cavity of mammals differs from that of the
Sauropsida in being completely separated from the abdomen by a muscular
partition, the “diaphragm,” attached to the vertebral column, the ribs,
and the sternum. This is much arched, with the convexity towards the
thorax, so that when its fibres contract and it is flattened the cavity
of the thorax is increased, and when they are relaxed the cavity is
diminished.
_Lungs._—The lungs are suspended freely in the thorax, one on each side
of the heart, being attached only by the root, which consists of the
bronchus or air-tube and pulmonary arteries and veins by which the blood
is passed backwards and forwards between the heart and the lungs. The
remaining part of the surface of each lung is covered by serous membrane,
the “pleura”; and whatever the state of distension or contraction of the
chest-wall, is accurately in contact with it. Inspiration is effected
by the contraction of the diaphragm and by the intercostal and other
muscles elevating or bringing forward the ribs, and thus throwing the
sternum farther away from the vertebral column. As the surface of the
lung must follow the chest-wall, the organ itself is expanded, and air
rushes in through the trachea to fill all the minute cells in which the
ultimate ramifications of the bronchi terminate. In ordinary expiration
very little muscular power is expended, the elasticity of the lungs and
surrounding parts being sufficient to cause a state of contraction and
thus drive out at least a portion of the air contained in the cells,
when the muscular stimulus is withdrawn. The lungs are sometimes simple
externally, as in the Sirenia (where they are greatly elongated) and the
Cetacea, but are more often divided by deep fissures into one or more
lobes. The right lung is usually larger and more subdivided than the
left. It often has a small distinct lobe behind, wanting on the left
side, and hence called _lobulus azygos_.
_Air-sacs._—Most mammals have in connection with the air passages certain
diverticuli or pouches containing air, the use of which is not always
easy to divine. The numerous air sinuses situated between the outer and
inner tables of the bones of the head, represented in Man by the antrum
of Highmore and the frontal and sphenoidal sinuses, and attaining their
maximum of development in the Indian Elephant, are obviously for the
mechanical purpose of allowing expansion of the osseous surface without
increase of weight. They are connected with the nasal passages. The
Eustachian tubes pass from the back of the pharynx to the cavity of
the tympanum, into which and the mastoid cells they allow air to pass.
In the _Equidæ_ there are large post-pharyngeal air-sacs in connection
with them. The Dolphins have an exceedingly complicated system of
air-sacs in connection with the nasal passages just within the nostrils,
and the Tapirs, Rhinoceroses, and Horses have blind sacs in the same
situation. In the males of some Seals (_Cystophora_ and _Macrorhinus_)
large pouches, which the animal can inflate with air, and which are not
developed in the young animal or the female, arise from the upper part
of the nasal passages, and lie immediately under the skin of the face.
These appear analogous, although not in the same situation, to the gular
pouch of the male Bustard. The larynx frequently has membranous pouches
in connection with it, into which air passes. These may be lateral and
opening just above the vocal cords, when they constitute the _sacculi
laryngis_, found in a rudimentary state in Man, and attaining an enormous
development, so as to reach to the shoulders and axillæ, in some of the
Anthropoid Apes; or they may be median, opening in front either above or
below the thyroid and cricoid cartilages, as in the Howling and other
Monkeys, and also in the Whalebone Whales and Great Anteater.
_Urinary Organs._—The kidneys of mammals are more compact and definite
in form than in other vertebrates, being usually more or less oval,
with an indent on the side turned towards the middle line, from and
into which the vessels and ducts pass. They are distinctly divided into
a cortical secretory portion, composed mainly of convoluted tubes, and
containing the so-called Malpighian bodies; and a medullary excreting
portion, formed of straight tubes converging towards a papilla, embraced
by the commencement of the ureter or duct of the organ. The kidneys of
some mammals, as most Monkeys, Carnivores, Rodents, etc., are simple,
with a single papilla into which all the renal tubuli enter. In others,
as Man, there are many pyramids of the medullary portion, each with its
papilla, opening into a division (calyx) of the upper end of the ureter.
Such kidneys, either in the embryonic condition only, or throughout
life, are lobulated on the surface. In some cases, as in Bears, Seals,
and especially the Cetacea, the lobulation is carried further, the whole
organ being composed of a mass of renules, loosely united by connective
tissue, and with separate ducts, which soon join to form the common
ureter.
_Bladder._—In all mammals except the Monotremes the ureters terminate
by slit-like valvular openings in the urinary bladder. This receptacle
when filled discharges its contents through the single median urethra,
which in the male is almost invariably included in the penis, and in
the females of some species of Rodents, Insectivores, and Lemurs has a
similar relation to the clitoris. In the Monotremes, though the bladder
is present, the ureters do not enter into it, but join the urino-genital
canal some distance below it, with the orifice of the genital duct
intervening.
VI. NERVOUS SYSTEM AND ORGANS OF SENSE.
_Brain._—The brain of mammals shows a higher condition of organisation
than that of other vertebrates. The cerebral hemispheres have a greater
preponderance compared with other parts, especially to the so-called
optic lobes, or corpora quadrigemina, which are completely concealed by
them. The commissural system of the hemispheres is much more complex,
both fornix and corpus callosum being present in some form; and when the
latter is rudimentary, as in Marsupials and Monotremes, its deficiency
is made up for by the great size of the anterior commissure. The lateral
lobes of the cerebellum, wanting in lower vertebrates, are well developed
and connected by a transverse commissure, the pons Varolii. The whole
brain, owing especially to the size of the cerebral hemispheres, is
considerably larger relatively to the bulk of the animal than in other
classes, but it must be recollected that the size of its brain depends
upon many circumstances besides the degree of intelligence which an
animal possesses, although this is certainly one. Man’s brain is many
times larger than that of all other known mammals of equal bulk, and
even three times as large as that of the most nearly allied Ape. Equal
bulk of body is here mentioned, because, in drawing any conclusions
from the size of the brain compared with that of the entire animal, it
is always necessary to take into consideration the fact that in every
natural group of closely allied animals the larger species have much
smaller brains relatively to their general size than the smaller species,
so that, in making any effective comparison among animals belonging to
different groups, species of the same size must be selected. It may be
true that the brain of a Mouse is, as compared with the size of its
body, larger than that of a Man, but, if it were possible to reduce an
animal having the general organisation of a Man to the size of a Mouse,
its brain would doubtless be very many times larger; and conversely, as
shown by the rapid diminution of the relative size of the brain in all
the large members of the Rodent order, a Mouse magnified to the size of
a Man would, if the general rule were observed, have a brain exceedingly
inferior in volume. Although the brain of the large species of Whales is,
as commonly stated, the smallest in proportion to the bulk of the animal
of any mammal, this does not invalidate the general proposition that
the Cetacea have very large brains compared with terrestrial mammals,
like the Ungulata, or even the aquatic Sirenia, as may be proved by
placing the brain of a Dolphin by the side of that of a Sheep, a Pig, or
a Manatee of equal general weight. It is only because the universally
observed difference between the slower ratio of increase of the brain
compared with that of the body becomes so enormous in these immense
creatures that they are accredited with small brains.
The presence or absence of “sulci” or fissures on the surface of
the hemisphere, dividing it into “convolutions” or “gyri,” and thus
increasing the superficies of the cortical gray matter, as well as
allowing the pia mater with its nutrient blood-vessels to penetrate
into the cerebral substance, follow somewhat similar rules. The sulci
are related partly to the high or low condition of organisation of
the species, but also in a great degree to the size of the cerebral
hemispheres. In very small species of all groups, even the Primates, they
are absent, and in the largest species of groups so low in the scale as
the Marsupials and Edentates they are found. They reach their maximum of
development in the Cetacea.
The accompanying woodcut (Fig. 23) shows the principal parts of a
mammalian brain, as seen from the superior, lateral, and inner surfaces.
The sylvian fissure (_sf_) is one of the most constant of the sulci found
in the hemispheres.
[Illustration: FIG. 23.—Brain of the Genet (_Genetta tigrina_). A, From
above; B, from the right side; C, inner surface of right hemisphere;
_cc_, corpus callosum; _c.m.s_, calloso-marginal sulcus; _c_, notch
representing central sulcus of other forms; _d_, depression on superior
lateral gyrus of hemisphere; _hg_, hippocampal gyrus; _i_, inferior
lateral gyrus of hemisphere; _m_, middle lateral gyrus of do.; _s_,
superior lateral gyrus of do.; _os_, supraorbital sulcus of do.; _sf_,
sylvian fissure of do.; _ol_, olfactory lobes. The deeply convoluted part
behind the cerebral hemisphere is the cerebellum, below which lies the
medulla oblongata, or commencement of the spinal cord. (Mivart, _Proc.
Zool. Soc._ 1882, p. 516.)]
The researches of Palæontologists, founded upon studies of casts of the
interior of the cranial cavity of extinct forms, have shown that, in
many natural groups of mammals, if not in all, the brain has increased
in size, and also in complexity of surface foldings, with the advance
of time,—indicating in this, as in so many other respects, a gradual
progress from a lower to a higher type of development.
_Nerves._—The twelve pairs of cranial nerves generally recognised in
vertebrates are usually all found in mammals, though the olfactory nerves
are excessively rudimentary, if not altogether absent, in the Toothed
Whales. The spinal cord, or continuation of the central nervous axis,
lies in the canal formed by the neural arches of the vertebræ, and gives
off the compound double-rooted nerves of the trunk and the extremities,
corresponding in number to the vertebræ, through the interspaces between
which they pass out to their destination. The cord is somewhat enlarged
at the two points where it gives off the great nerves to the anterior and
the posterior extremities, which, from their interlacements soon after
their origin, are called respectively the brachial and lumbar plexuses.
The ganglionic or sympathetic portion of the nervous system is well
developed, and presents few modifications.
_Sense of Touch._—The sense of touch is situated in the skin generally,
but is most acute in certain regions more or less specialised for the
purpose by the presence of tactile papillæ, such as portions of the face,
especially the lips and end of the snout, and the extremities of the
limbs when these are used for other purposes than mere progression, and
the under surface of the end of the tail in some Monkeys. The “vibrissæ”
or long stiff bristles situated on the face of many mammals are rendered
extremely sensitive to touch by the abundant supply of branches from
the fifth nerve to their basal papillæ. In Bats the extended wing
membranes, and probably also the large ears and the folds and prominences
of skin about the face of some species, are so sensitive as to receive
impressions even from the different degrees of resistance of the air, and
so enable the animals to avoid coming in contact with obstacles to their
nocturnal flight.
_Taste and Smell._—The organs of the other special senses are confined
to the head. Taste is situated in the papillæ scattered on the dorsal
surface of the tongue. The organ of smell is present in all mammals
except the Toothed Whales. It consists of a ramification of the olfactory
nerves over a plicated, moist, mucous membrane, supported by folded
plates of bone, placed on each side of the septum nasi in the roof, or
often in a partially distinct upper chamber, of the nasal passage, so
arranged that, of the air passing into the lungs in inspiration, some
comes in contact with it, causing the perception of any odorous particles
with which it may be charged. Many mammals possess intense powers of
smelling certain odours which others are quite unable to appreciate, and
the influence which this sense exercises over the well-being of many
species is very great, especially in indicating the proximity of others
of the same kind, and giving warning of the approach of enemies. The
development and modification of the sense of smell is probably associated
with that of the odorous secretion of the cutaneous glands.
_Sight._—The organ of sight is quite rudimentary, and even concealed
beneath the integument, in some burrowing Rodents and Insectivores,
and is most imperfectly developed in the _Platanista_, or Freshwater
Dolphin of the rivers of India. In all other mammals the eyeball has
the structure characteristic of the organ in the higher Vertebrata,
consisting of parts through which the rays of light are admitted,
regulated, and concentrated upon the sensitive expansion of the
optic nerve lining the posterior part of the ball. A portion of the
fibrovascular and highly pigmented layer, the choroid, which is
interposed between the retina and the outer sclerotic coat, is in many
mammals modified into a brilliantly-coloured light-reflecting surface,
the _tapetum lucidum_. There is never a pecten or marsupium like that
of the Sauropsida, nor is the sclerotic ever supported by a ring of
flattened ossicles, as is so frequently the case in the lower vertebrated
classes. The eyeball is moved in various directions by a series of
muscles—the four straight, two oblique, and, except in the higher
Primates, a posterior retractor muscle called choanoid. The superior
oblique muscle passes through a tendinous pulley fastened to the roof
of the orbit, which is a feature not found beyond the limits of the
mammalian class. The eye is protected by the lids, generally distinctly
separated into an upper and a lower movable flap, which, when closed,
meet over the front of the eye in a more or less nearly horizontal line:
but sometimes, as in the Sirenia, the lids are not distinct, and the
aperture is circular, closing to a point. In almost all mammals below the
Primates, except the Cetacea, a “nictitating membrane” or third eyelid is
placed at the inner corner of the eyeball, and works horizontally across
the front of the ball within the true lids. Its action is instantaneous,
being apparently for the purpose of cleaning the front of the transparent
cornea;—a function unnecessary in animals whose eyes are habitually
bathed in water, and which in Man and his nearest allies is performed
by winking the true eyelids. Except in Cetacea the surface of the eye
is kept moist by the secretion of the lachrymal gland, placed under the
upper lid at its outer side, and the lids are lubricated by the Harderian
and Meibornian glands, the former being situated at the inner side of the
orbit, and especially related to the nictitating membrane, the latter in
the lining membrane of the lids.
_Hearing._—The organ of hearing is inclosed in a bony capsule (periotic)
situated in the side of the head, intercalated between the posterior
(occipital) and the penultimate (parietal) segment of the skull. It
has, in common with other vertebrates, three semicircular canals and
a vestibule, but the cochlea is more fully developed than in the
Sauropsida, and, except in the Monotremes, spirally convoluted. The
tympanic cavity is often dilated below, forming a smooth rounded
prominence on the base of the skull, the auditory bulla (Fig. 8). The
three principal ossicles, the “malleus,” “incus,” and “stapes,” are
always present, but variable in characters. In the Sirenia, Cetacea, and
Seals they are massive in form, being in the first-named order of larger
size than in any other mammals. In the Cetacea the malleus is ankylosed
to the tympanic; but in other mammals it is connected only with the
membrana tympani. The stapes in the lower orders—Edentates, Marsupials,
and Monotremes—has a great tendency to assume the columnar form of the
corresponding bone in Sauropsida, its two rami entirely or partially
coalescing.[16] The tympanic membrane (drum of the ear) forms the outer
wall of the cavity. In the fœtal state it is level with the external
surface of the skull, and remains so permanently in a few mammals as the
American Monkeys; but commonly, by the growth of the squamosal bone, it
becomes deeply buried at the bottom of a bony tube (_meatus auditorus
externus_), which is continued to the surface of the skin in a fibrous
or fibro-cartilaginous form. In Whales, owing to the thickness of the
subcutaneous adipose tissue, this meatus is of great length, and is also
extremely narrow. In most aquatic and burrowing animals it opens upon
the surface by a simple aperture, but in the large majority of the class
there is a projecting fold of skin, strengthened by fibro-cartilages,
called the pinna, auricle, or “external ear,” of very variable size and
shape, generally movably articulated on the skull, and provided with
muscles to vary its position; this pinna helping to collect and direct
the vibrations of sound into the meatus.
VII. REPRODUCTIVE ORGANS.
_Testes._—In the male the testes retain nearly their primitive or
internal position throughout life in the Monotremata, Sirenia, Cetacea,
most Edentata, Hyracoidea, Proboscidea, and Seals, but, in other groups
they either periodically (as in Rodentia, Insectivora, and Chiroptera)
or permanently pass out of the abdominal cavity through the inguinal
canal, forming a projection beneath the skin of the perineum, or becoming
suspended in a distinct pouch of integument called the scrotum. All the
Marsupials have a pedunculated scrotum, the position of which differs
from that of other mammals, being in front of, instead of behind, the
preputial orifice. As regards the presence, absence, or comparative size
and number of the accessory generative glands—prostate, vesicular, and
Cowper’s glands, as they are called—there is much variation in different
groups of mammals.
_Penis._—The penis is almost always completely developed, consisting of
two corpora cavernosa attached to the ischial bones, and of a median
corpus spongiosum enclosing the urethra, and forming the glans at
the distal portion of the organ. In Marsupials, Monotremes, and the
Sloths and Anteaters, the corpora cavernosa are not attached directly
to the ischia, and in the last-named the penis is otherwise of a very
rudimentary character, the corpus spongiosum not being present. In many
Marsupials the glans penis is bifurcated. In most Primates, Carnivora,
Rodentia, Insectivora, and Chiroptera, but in no other orders, an _os
penis_ is present.
_Ovaries and Oviduct._—In the female, the ovaries permanently retain
their original abdominal position, or only descend a short distance
into the pelvis. They are of comparatively smaller size than in other
vertebrates, have a definite flattened oval form, and are enclosed in a
more or less firm “tunica albiginea.” The oviduct has a trumpet-like, and
usually fimbriated abdominal aperture, and is more or less differentiated
into three portions:—(1) a contracted upper part, called in Man and
the higher mammals the “Fallopian tube”; (2) an expanded part with
muscular walls, in which the ovum undergoes the changes by which it is
developed into the fœtus, called the “uterus”; (3) a canal, the “vagina,”
separated from the last by a valvular aperture, and terminating in the
urino-genital canal, or common urinal and genital passage, which in
higher mammals is so short as scarcely to be distinct from the vagina.
The complete distinction of the oviducts of the two sides throughout
their whole length, found in all lower vertebrates, only occurs in this
class in Monotremes; a prevailing mammalian characteristic being their
more or less perfect coalescence in the middle line to form a single
median canal. In the Marsupials this union only includes the lower part
of the vagina; but in most Placentals it extends to the whole vagina
and a certain portion of the uterus, which cavity is then described as
“bicornuate.” In the higher mammals, as in Man, and also in some of
the Edentates, the whole of the uterus is single, the contracted upper
portion of the oviducts or Fallopian tubes, as they are then called,
entering its upper lateral angles by small apertures. In certain lower
forms the urino-genital canal opens with the termination of the rectum
into a common cloaca, as in other vertebrates; but it is characteristic
of the majority of the class that the two orifices are more or less
distinct externally.
_Mammary Glands._—Mammary glands secreting the milk by which the young
are nourished during the first portion of their existence after birth,
are present in both sexes in all mammals, though usually only functional
in the female. In the Monotremes alone their orifices are mere scattered
pores in the skin, but in all other forms they are situated upon the
end of conical elevations, called mammillæ, or teats, which, taken into
the mouth of the young animal, facilitate the process of sucking. These
are always placed in pairs upon some part of the ventral surface of the
body, but vary greatly in number and position in different groups. In the
Cetacea, where the prolonged action of sucking would be incompatible with
their subaqueous life, the ducts of the glands are dilated into large
reservoirs from which the contents are injected into the mouth of the
young animal by the action of a compressor muscle.
_Secondary Sexual Characters._—Secondary sexual characters, or
modifications of structure peculiar to one sex, but not directly related
to the reproductive function, are very general in mammals. They almost
always consist of the acquisition or perfection of some character by
the male as it attains maturity, which is not found in the female or
the young in either sex. In a large number of cases these clearly
relate to the combats in which the males of many species engage for
the possession of the females during the breeding season; others are
apparently ornamental, and of many it is still difficult to apprehend
the meaning. Many suggestions on this subject will, however, be found in
the chapters devoted to it in Darwin’s work on _The Descent of Man and
Selection in Relation to Sex_, where most of the best-known instances are
collected. Superiority of size and strength in the male of many species
is a well-marked secondary sexual character related to the purpose
indicated above, being probably perpetuated by the survivors or victors
in combats transmitting to their descendants those qualities which gave
them advantages over others of their kind. To the same category belong
the great development of the canine teeth of the males of many species
which do not use these organs in procuring their food, as the Apes,
Swine, Musk and some other Deer, the tusk of the male Narwhal, the
antlers of Deer, which are present in most cases only in the males, and
the usual superiority in size and strength of the horns of the _Bovidæ_.
Other secondary sexual characters, the use of which is not so obvious, or
which may only relate to ornament, are the presence of masses or tufts
of long hair on different parts of the body, as the mane of the male
Lion and Bison, the beards of some Ruminants and Bats (as _Taphozous
melanopogon_), Monkeys, and of Man, and all the variations of coloration
in the sexes, in which, as a general rule, the adult male is darker and
more vividly coloured than the female. Here may also be mentioned the
presence or the greater development of odoriferous glands in the male,
as in the Musk Deer, and the remarkable perforated spur with its glands
and duct, so like the poison-tooth of the venomous serpents, found in the
males of both _Ornithorhynchus_ and _Echidna_, the use of which is at
present unknown.
_Placenta._—The development of the mammalian ovum, and the changes which
the various tissues and organs of the body undergo in the process of
growth, are too intricate subjects to be explained without entering into
details incompatible with the limits of this work, especially as they
scarcely differ, excepting in their later stages, from those of other
vertebrates, upon which, owing to the greater facilities these present
for examination and study, the subject has been more fully worked out.
There are, however, some points which require notice, as peculiar to the
mammalian class, and as affording at least some hints upon the difficult
subject of the affinities and classification of the members of the group.
The nourishment of the fœtus during intra-uterine life takes place
through the medium of certain structures, partly belonging to the fœtus
itself and partly belonging to the inner parietes of the uterus of the
parent. These in their complete form constitute the complex organ called
the “placenta,” serving as the medium of communication between the mother
and fœtus, and in which the physiological processes that are concerned
in the nutrition of the latter take place; but as we shall see, though
a placenta, in the usual acceptation of the term, is peculiar to the
mammalian class, it is not in all of its members that one is developed.
The structures to which we shall have especially to refer are the outer
tunic of the ovum, to which, however formed, the term “chorion” is
commonly applied, and two sac-like organs connected with the body-cavity
of the embryo, both formed from the splanchnic mesoblast, lined by a
layer of the hypoblast. These are the “umbilical vesicle” or “yolk-sac”
and the “allantois.”
The umbilical vesicle is a thin membrane enclosing the yolk, which by
the doubling in of the ventral walls of the embryo becomes gradually
formed into a distinct sac external to the body, with a pedicle (the
omphalo-enteric duct) by which for a time a communication is maintained
between its cavity and the intestinal canal. In the walls of this sac
blood-vessels (omphalo-meseraic or vitelline) are developed in connection
with the vascular system of the embryo, through which, either by their
contact with the outer surface of the walls of the ovum, or by the
absorption through them of the contents of the yolk-sac, the nutrition
of the embryo in the lower vertebrates chiefly takes place. In mammals
the umbilical vesicle plays a comparatively subordinate part in the
nourishment of the fœtus, its function being generally superseded by the
allantois.
The last-named sac commences at a very early period as a diverticulum
from the hinder end of the alimentary tract of the embryo. Its proximal
portion afterwards becomes the urinary bladder, the contracted part
between this and the cavity of the allantois proper constituting the
urachus, which passes out of the body of the fœtus at the umbilicus
together with the vitelline duct. The mesoblastic tissue of the walls of
the allantois soon becomes vascular; its arteries are supplied with fœtal
blood by the two hypogastric branches of the iliacs, or main divisions of
the abdominal aorta, and the blood is returned by venous trunks uniting
to form the single umbilical vein which runs to the under surface of the
liver, where, part of it joining the portal vein and part entering the
vena cava directly, it is brought to the heart. These are the vessels
which, with their surrounding membranes, constitute the umbilical
cord—the medium of communication between the fœtus and the placenta, when
that organ is fully developed.
The egg membranes of the Monotremes present many points of agreement
with those of the ovum of the Marsupials,[17] and differ from those of
the Placental types. Thus Monotremes and Marsupials agree in having
a vitelline membrane, which appears between the young ovum and the
follicular epithelium, persisting in the one case until the time of
hatching, and in the other till a late uterine stage. There are also
several other common features fully described in Mr. Caldwell’s memoir,
but which cannot be detailed in this work.
In the Marsupialia the observations made many years ago by Sir R. Owen
upon the development of the Kangaroo have been confirmed by those of Dr.
H. C. Chapman,[18] while Dr. Selenka,[19] and Professor H. F. Osborn[20]
have contributed important evidence as to the structure and relations
of the fœtal membranes of the Opossums and others. It thus appears that
up to the period of the very premature birth of these animals the outer
covering of the ovum, or false chorion, is free from persistent villi,
and not adherent to the epithelium of the uterine walls; for, although
fitting into the folds of the latter, it is perfectly and readily
separable in its entire extent from them. The umbilical vesicle or
yolk-sac is large, vascular, and adherent to a considerable portion of
the false chorion or subzonal membrane, while the allantois is relatively
small, and although the usual blood-vessels can be traced into it, it
does not appear to contract any connection with the false chorion, and,
therefore, much less with the walls of the uterus, of such a nature as
to constitute a placenta. In some forms, however, such as the Opossums,
the umbilical vesicle or yolk-sac develops temporary villi, which unite
with the subzonal membrane, or false chorion, to form a disc-like area
closely attached to the cells covering the utricular glands of the
uterine epithelium, and thus forming a so-called _yolk-sac placenta_.
The function of this organ is considered to be the transmission of
the secretions of the utricular glands to the embryo by means of the
umbilical vesicle; the function of the allantois being either respiratory
or the absorption of the fluid secreted in the uterine cavity by the
utricular glands.
While in the uterus the nourishment of the fœtus seems, therefore, to be
derived from the umbilical vesicle, as in reptiles and birds, rather
than from the uterine walls by means of the allantoic vessels, as in
the higher mammals. The latter vessels, in fact, play even a much less
important part in the development of these animals, not only than in
the placental mammals, but even than in the Sauropsida, for they can
scarcely have the respiratory function assigned to them in that group:
pulmonary respiration and the lacteal secretion of the mother very early
superseding all other methods of providing the due supply, both of oxygen
and of food required for the development and growth of the young animal.
In this sense the Marsupials may be looked upon as the most typically
“mammalian” of the whole class. In no other group do the milk-secreting
glands play such an important part in providing for the continuity of the
race.
In the third primary division of the Mammalia, the so-called Placentalia,
the umbilical vesicle generally does not quite unite with the chorion,
and disappears as development proceeds, so that no trace of it can be
seen in the membranes of an advanced embryo; but it may persist until the
end of the intra-uterine life as a distinct sac in the umbilical cord, or
lying between the allantois and amnion. The disappearance or persistence
of the umbilical vesicle does not, according to our present knowledge,
appear to be correlated with a higher or lower general grade of
development, as might be presupposed. It is stated to have been found in
Man even up to the end of intra-uterine life, and also in the Carnivora,
while in the Ungulata and Cetacea it disappears at an earlier age. In
many, if not all, of the Rodentia, Insectivora, and Chiroptera, it plays
a more important part, becoming adherent to a considerable part of the
inner surface of the chorion, to which it conveys blood-vessels, although
villi do not appear to be developed from the surface of this part, as
they are on the portion of the chorion supplied by the allantoic vessels.
These orders thus present to a certain extent a transitional condition
from the Marsupials, although essentially different, in possessing the
structures next to be described.
The special characteristic of the whole of the placental mammals
constituting the majority of the class, is that the allantois and its
vessels become intimately blended with a smaller or greater part of the
parietes of the ovum, forming a structure on the outer surface of which
villi are developed, and which, penetrating into corresponding cavities
of the “decidua,” or soft, vascular, hypertrophied lining membrane of
the uterus, constitutes the placenta. This organ may be regarded, as Sir
William Turner says, both in its function and in the relative arrangement
of its constituent textures, as a specially modified secreting gland, the
ducts of which are represented by the extremities of the blood-vessels
of the fœtal system. The passage of material from the maternal to the
fœtal-system of vessels is not a simple percolation or diffusion through
their walls, but is occasioned by the action of a layer of cells derived
from the maternal or uterine structures, and interposed between the
blood-vessels of the maternal part of the placenta and those of the villi
covering the chorion, in which the embryonic vessels ramify.
The numerous modifications in the details of the structure of this
organ relate to augmenting the absorbing capacity of the vessels of the
chorion, and are brought about either by increasing the complexity of the
fœtal villi and maternal crypts over a limited area, or by increasing the
area of the part of the chorion covered by the placental villi, or by
various combinations of the two methods.
The first class of variations has given rise to a distinction into
two principal kinds of placenta: (1) simple or non-deciduate, and
(2) deciduate. In the former the fœtal villi are received into
corresponding depressions of the maternal surface, from which at the
period of parturition they are simply withdrawn. In the second, or more
complex form, the relation is more intimate, a layer of greater or
less thickness of the lining membrane of the uterus, called “decidua,”
becoming so intimately blended with the chorion as to form part of the
placenta proper, or that structure which is cast off as a solid body
at parturition. In other words, in the one case the line of separation
between the placenta and uterus at birth takes place at the junction of
the fœtal and maternal structures, in the other through the latter, so
that a portion of them, often of considerable thickness, and containing
highly organised structures, is cast off with the former. It was once
thought that the distinction between these two forms of placentation is
so important as to constitute a sufficiently valid basis for a primary
division of the placental mammals into two groups. It has, however, been
shown that the distinction is one rather of degree than of kind, as
intermediate conditions may exist, and it is probable that in different
primary groups the simpler, non-deciduate form may have become developed
independently into one or other of the more complex kinds.
Apart from its intimate structure, the placenta may be met with of very
varied general form. It may consist of villi scattered more or less
regularly over the greater part of the surface of the chorion, the two
extremities or poles being usually more or less bare. This form is called
the “diffused placenta.” It is probably a primitive condition, from which
most of the others are derived, although its existence must presuppose
the absence of the umbilical vesicle as a constituent of the chorionic
wall. It is found at present in the Manis among Edentates, the Cetacea,
the Perissodactyle Ungulates, and the Camels, Pigs, and Chevrotains
among the Artiodactyles. Such placentæ are always non-deciduate. Recent
observations by Sir W. Turner on the placentation of the Dugong show that
the Sirenia present the peculiarity of having a zonary placenta, which
is either entirely or in great part non-deciduate, and is, therefore,
transitional between the diffused and the true zonary type.
In the true Ruminants or Pecora, among the Artiodactyle Ungulates,
the villi are aggregated in masses called cotyledons, with bare
spaces between. Such a placentation is called “polycotyledonary.” In
another modification the villi are collected in a more or less broad
band encircling the chorion, leaving a very large portion of the two
poles bare, constituting the “zonary placenta,” characteristic of the
Carnivora, and also occurring in the Elephant, Hyrax, and Orycteropus.
The fact of the form of the placenta of these three last-named animals
agreeing together, and with that of the Carnivora, does not, however,
necessitate the ascription of zoological affinities, as the same ultimate
form may have been attained by different processes of development.
In another form one pole only of the chorion is non-vascular, the
placenta assuming a dome or bell shape, as in the Lemurs and the
Sloths. The transition from this, by the gradual restriction of the
vascular area, is easy to the oval or discoidal form of placenta of
the Anteaters, Armadillos, and higher Primates. The discoidal placenta
of the Rodents, Insectivores, and Chiroptera, though showing so much
superficial resemblance to that of the last-named order as to have led
to the inclusion of all these forms in one primary group, is now known
to be developed in another manner, not by the concentration of villi
from a diffused to a limited area, but by retaining the area to which
it was originally restricted in consequence of the large surface of the
chorion occupied, as before mentioned, by the umbilical vesicle. To
compensate for the smallness of area, the complex or deciduate structure
has been developed. Among some Rodents there is evidence to show that the
discoidal placenta has been derived from a zonary one, of which distinct
vestiges have been detected in the Mouse. We may conclude that, although
the characters and arrangement of the fœtal structures may not have that
extreme importance which has been attributed to them by some zoologists,
they will form, especially when more completely understood, valuable aids
in the study of the natural affinities and evolution of the Mammalia.[21]
CHAPTER III
ORIGIN AND CLASSIFICATION OF THE MAMMALIA
_Origin._—Although, as stated in the first chapter, the mammalian
class, as at present known either by existing or extinct forms, is
completely isolated from all other groups of the animal kingdom, yet it
is impossible to refrain from speculating as to its origin and nearest
affinities. In arranging the classes of vertebrates in a linear series
it is customary to place them in the following order—Pisces, Amphibia,
Reptilia, Aves, Mammalia,—an order which probably indicates the relative
degree of elevation to which the most highly developed members of each
class has attained. Such an arrangement appears to express the true
relationship of the first four classes to one another, but it is quite
clear that the Mammalia have no sort of affinity with the Aves. Writing
in 1879, Professor Huxley[22] came to the conclusion that, in looking
among vertebrates for the progenitors of the Mammalia, we must pass
over all known forms of birds and reptiles, and go straight down to the
Amphibia. In addition to the characters derived from the conformation of
the pelvis upon which the argument was primarily based, the following
reasons were given for this conclusion: “The Amphibia are the only
air-breathing Vertebrata which, like mammals, have a dicondylian skull.
It is only in them that the articular element of the mandibular arch
remains cartilaginous, while the quadrate ossification is small, and the
squamosal extends down over it to the osseous elements of the mandible,
thus affording an easy transition to the mammalian condition of those
parts. The pectoral arch [girdle] of the Monotremes is as much amphibian
as it is sauropsidian; the carpus and the tarsus of all Sauropsida,
except the Chelonia, are modified away from the Urodele type, while those
of the mammal are directly reducible to it. Finally, the fact that in
all Sauropsida it is a right aortic arch which is the main conduit of
arterial blood leaving the heart, while in mammals it is a left aortic
arch which performs this office, is a great stumbling-block in the way
of the derivation of the Mammalia from any of the Sauropsida. But, if
we suppose the earliest forms of both the Mammalia and the Sauropsida
to have had a common Amphibian origin, there is no difficulty in the
supposition that, from the first, it was a left aortic arch in the one
series, and the corresponding right aortic arch in the other, which
became the predominant feeder of the arterial system.” Subsequently
Professor E. D. Cope[23] in a suggestive paper called attention to the
remarkable resemblances to the Monotremes presented by the skeleton of
that group of early secondary reptiles which he then designated the
Theromorpha, but which may be included in the Anomodontia of Sir R. Owen,
and came to the conclusion that in that group we have the true ancestors
of the Mammalia. This conclusion was, however, disputed by Dr. Baur,[24]
who considered that the Anomodontia were too specialised to have been
the actual progenitors of the Mammalia, and that they should rather be
regarded as a divergent branch of the stem which had given origin to the
Mammalia. Since that date observations made on the structure of the South
African Anomodonts have shown such an intimate connection between that
group and the Labyrinthodont Amphibians, that there can be no hesitation
in regarding the one as the direct descendant of the other; and we may
probably regard the Mammalia as having originated from the same ancestral
stock at the time the Amphibian type was passing into the Reptilian. From
this point of view, some of the mammalian features found in the more
specialised Anomodonts may probably be regarded as having been acquired
during a parallel line of development.
Both the Anomodontia and the Mammalia differ from the Amphibians in the
loss of the splint-like parasphenoid which underlies the basisphenoid
axis of the skull, and by the ossification of that axis; but while the
former have become monocondylic by the participation of the basioccipital
in the support of the cranium, the latter retain the Amphibian dicondylic
plan. The skull of the Anomodonts presents mammalian resemblances not
found in any other Reptiles, this being especially noticeable in the
region of the squamosal; and it is only in this group and mammals that
the temporal or zygomatic arch is a squamoso-maxillary one (see p.
37). The resemblance between the pectoral and pelvic girdles of the
Anomodonts and those of the Monotreme Mammals is noticed under the head
of the latter, where reference is also made to the similarity in the
structure of the humerus in the two groups. The pes of the Amphibia and
Anomodontia agree in having a distinct intermedium, tibiale, fibulare,
and centrale, whereas in other Reptiles these bones are not generally
distinct; in Mammals the intermedium, fibulare, and centrale are
distinct, and according to Cope’s interpretation there may be a distinct
tibiale.
_Classification._—In the present condition of the world, mammals
have become so broken up into distinct groups by the extinction of
intermediate forms, that a systematic classification is perfectly
practicable. Most of the associations of species, which we call “orders,”
and even the “suborders” and “families,” are natural groups. In
isolating, defining, and naming them, we are really dealing with facts
of nature of a totally different order from the artificial and fanciful
divisions formed in the infancy of zoological science.
When, however, we pass to the extinct world, all is changed. In many
cases the boundaries of our groups become enlarged until they touch those
of others. New forms are discovered which cannot be placed within any of
the existing divisions. As the horizon of our vision is thus expanded,
the principles upon which a scheme of classification is constructed must
be altogether changed. Our present divisions and terminology are no
longer sufficient for the purpose; and some other method will have to be
invented to show the complex relationships existing between different
animal forms when viewed as a whole. The present time, pre-eminently
distinguished by the rapidly changing and advancing knowledge of extinct
forms, is scarcely one in which this can be done with any satisfactory
result; so that all attempts to form a classification embracing even the
already known extinct species must be only of a provisional and temporary
nature.
In systematic descriptions in books, in lists, and catalogues, and in
arranging collections, the objects dealt with must be placed in a single
linear series. But by no means whatever can such a series be made to
coincide with natural affinities. The artificial character of such an
arrangement, the constant violation of all true relationships, are the
more painfully evident the greater the knowledge of the real structure
and affinities. But the necessity is obvious; and all that can be done is
to make such an arrangement as little as possible discordant with facts.
The following table contains a list of the orders, suborders, and
families of existing mammals as recognised by the authors, and placed
in the order in which they will be treated of in this work. The more
important of the groups containing only extinct forms are added in a
different type, being interpolated, as near as may be, among those that
appear to be their existing relatives.
A few explanatory remarks upon the mutual relations of some of the
principal groups mentioned in the table may be useful here, but the
subject will be more fully developed in treating separately of each
division.
One of the most certain and fundamental points in the classification
of the Mammalia is, that all the animals now composing the class can
be grouped primarily into three natural divisions, which, presenting
very marked differential characters, and having no existing, or yet
certainly demonstrated extinct, intermediate, or transitional forms,
may be considered as subclasses of equal value, taxonomically speaking,
though very different in the numbers and importance of the animals at
present composing them. These three groups are often called by the names
originally proposed for them by Blainville—(1) _Ornithodelphia_, (2)
_Didelphia_, (3) _Monodelphia_—the first being equivalent to the order
_Monotremata_, the second to the _Marsupialia_, and the third including
all the remaining members of the class. Although actual palæontological
proof is wanting, there is much reason to believe that each of these, as
now existing, are survivors of distinct branches to which the earliest
forms of mammals have successively given rise, and for which hypothetical
branches Professor Huxley has proposed the names of _Prototheria_,
_Metatheria_, and _Eutheria_, names which, being far less open to
objection than those of Blainville, are here used as equivalents of the
latter.
The only known existing PROTOTHERIA, although agreeing in many important
characters, evidently represent two very divergent stocks, perhaps as
far removed as are the members of some of the accepted orders of the
Eutheria. It would, however, be merely encumbering zoological science
with new names to give them any other than the ordinarily known family
designations of _Ornithorhynchidæ_ and _Echidnidæ_.
Similarly with regard to the METATHERIA, although the great diversity
in external form, in anatomical characters, and in mode of life of the
various animals of this section might lead to their division into groups
equivalent to the orders of the Eutheria, we do not think it advisable to
depart from the usual custom of treating them all as forming one order,
called Marsupialia, the limits of which are equivalent to those of the
subclass. The characters of the six families which compose the group are
extremely well marked and easily defined; and since they form a regular
gradation between two extreme types, they can be satisfactorily arranged
in a serial order. A marked distinction in the dentition enables us to
divide them into primary groups or suborders.
The remaining mammals are included in the EUTHERIA, PLACENTALIA, or
MONODELPHIA. Their affinities with one another are so complex that
it is impossible to arrange them serially with any regard to natural
affinities. Indeed each order is now so isolated that it is almost
impossible to say what its affinities are; and none of the hitherto
proposed associations of the orders into larger groups stand the test
of critical investigation. All serial arrangements of the orders are
therefore perfectly arbitrary; and although it would be of very great
convenience for reference in books and museums if some general sequence,
such as that here proposed, were generally adopted, such a result can
scarcely be expected, since equally good reasons might be given for
almost any other combination of the various elements of which the series
is composed. In fact, we have already seen reason to depart in some
respects from that used in the “Encyclopædia.”
The Edentata, Sirenia, and Cetacea stand apart from all the rest in the
fact that their dentition does not conform to the general heterodont,
diphyodont type to which that of all other Eutheria can be reduced, and
which is such a close bond of union between them. In all three orders,
however, some indications may be traced of relationship, however distant,
with the general type.
With regard to the Edentata, reasons will be given for believing
that both the Sloths and Anteaters are nearly related, and that the
Armadillos, though much modified, belong to the same stock, but that the
Pangolins and the Aard-varks represent very isolated forms.
There is no difficulty about the limits of the order Sirenia, comprising
aquatic, vegetable-eating animals, with complete absence of hind limbs,
and low cerebral organisation, represented in our present state of
knowledge only by two existing genera, _Halicore_ and _Manatus_, and a
few extinct forms, which, though approaching a more generalised mammalian
type, show no special characters allying them to any of the other orders.
The few facts as yet collected relating to the former history of the
Sirenia leave us as much in the dark as to the origin and affinities of
this peculiar group of animals as we were when we only knew the living
members. They lend no countenance to their association with the Cetacea;
and, on the other hand, their supposed affinity with the Ungulata
receives no very material support from them.
Another equally well-marked and equally isolated, though far more
numerously represented and diversified order, is that of the Cetacea,
placed simply for convenience next to the Sirenia; with which, except in
their fish-like adaptation to aquatic life, they have little in common.
The old association of these orders in one group can only be maintained
either in ignorance of their structure or in an avowedly artificial
system. Among the existing members of the order, there are two very
distinct types, the toothed Whales or Odontoceti, and the Baleen Whales
or Mystacoceti, which present as many marked distinguishing structural
characters as are found between many other divisions of the Mammalia
usually reckoned as orders. Since the extinct Zeuglodonts, so far as
their characters are known, do not fall into either of these groups, but
are in some respects annectant forms, we have placed them provisionally,
at least, in a third group by themselves, named Archæoceti. There is
nothing known at present to connect the Cetacea with any other order of
Mammals; but it is quite as likely that they are offsets of a primitive
Ungulate as of a Carnivorous type, or perhaps of a still more generalised
mammalian stock.
The remaining Eutherian mammals are clearly united by the characters
of their teeth, being all heterodont and diphyodont, with their dental
system reducible to a common formula.
Although older views of the relationship of Ungulate mammals expressed
by the terms _Pachydermata_, _Ruminantia_, and so forth, still
linger in some corners of zoological literature, no single point in
zoological classification can be considered so firmly established as
the distinction between the Perissodactyle and Artiodactyle Ungulates;
both being in the existing fauna of the world perfectly natural and
distinctly circumscribed groups. The breaking-up of the latter into
four equivalent sections, the Pecora, Tylopoda, Tragulina, and Suina,
is equally in accordance with all known facts. Less certain, however,
is the association of the Proboscidea and the Hyracoidea with the true
Ungulates. By many zoologists they are each, although containing so very
few existing species, made into distinct orders; and much is to be said
in favour of this view. The discovery, however, of a vast number of
extinct species of Ungulates which cannot be brought under the definition
of either Perissodactyla or Artiodactyla, and yet are evidently allied
to both, and to a certain extent bridge over the interval between them
and the isolated groups just mentioned, make it necessary either to
introduce a number of new and ill-defined ordinal divisions, or so to
widen the scope of the original order as to embrace them all, considering
the Elephants and the Hyraces as representing suborders equivalent to
the great Perissodactyle and Artiodactyle groups. It is the latter
alternative that we have adopted.
The Rodentia, although generally presenting a low grade of development,
are a very specialised and distinct group. The position here assigned to
them would accord with apparent relationships with the Ungulates, through
the Elephant on the one hand and the extinct _Typotherium_ on the other.
In the present state of the fauna of the earth, the Carnivora form a
very distinct order, though naturally subdivided into two groups, the
members of the one being more typical, while those of the other (the
_Pinnipedia_) are aberrant, having the whole of their organisation
specially modified for living habitually in the water.
The Insectivora comprise various lowly organised and generalised forms,
exhibiting considerable divergence of character, and apparently connected
through transitional extinct species with the Carnivora. As no other
order can claim the family _Galeopithecidæ_, it is placed here, but
rather for convenience than for any other consideration, since it has but
little if any relationship with any of the other members. Its isolated
position is indicated by assigning it a distinct subordinal rank.
The Chiroptera have always been placed near the Insectivora; but they
are really a highly specialised group, as much isolated from all other
mammals by the modification of their anterior limbs in adaptation to
aerial locomotion, as the Cetacea and the Sirenia, by the absence of hind
limbs, are specially adapted for an aquatic life.
Lastly, the Primates, which in any natural system must be placed at the
head of the series, are divisible into two very distinct groups—one
containing the various forms of Lemurs (Lemuroidea), and the other the
Monkeys and Man (Anthropoidea). Whether the Lemuroidea should form part
of the Primates (according to the traditional view), or a distinct order
altogether removed from it, is as yet an undetermined question, for both
sides of which there is much to be said. There can, however, be no doubt
that the Anthropoidea form a perfectly natural group, presenting a series
of tolerably regular gradations from the Marmosets (_Hapale_) to Man.
Certain breaks in the series, however, enable us to divide it into five
distinct families:—_Hapalidæ_ or Marmosets: _Cebidæ_ or American Monkeys,
with three premolar teeth on each side of each jaw; _Cercopithecidæ_,
containing the majority of Old-world Monkeys; _Simiidæ_, consisting of
the genera _Hylobates_, _Simia_, _Gorilla_, and _Anthropopithecus_, the
true Man-like Apes; and, lastly, _Hominidæ_, containing the genus _Homo_
alone.
Subclass I. PROTOTHERIA.
Order i. MONOTREMATA—Monotremes.
Fam. 1. _Ornithorhynchidæ_—Duck-bill.
2. _Echidnidæ_—Spiny Anteater.
Group. =MULTITUBERCULATA.=[25]
Fam. 1. =Plagiaulacidæ=—Plagiaulax.
2. =Polymastodontidæ=—Polymastodon.
3. =Tritylodontidæ=—Tritylodon.
Subclass II. METATHERIA.
Order ii. MARSUPIALIA—Marsupials.
Suborder 1. POLYPROTODONTIA—Polyprotodonts.
Fam. 1. =Dromatheriidæ=—Dromatherium.
2. =Amphitheriidæ=—Amphitherium, etc.
3. =Spalacotheriidæ=—Spalacotherium.
4. =Tritylodontidæ=—Tritylodon.
5. _Didelphyidæ_—Opossums.
6. _Dasyuridæ_—Thylacine and Dasyures.
7. _Peramelidæ_—Bandicoots.
Suborder 2. DIPROTODONTIA—Diprotodonts.
Fam. 8. _Phascolomyidæ_—Wombats.
9. _Phalangeridæ_—Phalangers.
10. =Diprotodontidæ=—Diprotodon.
11. =Nototheriidæ=—Notothere.
12. _Macropodidæ_—Kangaroos.
Subclass III. EUTHERIA.
Order iii. EDENTATA—Edentates.
Fam. 1. _Bradypodidæ_—Sloths.
2. =Megatheriidæ=—Ground Sloths.
3. _Myrmecophagidæ_—Anteaters.
4. _Dasypodidæ_—Armadillos.
5. =Glyptodontidæ=—Glyptodonts.
6. _Manidæ_—Pangolins.
7. _Orycteropodidæ_—Aard-varks.
Order iv. SIRENIA—Sirenians.
Fam. 1. _Manatidæ_—Manatees.
2. =Rhytinidæ=—Rhytina.
3. _Halicoridæ_—Dugongs.
4. =Halitheriidæ=—Halithere.
Order v. CETACEA—Cetaceans.
Suborder 1. MYSTACOCETI—Baleen Whales.
Fam. 1. _Balænidæ_—Greenland Whale, etc.
Suborder 2. =ARCHÆOCETI.=
Fam. 2. =Zeuglodontidæ=—Zeuglodonts.
Suborder 3. ODONTOCETI—Toothed Whales.
Fam. 3. _Physeteridæ_—Sperm Whale.
4. _Platanistidæ_—Freshwater Dolphins.
5. _Delphinidæ_—Dolphins, Porpoises, etc.
Order vi. UNGULATA—Hoofed Mammals.
Suborder 1. ARTIODACTYLA—Artiodactyles.
Section A. SUINA—Pig-like Artiodactyles.
Fam 1. _Hippopotamidæ_—Hippopotamus.
2. _Suidæ_—Pigs and Peccaries.
Annectant types.
{ 3. =Chœropotamidæ=—Chœropotamus.
{ 4. =Anthracotheriidæ=—Anthracothere.
{ 5. =Merycopotamidæ=—Merycopotamus.
{ 6. =Cotylopidæ=—Oreodonts.
{ 7. =Anoplotheriidæ=—Anoplothere.
{ 8. =Dichodontidæ=—Dichodon.
Section B. TRAGULINA—Chevrotains.
9. _Tragulidæ_—Chevrotains.
Section C. TYLOPODA—Camels.
10. _Camelidæ_—Camels and Llamas.
11. =Poebrotheriidæ=—Poëbrotherium.
Section D. PECORA—True Ruminants.
12. _Cervidæ_—Deer.
13. _Giraffidæ_—Giraffe.
14. _Antilocapridæ_—Prong-buck.
15. _Bovidæ_—Sheep, Cattle, etc.
Suborder 2. PERISSODACTYLA—Perissodactyles.
Fam. 16. _Tapiridæ_—Tapirs.
17. =Lophiodontidæ=—Lophiodonts.
18. =Palæotheriidæ=—Palæotheres.
19. _Equidæ_—Horses.
20. _Rhinocerotidæ_—Rhinoceroses.
21. =Lambdotheriidæ=—Palæosyops.
22. =Chalicotheriidæ=—Chalicothere.
23. =Titanotheriidæ=—Titanothere.
24. =Macraucheniidæ=—Macrauchenia.
Suborder 3. =TOXODONTIA=—Toxodonts.
Fam. 25. =Toxodontidæ=—Toxodon.
26. =Typotheriidæ=—Typothere.
Suborder 4. =CONDYLARTHRA.=
Fam. 27. =Periptychidæ=—Periptychus.
28. =Phenacodontidæ=—Phenacodus.
29. =Meniscotheriidæ=—Meniscothere.
Suborder 5. HYRACOIDEA—Hyraces.
Fam. 30. _Hyracidæ_—Hyrax.
Suborder 6. =AMBLYPODA.=
Fam. 31. =Pantolambdidæ=—Pantolambda.
32. =Coryphodontidæ=—Coryphodon.
33. =Uintatheriidæ=—Uintathere.
Suborder 7. PROBOSCIDEA—Proboscideans.
Fam. 34. =Dinotheriidæ=—Dinothere.
35. _Elephantidæ_—Elephants.
Group. =TILLODONTIA=—Tillodonts.
Fam. =Anchippodontidæ=—Anchippodus.
=Calamodontidæ=—Calamodon.
Order vii. RODENTIA—Rodents.
Suborder 1. SIMPLICIDENTATA.
Fam. 1. _Anomaluridæ_—Anomalurus.
2. _Sciuridæ_—Squirrels and Marmots.
3. _Haplodontidæ_—Haplodon.
4. =Ischyromyidæ=—Ischyromys.
5. _Castoridæ_—Beavers.
6. _Myoxidæ_—Dormice.
7. _Lophiomyidæ_—Lophiomys.
8. _Muridæ_—Rats, Mice, and Voles.
9. _Spalacidæ_—Mole-rats.
10. _Geomyidæ_—Pouched Rats.
11. _Dipodidæ_—Jerboas.
12. =Theridomyidæ=—Theridomys.
13. _Octodontidæ_—Spiny Mice.
14. =Castoroididæ=—Castoroides.
15. _Hystricidæ_—Porcupines.
16. _Chinchillidæ_—Chinchillas.
17. _Dinomyidæ_—Dinomys.
18. _Caviidæ_—Cavies.
19. _Dasyproctidæ_—Agouties.
Suborder 2. DUPLICIDENTATA.
Fam. 20. _Lagomyidæ_—Picas.
21. _Leporidæ_—Hares and Rabbits.
Order viii. CARNIVORA—Carnivores.
Suborder 1. CARNIVORA VERA—Fissipedes.
Fam. 1. _Felidæ_—Cats.
2. _Hyænidæ_—Hyænas.
3. _Proteleidæ_—Earth-wolf.
4. _Viverridæ_—Civets and Ichneumons.
5. _Canidæ_—Wolves and Foxes.
6. _Ursidæ_—Bears.
7. _Mustelidæ_—Weasels and Otters.
8. _Procyonidæ_—Raccoons and Cat-bear.
Suborder 2. PINNIPEDIA—Pinnipedes.
Fam. 9. _Otariidæ_—Eared Seals.
10. _Trichechidæ_—Walrus.
11. _Phocidæ_—Seals.
Suborder 3. =CREODONTA=—Creodonts.
Fam. 12. =Hyænodontidæ=—Hyænodon.
13. =Proviverridæ=—Proviverra.
14. =Arctocyonidæ=—Arctocyon.
15. =Mesonychidæ=—Mesonyx.
Order ix. INSECTIVORA—Insectivores.
Suborder 1. INSECTIVORA VERA.
Fam. 1. _Tupaiidæ_—Tupaias.
2. _Macroscelididæ_—Elephant-Shrews.
3. _Erinaceidæ_—Hedgehogs.
4. _Soricidæ_—Shrews.
5. _Talpidæ_—Moles.
6. _Potamogalidæ_—Potamogale.
7. _Solenodontidæ_—Solenodon.
8. _Centetidæ_—Centetes.
9. _Chrysochloridæ_—Golden Moles.
Suborder 2. DERMOPTERA.
Fam. 10. _Galeopithecidæ_—Galeopithecus.
Order x. CHIROPTERA—Bats.
Suborder 1. MEGACHIROPTERA—Frugivorous Bats.
Fam. 1. _Pteropodidæ_—Flying Foxes.
Suborder 2. MICROCHIROPTERA—Insectivorous Bats.
Fam. 2. _Vespertilionidæ_—Common Bats.
3. _Nycteridæ_—Nycteris.
4. _Rhinolophidæ_—Leaf-nosed Bats.
5. _Emballonuridæ_—Emballonura.
6. _Phyllostomatidæ_—Vampyres.
Order xi. PRIMATES.
Suborder 1. LEMUROIDEA—Lemuroids.
Fam. 1. =Hyopsodontidæ=—Hyopsodus.
2. _Chiromyidæ_—Aye-Aye.
3. _Tarsiidæ_—Tarsier.
4. _Lemuridæ_—Lemurs.
Suborder 2. ANTHROPOIDEA—Anthropoids.
Fam. 5. _Hapalidæ_—Marmosets.
6. _Cebidæ_—American Monkeys.
7. _Cercopithecidæ_—Old World Monkeys.
8. _Simiidæ_—Gibbons and Man-like Apes.
9. _Hominidæ_—Man.
The distinctive character of these subclasses and orders, with an account
of their subdivisions and the principal forms contained in each, will be
given in subsequent chapters.
CHAPTER IV
GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION
I. GEOGRAPHICAL DISTRIBUTION.[26]
In considering the present distribution of mammals over the globe, we
may, in the first place, direct our attention to terrestrial or land
types, reserving the consideration of aerial types, like the Bats, and
aquatic forms, as exemplified by the Cetaceans, Sirenians, and Seals, to
separate sections.
Among terrestrial forms each species has a certain definite area of
distribution in space, which may be of very wide extent, or may be
confined to a restricted region. This distributional area is, however,
always connected, or continuous; that is to say, that although we may
have a single species inhabiting two continents, like the Lion in Asia
and Africa, or dwelling both on a continent and adjacent continental
islands, like the Javan Rhinoceros of India, Java, and Borneo, yet we
shall always find that such areas, if not still connected, show evident
signs of having been so connected in comparatively late geological
epochs; and we never find instances of the same species inhabiting
totally disconnected areas, such as India and South America. As examples
of mammals with a wide distribution we may mention the Lion and the
Leopard, which are now found throughout Africa, and also occur in India,
as well as in the intervening areas of Arabia and Persia. In the case of
the former species, palæontology further teaches us that its distribution
in the last geological epoch was even more extensive, since we have good
evidence to show that it formerly ranged over the greater part of Europe,
including the British Isles. The Jackal affords another well-known
instance of a species common to India and Africa. The American Puma,
again, may be cited as an example of a mammal having a very wide range
in latitude, since it is found from Patagonia in the south to Canada in
the north. As instances of wide range in the opposite direction we have
only to mention the Reindeer and the Elk or Moose, found in the northern
regions of both the Old and New Worlds, which are only separated from one
another by the narrow channel of Behring Strait.
Of mammals with extremely restricted distributional areas, we may mention
many of the Insectivora, such as the Desman of the Pyrenees, and some of
the Madagascar types of this order, the Lemurs from the same island, some
of the species of Marmots, the remarkable bear-like _Æluropus_ of Eastern
Tibet, one species of Zebra, and other Ungulates from Africa.
The distribution of a genus (except of course when the genus is
represented only by a single form) is very generally more extensive
than that of a species; and this may be markedly the case when there
are only some two or three species in a genus. In genera, moreover, we
meet with what is known as discontinuous distribution, that is, where
the distributional area of one or more species is totally separated from
that of others. The best instance of this occurs in the case of the
Tapirs, where we find one species inhabiting the Malayan Peninsula, and
no others anywhere in the world, with the exception of South America.
The explanation of such an apparently anomalous feature in distribution
is to be found in the past history of the globe, which shows us that
Tapirs once existed in China, Europe, and North America, and, therefore,
indicates that the existing isolated species are the sole survivors of a
group once spread over a large portion of the earth’s surface. In regard
to generic distribution it must, however, be mentioned that this depends
to a great extent on the limits which we are disposed to assign to genera
themselves.
As the distributional area of a genus generally exceeds that of a
species, so that of a family, or group of genera, is larger than that of
a single genus; and similarly the distribution of an order, or assemblage
of families, usually occupies a larger area than that of a single family.
Thus, for instance, the genus _Thylacinus_, represented only by the
so-called Tasmanian Wolf or Thylacine, is now entirely restricted to
Tasmania; but the family _Dasyuridæ_, to which that genus belongs, ranges
all over Australia, while the order Marsupialia, which includes the
_Dasyuridæ_, is found both in Australia and America, and in past epochs
was probably spread over the entire globe.
A remarkable feature in connection with the distribution of the
terrestrial Mammalia is the circumstance that, with the exception of
certain species introduced by human agency, and small forms which can
easily have been transported on floating timber or other similar means,
they are totally absent from what are known as oceanic islands—that is
islands arising from great depths in the ocean, mainly composed of coral
or volcanic rocks, and showing no signs of having ever been connected
with the existing continents, or the larger and so-called continental
islands. The obvious explanation of this feature is, that from their
total isolation these islands have never been able to receive a mammalian
fauna from the great continental areas on which mammalian life was
probably first developed.
As an intermediate step between these islands which are practically void
of mammalian life and the continents which teem with such a variety of
forms, are certain larger islands and portions of continents containing
a mammalian fauna more or less markedly distinct from that of the
whole of the other regions of the globe. The best instance of this is
Australia, which, with the exception of one dog—the Dingo—and certain
_Muridæ_ and Bats, has no mammals except Monotremes and Marsupials. The
latter are, moreover, perfectly distinct from those of America, which,
if we exclude the islands in the neighbourhood of Australia, is the
only other region which now possesses any Marsupials at all. Here also
we have a ready and full explanation which accords with all the facts;
since it is evident that Australia has been isolated from the Asiatic
continent from some very remote geological epoch, at which period it is
probable that Monotremes and Marsupials were the dominant if not the sole
representatives of the Mammalia then existing. Consequently Australia has
never been able to receive an influx of the Eutherian orders, which have
probably swept away all the Marsupials except the small American Opossums
from the rest of the globe. Again, the large island of Madagascar, which
has a fauna of an African type, but still very markedly different from
that of the mainland, may be considered to have been connected with the
latter at a time when the Eutheria had become the dominant forms, but has
been separated for a sufficiently long period to have enabled a large
number of its species and genera to have become distinct from those of
the adjacent continent. Similarly, there is evidence to show that South
America was probably cut off for a considerable period from the northern
half of the American continent, in consequence of which its lowly
organised fauna of Edentates were enabled to attain such a remarkable
development in the later geological periods.
In contrast to the mammalian fauna of islands of the preceding type is,
or rather was, that of the British Islands, which in the early historic
and prehistoric periods was identical with that of the Continent. This
leads to the inference that at a comparatively late epoch there was a
direct land communication between Britain and the Continent, which is
shown by geological evidence to have actually been the case.
The above instances are sufficient to show what an important influence
the date of separation of islands from the adjacent continents has
had upon their existing mammalian fauna, and how largely the present
distribution of mammalian life is bound up with the past history of our
globe. We must, however, not omit to mention another very important
agency of past times which has likewise had great influence on the
present distribution of the various faunas of the northern hemisphere.
This is the so-called glacial epoch, which took place immediately before
the establishment of the present condition of things, and appears to have
been the cause of the extinction of many of the larger mammalian types
which formerly inhabited Europe, and whose retreat to the warmer regions
of the south was apparently cut off by the Mediterranean.
_Zoological Regions._—Zoologists are now generally agreed in dividing
the land surfaces of the globe into a number of zoological regions or
provinces, characterised by a more or less distinctly marked general
_facies_ of their fauna as a whole. Some of these regions are much more
distinctly defined than the others; and in the majority of cases there is
a kind of neutral ground or No-man’s-land at the junction between any two
of these regions. It must also be remembered that in the Old World proper
as we go back in time we find a gradual assimilation in the mammalian
faunas of the different regions, indicating that originally there was one
large fauna of a generally similar type occupying the greater portion of
this area. Thus we find that Hippopotami, Giraffes, Kudus, Elands, and
other types of Antelopes now restricted to Africa, formerly extended to
Europe and India, while there is also evidence to show that the group
of large anthropoid Apes, now found only in Africa and the Bornean
region, were likewise spread over a large part of the south-western half
of the Old World. Moreover, while at the present day there is a marked
connection between the mammals of the northern regions of both the Old
and New Worlds, in the Tertiary period it appears that the fauna of the
whole of North America was much more nearly allied to that of the central
regions of the Old World than is now the case. Thus in the Tertiary rocks
of America we meet with remains of what we are accustomed to regard as
such essentially Old World genera as Horses and Rhinoceroses. On the
other hand there are no traces in America of the existence at any period
of Apes, Giraffes, Hippopotami, or Hyænas, while that continent has
yielded evidence of groups of Ungulates totally unrepresented in the
eastern hemisphere.
The chief zoological regions of the globe, proposed by Mr. Sclater in
1857, and now recognised by the majority of authorities, are six in
number, and are named as follows. Firstly, the Palæarctic region,
embracing the whole of Europe, Persia, Northern Arabia, and all of Asia
northward of the line of the Himalaya proper, Japan, that part of Africa
lying northward of the Sahara Desert, and the oceanic islands of the
North Atlantic. Secondly, the Ethiopian region, which comprises all
Africa lying to the south of the Sahara, the southern part of Arabia,
Madagascar, and the Mascarene Islands. Thirdly, the Oriental or Indian
region, which is taken to include India south of the Himalaya, and to the
north-west as far as Beluchistan, the Malay peninsula, southern China,
Sumatra, Java, Borneo, and the Philippines. Fourthly, the Australasian
region, which is usually defined as being bounded to the north-west by
the deep sea channel lying between Borneo and Celebes known as Wallace’s
line, and is taken to include Celebes, Lumbok, New Guinea, Australia,
Tasmania, New Zealand, and the host of oceanic islands in the South
Pacific. Several writers, however, prefer to regard Celebes and some
of the adjacent islands as representing a transitional Austro-Malayan
region. Fifthly, the Nearctic region, comprising Greenland and North
America as far south as the north of Mexico. And sixthly, the Neotropical
region, which embraces the remaining portion of the American continent
and the West Indies.
Various minor modifications of this scheme have been proposed. Thus some
writers are disposed to raise India to the rank of a distinct primary
region, while others propose the same for New Zealand. The Palæarctic and
Nearctic regions have a large number of common types, more especially
among the mammals, and Dr. A. Heilprin[27] has expressed his opinion that
they should be regarded as a single primary region under the name of the
Holarctic. The same writer would also separate the South Pacific Islands
as constituting a Polynesian region.
Minor divisions or sub-regions have also been marked out, but it will
be unnecessary to indicate their limits in the present work. We may,
however, mention the Mediterranean sub-region of the Palæarctic, which
includes the peninsular portion of southern Europe, North Africa, Asia
Minor, Persia, Afghanistan, Beluchistan, and Northern Arabia, as a good
instance of the transition from one region to another, since its fauna
has a mingling of Palæarctic, Ethiopian, and Oriental types, the former
being, however, the predominant ones.
Of the chief mammalian types characteristic of these various regions only
a brief sketch can be given in this work.
_Palæarctic Region._—The Palæarctic region is of enormous extent,
and includes countries varying greatly in their flora, climate, and
elevation. Thus it embraces the Arctic plains of Siberia, the warm
regions of Italy, Southern France, and Northern Africa, the forest-clad
slopes of the outer Himalaya, and the lofty arid plains of Turkestan
and Tibet, scorched by a burning sun in summer and chilled by a still
more terrible cold in winter. Its extreme limits in the west are marked
by the Canaries and Azores, and in the east by distant Japan; and yet
throughout this vast expanse we find a great uniformity of life, as
exemplified by the large number of British genera which occur also in
Japan. The mammals which are on the whole the most characteristic of this
region are the Sheep and Goats, forming a section of the great family
of _Bovidæ_; nearly all the species of which are Palæarctic, although
we meet with one Goat (_Capra_) in the Nilgherries of Southern India,
and a Sheep (_Ovis_) in the Nearctic region. The Musk Ox (_Ovibos_)
is characteristic of the Palæarctic and Nearctic regions. At least
one species of Camel is characteristic of this region, and it is not
improbable that the second may also have originated in it. There are
a few characteristic types of Antelopes, such as the Alpine Chamois
(_Rupicapra_), the Saiga of Tartary, and the Chiru (_Pantholops_) of
Tibet, each of which is represented by only a single species: and we
miss the host of Antelopes so characteristic of the Ethiopian region.
Deer (_Cervus_) are abundant, although by no means confined to this
region; and the Musk Deer (_Moschus_), the sole representative of the
subfamily _Moschinæ_, is exclusively Palæarctic. Monkeys, as a rule,
are absent, although we meet with one species of _Macacus_ in Northern
Africa and at Gibraltar, and some other types on the southern border of
Tibet. The Moles (_Talpa_) are mainly Palæarctic, although one species
enters Northern India, while the Desmans (_Myogale_) of the Pyrenees
and Southern Russia are unknown beyond the limits of this region. The
Water-shrew (_Nectogale_) is likewise a peculiar eastern Palæarctic
type. Among the Rodents, the Picas or Tailless Hares (_Lagomys_) and
the Dormice (_Myoxus_) are essentially Palæarctic forms, only one
species of each being found beyond the limits of the region, and the one
extra-Palæarctic species of _Lagomys_ occurring in the cognate Nearctic
region. The Mice and Rats are represented by the typical genus _Mus_ and
other types, and Hares (_Lepus_) and one species of Squirrel (_Sciurus_)
are common. The Carnivora include two species of Bears (_Ursus_), Wolves
and Foxes (_Canis_), a Lynx and a few species of Cats (_Felis_), as well
as numerous weasels (_Mustela_), and some other types.
_Ethiopian Region._—The Ethiopian region is of great interest to the
student of mammals, since it is inhabited by a number of forms remarkable
for their large size. A considerable portion of the area consists of
desert, especially in the north; but there is also a wide extent of
grassy plains (veltd), as well as vast tracts of equatorial forests of
great density. Perhaps the most striking feature in the Ethiopian fauna
is the number of Ungulates, both of the Artiodactyle and Perissodactyle
sections. In the former section we have the Giraffes (_Giraffa_)
represented by one species, which is the type of a family, and is
unknown elsewhere. Equally characteristic are the Hippopotami, which
likewise form the type of a family, while the Pigs are represented by
the Wart-hogs (_Phacochœrus_) and the River-hogs, forming an aberrant
group of the genus _Sus_. The Oxen (_Bos_) are represented by Buffaloes,
but there are no species of true Oxen or Bison. The Antelopes attain
an extraordinary development, the number of species being estimated at
from eighty to ninety, which are referred to a large number of genera,
although several of these are more or less ill-defined. Most of these
genera are peculiar to this region, but the Gazelles (_Gazella_) are also
found in the desert regions of other parts of the Old World, and _Oryx_
ranges into Arabia and Persia. In contrast to this abundance of Antelopes
is the total absence of the Deer family, or _Cervidæ_, which are so
characteristic of the Palæarctic and Oriental regions. The Chevrotains
or _Tragulidæ_ are, however, represented by _Dorcatherium_.[28] In the
Perissodactyle section we may notice the presence of two species of
_Rhinoceros_, both furnished with two horns, and distinguished from
those of the Oriental region by the absence of incisor and canine teeth.
The Horse family (_Equidæ_) is also represented by several species, and
includes the peculiar group of Zebras, characterised by their beautifully
striped skins. Of other Ungulates the Elephants, which, like the
Rhinoceroses, are now peculiar to the Ethiopian and Oriental regions,
have one species, which is widely different from its Indian congener.
The Hyraces are mainly characteristic of this region, although one
species occurs in Syria and Palestine. The Carnivora include some forms
like the Lion, Leopard, and Jackal, common to the Oriental region, but
likewise include certain peculiar types like the Earth-wolf (_Proteles_),
which may be regarded as the type of a distinct family, and two species
Hyænas, which are referred by some authorities to a distinct genus
(_Crocuta_). There is also the Hunting dog (_Lycaon_), and the peculiar
group of Foxes known as the Fennecs, together with _Otocyon_. Bears,
Wolves, and true Foxes are absent; but Civets, etc., are abundant,
although not characteristic of the region. The Primates yield several
very characteristic types, such as the Gorilla and the Chimpanzee
(_Anthropopithecus_) among the _Simiidæ_, which, with the exception of
the Orangs of Borneo, are the only existing large man-like Apes, and the
group of Dog-faced Baboons (_Cynocephalus_) in the _Cercopithecidæ_.
The genus _Colobus_ is also a group of the latter family, absolutely
characteristic of the region. Lemurs, again, occur on the continent of
Africa, but the great development of this group is in the adjacent island
of Madagascar, where several peculiar genera occur, and where the larger
Carnivora and Ungulata are absent. These peculiarities of the fauna of
Madagascar apparently point, as previously mentioned, to its separation
from the mainland before the latter was overrun by the larger types, and
at a time when its chief mammals were Lemurs and Insectivores. There
are two genera of Edentates, the Pangolins (_Manis_), and the Aard-vark
(_Orycteropus_), the latter being peculiar.