Chapter IV: Part 4

_Digestive System._--As already mentioned, mammals are specially
characterized by the division of the body-cavity into two main
chambers, by means of the horizontal muscular partition known as the
diaphragm, which is perforated by the great blood-vessels and the
alimentary tube. The mouth of the great majority of mammals is
peculiar for being guarded by thick fleshy lips, which are, however,
absent in the Cetacea; their principal function being to seize the
food, for which purpose they are endowed, as a rule, with more or less
strongly marked prehensile power. The roof of the mouth is formed by
the palate, terminating behind by a muscular, contractile arch, having
in man and a 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 layer which separates 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 two
branches of the lower jaw, and supported behind by the hyoid
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 by reason of 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 numerous. Between the long, extensile,
worm-like tongue of the anteaters, essential to the peculiar mode of
feeding of those animals, and the short, immovable and almost
functionless tongue of the porpoise, every intermediate condition is
found. Whatever the form, the upper surface is, however, covered with
numerous fine papillae, in which the terminal filaments of the
taste-nerve are distributed. In some mammals, notably lemurs, occurs a
hard structure known as the sublingua, which may terminate in a free
horny tip. If, as has been suggested, this organ represents the tongue
of reptiles, the mammalian tongue will obviously be a super-added
organ distinctive of the class.

o, oesophagus;
st, stomach;
p, pylorus;
ss, small intestine (abbreviated);
c, caecum;
ll, large intestine or
colon, ending in
r, the rectum.]

Salivary glands, of which the most constant are the parotid and the
submaxillary, are always present in terrestrial mammals. Next in
constancy are the "sublingual," closely associated with the
last-named, at all events in the locality in which the secretion is
poured out; and the "zygomatic," found only in some mammals in the
cheek, just under cover of the anterior part of the zygomatic arch,
the duct entering the mouth-cavity near that of the parotid.

The alimentary, or intestinal, canal varies greatly in relative length
and capacity in different mammals, and 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 a lower and wider portion, called
respectively the small and the large intestine; the former being
arbitrarily divided into duodenum, jejunum and ileum, and the latter
into colon and rectum. One of the most striking peculiarities of this
part of the canal is the frequent presence of a blind pouch, "caecum,"
situated at the junction of the large and the small intestine. Their
structure presents 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 remainder of the
alimentary canal. It is only in herbivorous mammals that the caecum is
developed to this great extent, and among these there is a
complementary relationship between the size and complexity of the
organ and that of the stomach. Where the latter is simple the caecum
is generally the largest, and vice versa. In vol. xvii. (1905) of the
_Transactions_ of the Zoological Society of London, Dr P. Chalmers
Mitchell has identified the paired caeca, or blind appendages, of the
intestine of birds with the usually single caecum of mammals. These
caeca occur in birds (as in mammals) at the junction of the small with
the large intestine; and while in ordinary perching-birds they are
reduced to small nipple-like buds of no functional importance, in many
other birds--owls for instance--they form quite long receptacles.
Among mammals, the horse and the dog may be cited as instances where
the single caecum is of large size, this being especially the case of
the former, where it is of enormous dimensions; in human beings, on
the other hand, the caecum is rudimentary, and best known in connexion
with "appendicitis." The existence of paired caeca was previously
known in a few armadillos and anteaters, but Dr Mitchell has shown
that they are common in these groups, while he has also recorded their
occurrence in the hyrax and the manati. With the aid of these
instances of paired caeca, coupled with the frequent existence of a
rudiment of its missing fellow when only one is functional, the author
has been enabled to demonstrate conclusively that these double organs
in birds correspond in relations with their normally single
representative in mammals.

In mammals both caecum 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
contrivances for increasing the absorbing surface without adding to
the general bulk of the organ, such as the numerous small tags, or
"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 "valvulae
conniventes" of man form well-known examples. Besides the crypts of
Lieberkühn 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 more
commonly known by the name of "Peyer's patches." Of the liver little
need be said, except that in all living mammals it has been divided
into a number of distinct lobes, which have received separate names.
It has, indeed, been suggested that in the earlier mammals the liver
was a simple undivided organ. This, however, is denied by G. Ruge
(vol. xxix. of Gegenbaur's _Morphologisches Jahrbuch_).

_Origin of Mammals._--That mammals have become differentiated from a lower type of vertebrates at least as early as the commencement of the Jurassic period is abundantly testified by the occurrence of the remains of small species in strata of that epoch, some of which are mentioned in the articles MARSUPIALIA and MONOTREMATA (q.v.). Possibly mammalian remains also occur in the antecedent Triassic epoch, some palaeontologists regarding the South African _Tritylodon_ as a mammal, while others consider that it was probably a reptile. Whatever may be the true state of the case with regard to that animal probably also holds good in the case of the approximately contemporaneous European _Microlestes_. Of the European Jurassic (or Oolitic) mammals our knowledge is unfortunately very imperfect; and from the scarcity of their remains it is quite probable that they are merely stragglers from the region (possibly Africa) where the class was first differentiated. It is not till the early Eocene that mammals become a dominant type in the northern hemisphere.

It is now practically certain that mammals are descended from reptiles. Dr H. Gadow, in a paper on the origin of mammals contributed to the _Zeitschrift für Morphologie_, sums up as follows: "Mammals are descendants of reptiles as surely as they [the latter] have been evolved from Amphibia. This does not mean that any of the living groups of reptiles can claim their honour of ancestry, but it means that the mammals have branched where the principal reptilian groups meet, and that is a long way back. The Theromorpha, especially small Theriodontia, alone show us what these creatures were like." It may be explained that the Theromorpha, or Anomodontia, are those extinct reptiles so common in the early Secondary (Triassic) deposits of South Africa, some of which present a remarkable resemblance in their dentition and skeleton to mammals, while others come equally near amphibians. A difficulty naturally arises with regard to the fact that in reptiles the occipital condyle by which the skull articulates with the vertebral column is single, although composed of three elements, whereas in amphibians and mammals the articulation is formed by a pair of condyles. Nevertheless, according to Professor H. F. Osborn, the tripartite reptilian condyle, by the loss of its median element, has given rise to the paired mammalian condyles; so that this difficulty disappears. The fate of the reptilian quadrate bone (which is reduced to very small dimensions in the Anomodontia) has been referred to in an earlier section of the present article, where some mention has also been made of the disappearance in mammals of the hinder elements of the reptilian lower jaw, so as to leave the single bone (dentary) of each half of this part of the skeleton in mammals.

Most of the earliest known mammals appear to be related to the Marsupialia and Insectivora. Others however (inclusive of _Tritylodon_ and _Microlestes_, if they be really mammals), seem nearer to the Monotremata; and the question has yet to be decided whether placentals and marsupials on the one hand, and monotremes on the other are not independently derived from reptilian ancestors.

With regard to the evolution of marsupials and placentals, it has been pointed out that the majority of modern marsupials exhibit in the structure of their feet traces of the former opposability of the thumb and great toe to the other digits; and it has accordingly been argued that all marsupials are descended from arboreal ancestors. This doctrine is now receiving widespread acceptation among anatomical naturalists; and in the _American Naturalist_ for 1904, Dr W. D. Matthew, an American palaeontologist, considers himself provisionally justified in so extending it as to include all mammals. That is to say, he believes that, with the exception of the duckbill and the echidna, the mammalian class as a whole can lay claim to descent from small arboreal forms. This view is, of course, almost entirely based upon palaeontological considerations; and these, in the author's opinion, admit of the conclusion that all modern placental and marsupial mammals are descended from a common ancestral stock, of which the members were small in bodily size. These ancestral mammals, in addition to their small size, were characterized by the presence of five toes to each foot, of which the first was more or less completely opposable to the other four. The evidence in favour of this primitive opposability is considerable. In all the groups which are at present arboreal, the palaeontological evidence goes to show that their ancestors were likewise so; while since, in the case of modern terrestrial forms, the structure of the wrist and ankle joints tends to approximate to the arboreal type, as we recede in time, the available evidence, so far as it goes, is in favour of Dr Matthew's contention.

The same author also discusses the proposition from another standpoint, namely, the condition of the earth's surface in Cretaceous times. His theory is that in the early Cretaceous epoch the animals of the world were mostly aerial, amphibious, aquatic or arboreal; the flora of the land being undeveloped as compared with its present state. On the other hand, towards the close of the Cretaceous epoch (when the Chalk was in course of deposition), the spread of a great upland flora vastly extended the territory available for mammalian life. Accordingly, it was at this epoch that the small ancestral insectivorous mammals first forsook their arboreal habitat to try a life on the open plains, where their descendants developed on the one hand into the carnivorous and other groups, in which the toes are armed with nails or claws, and on the other into the hoofed group, inclusive of such monsters as the elephant and the giraffe. The hypothesis is not free from certain difficulties, one of which will be noticed later.

_Classification._--Existing mammals may be primarily divided into three main groups, or subclasses, of which the second and third are much more closely related to one another than is either of them to the first. These three classes are the Monotremata (or Prototheria), the Marsupialia (Didelphia, or Metatheria), and the Placentalia (Monodelphia, or Eutheria); the distinctive characters of each being given in separate articles (see MONOTREMATA, MARSUPIALIA and MONODELPHIA.)

The existing monotremes and marsupials are each represented only by a
single order; but the placentals are divided into the following
ordinal and subordinal groups, those which are extinct being marked
with an asterisk (*):--

1. Insectivora (Moles, Hedgehogs, &c.).
2. Chiroptera (Bats).
3. Dermoptera (Colugo, or Flying Lemur).
4. Edentata:--
a. Xenarthra (Anteaters, Sloths and Armadillos).
b. Pholidota (Pangolins).
c. Tubulidentata (Ant-bears, or Aard-varks).
5. Rodentia (Gnawing Mammals):--
a. Duplicidentata (Hares and Picas).
b. Simplicidentata (Rats, Beavers, &c.).
6. *Tillodontia (_Tillotherium_).
7. Carnivora:--
a. Fissipedia (Cats, Dogs, Bears, &c.).
b. Pinnipedia (Seals and Walruses).
c. *Creodonta (_Hyaenodon_, &c.).
8. Cetacea (Whales and Dolphins):--
a. *Archaeoceti (_Zeuglodon_, &c.).
b. Odontoceti (Spermwhales and Dolphins).
c. Mystacoceti (Whalebone Whales).
9. Sirenia (Dugongs and Manatis).
10. Ungulata (Hoofed Mammals):--
a. Proboscidea (Elephants and Mastodons).
b. Hyracoidea (Hyraxes).
c. *Barypoda (_Arsinöitherium_).
d. *Toxodontia (_Toxodon_, &c.).
e. *Amblypoda (_Uintatherium_, &c.).
f. *Litopterna (_Macrauchenia_, &c.).
g. *Ancylopoda (_Chalicotherium_, &c.).
h. *Condylarthra (_Phenacodus_, &c.).
i. Perissodactyla (Tapirs, Horses, &c.).
j. Artiodactyla (Ruminants, Swine, &c.).
11. Primates:--
a. Prosimiae (Lemurs and Galagos).
b. Anthropoidea (Monkeys, Apes and Man).

Separate articles are devoted to each of these orders, where
references will be found to other articles dealing with some of the
minor groups and a number of the more representative species.

_Relationships of the Groups._--As we recede in time we find the
extinct representatives of many of these orders approximating more and
more closely to a common generalized type, so that in a large number
of early Eocene forms it is often difficult to decide to which group
they should be assigned.

The Insectivora are certainly the lowest group of existing placental
mammals, and exhibit many signs of affinity with marsupials; they may
even be a more generalized group than the latter. From the Insectivora
the bats, or Chiroptera, are evidently a specialized lateral offshoot;
while the Dermoptera may be another branch from the same stock. As to
the Edentata, it is still a matter of uncertainty whether the
pangolins (Pholidota) and the ant-bears (Tubulidentata) are rightly
referred to an order typically represented by the sloths, anteaters,
and armadillos of South and Central America, or whether the two
first-named groups have any close relationship with one another. Much
uncertainty prevails with regard to the ancestry of the group as a
whole, although some of the earlier South American forms have a
comparatively full series of teeth, which are also of a less
degenerate type than those of their modern representatives.

An almost equal degree of doubt obtains with regard to the ancestry of
that very compact and well-defined group the Rodentia. If, however,
the so-called Proglires of the lower Eocene are really ancestral
rodents, the order is brought into comparatively close connexion with
the early generalized types of clawed, or unguiculate mammals. Whether
the extinct Tillodontia are most nearly allied to the Rodentia, the
Carnivore or the Ungulata, and whether they are really entitled to
constitute an ordinal group by themselves, must remain for the present
open questions.

The Carnivora, as represented by the (mainly) Eocene Creodonta, are
evidently an ancient and generalized type. As regards the number and
form of their permanent teeth, at any rate, creodonts present such a
marked similarity to carnivorous marsupials, that it is difficult to
believe the two groups are not allied, although the nature of the
relationship is not yet understood, and the minute internal structure
of the teeth is unlike that of marsupials and similar to that of
modern Carnivora. There is the further possibility that creodonts may
be directly descended from the carnivorous reptiles; a descent which
if proved might introduce some difficulty with regard to the
above-mentioned theory as to the arboreal ancestry of mammals
generally. Be this as it may, there can be little doubt that the
creodonts are related to the Insectivora, which, as stated above, show
decided signs of kinship with the marsupials.

A much more interesting relationship of the creodont carnivora has,
however, been established on the evidence of recent discoveries in
Egypt. From remains of Eocene age in that country Dr E. Fraas, of
Stuttgart, has demonstrated the derivation of the whale-like
_Zeuglodon_ from the creodonts. Dr C. E. Andrews has, moreover, not
only brought forward additional evidence in favour of this most
remarkable line of descent, but is confident--which Professor Fraas
was not--that _Zeuglodon_ itself is an ancestral cetacean, and
consequently that whales are the highly modified descendants of
creodonts. It must be admitted, however, that the links between
_Zeuglodon_ and typical cetaceans are at present unknown; but it may
be hoped that these will be eventually brought to light from the
deposits of the Mokattam Range, near Cairo. Whales and dolphins being
thus demonstrated to be nothing more than highly modified Carnivora,
might almost be included in the same ordinal group.

An analogous statement may be made with regard to the sea-cows, or
Sirenia, which appear to be derivates from the great herbivorous order
of Ungulata, and might consequently be included in that group, as
indeed has been already done in Dr Max Weber's classification. It is
with the proboscidean suborder of the Ungulata to which the Sirenia
are most nearly related; the nature of this relationship being
described by Dr Andrews as follows:--

"In the first place, the occurrence of the most primitive Sirenians
with which we are acquainted in the same region as the most
generalized proboscidean, _Moeritherium_, is in favour of such a view,
and this is further supported by the similarity of the brain-structure
and, to some extent, of the pelvis in the earliest-known members of
the two groups. Moreover, in the anatomy of the soft-parts of the
recent forms there are a number of remarkable points of resemblance.
Among the common characters may be noted the possession of: (1)
pectoral mammae; (2) abdominal testes; (3) a bifid apex of the heart;
(4) bilophodont molars with a tendency to the formation of an
additional lobe from the posterior part of the cingulum. The peculiar
mode of displacement of the teeth from behind forwards in some members
of both groups may perhaps indicate a relationship, although in the
case of the Sirenia the replacement takes place by means of a
succession of similar molars, while in the Proboscidea the molars
remain the same numerically, but increase greatly in size and number
of transverse ridges."

These and certain other facts referred to by the same author point to
the conclusion that not only are the Sirenia and the Proboscidea
derived from a single ancestral stock, but that the Hyracoidea--and so
_Arsinöitherium_--are also derivatives from the same stock, which must
necessarily have been Ethiopian.

Of the other suborders of ungulates, the Toxodontia and Litopterna are
exclusively South American, and while the former may possibly be
related to the Hyracoidea and Barypoda, the latter is perhaps more
nearly akin to the Perissodactyla. The Amblypoda, on the other hand,
are perhaps not far removed from the ancestral Proboscidea, which
depart comparatively little from the generalized ungulate type. The
latter is represented by the Eocene Condylarthra, which undoubtedly
gave rise to the Perissodactyla and Artiodactyla, and probably to
most, if not all, of the other groups. The Condylarthra, in their
turn, approximate closely to the ancestral Carnivora, as they also do
in some degree to the ancestral Primates. As regards the latter order,
although we are at present unacquainted with all the connecting links
between the lemurs and the monkeys, there is little doubt that the
ancestors of the former represent the stock from which the latter have
originated. C. D. Earle, in the _American Naturalist_ for 1897,
observes that "so far as the palaeontological evidence goes it is
decidedly in favour of the view that apes and lemurs are closely
related. Beginning with the earliest known lemur, _Anaptomorphus_,
this genus shows tendencies towards the anthropoids, and, when we pass
up into the Oligocene of the Old World, _Adapis_ is a decidedly mixed
type, and probably not far from the common stem-form which gave origin
to both suborders of the Primates. In regard to _Tarsius_, it is
evidently a type nearly between the lemurs and apes, but with many
essential characters belonging to the former group."

_Distribution._--For an account of the "realms" and "regions" into which the surface of the globe has been divided by those who have made a special study of the geographical distribution of animals, see ZOOLOGICAL DISTRIBUTION. For the purposes of such zoo-geographical divisions, mammals are much better adapted than birds, owing to their much more limited powers of dispersal; most of them (exclusive of the purely aquatic forms, such as seals, whales, dolphins and sea-cows) being unable to cross anything more than a very narrow arm of the sea. Consequently, the presence of nearly allied groups of mammals in areas now separated by considerable stretches of sea proves that at no very distant date such tracts must have had a land-connexion. In the case of the southern continents the difficulty is, however, to determine whether allied groups of mammals (and other animals) have reached their present isolated habitats by dispersal from the north along widely sundered longitudinal lines, or whether such a distribution implies the former existence of equatorial land-connexions. It may be added that even bats are unable to cross large tracts of sea; and the fact that fruit-bats of the genus _Pteropus_ are found in Madagascar and the Seychelles, as well as in India, while they are absent from Africa, is held to be an important link in the chain of evidence demonstrating a former land-connexion between Madagascar and India.

There is another point of view from which mammals are of especial importance in regard to geographical distribution, namely their comparatively late rise and dispersal, or "radiation," as compared with reptiles.

As regards terrestrial mammals (with which alone we are at present concerned), one of the most striking features in their distribution is their practical absence from oceanic islands; the only species found in such localities being either small forms which might have been carried on floating timber, or such as have been introduced by human agency. This absence of mammalian life in oceanic islands extends even to New Zealand, where the indigenous mammals comprise only two peculiar species of bats, the so-called Maori rat having been introduced by man.

One of the leading features in mammalian distribution is the fact that
the Monotremata, or egg-laying mammals, are exclusively confined to
Australia and Papua, with the adjacent islands. The marsupials also
attain their maximum development in Australia ("Notogaea" of the
distributionists), extending, however, as far west as Celebes and the
Moluccas, although in these islands they form an insignificant
minority among an extensive placental fauna, being represented only by
the cuscuses (_Phalanger_), a group unknown in either Papua or
Australia. Very different, on the other hand, is the condition of
things in Australia and Papua, where marsupials (and monotremes) are
the dominant forms of mammalian life, the placentals being represented
(apart from bats, which are mainly of an Asiatic type) only by a
number of more or less aberrant rodents belonging to the mouse-tribe,
and in Australia by the dingo, or native dog, and in New Guinea by a
wild pig. The dingo was, however, almost certainly brought from Asia
by the ancestors of the modern natives; while the Papuan pig is also
in all probability a human introduction, very likely of much later
date. The origin of the Australasian fauna is a question pertaining to
the article ZOOLOGICAL DISTRIBUTION. The remaining marsupials (namely
the families _Didelphyidae_ and _Epanorthidae_) are American, and
mainly South and Central American at the present day; although during
the early part of the Tertiary period representatives of the
first-named family ranged all over the northern hemisphere.

The Insectivora (except a few shrews which have entered from the
north) are absent from South America, and appear to have been mainly
an Old World group, the only forms which have entered North America
being the shrew-mice (_Soricidae_) and moles (_Talpidae_). The
occurrence of one aberrant group (_Solenodon_) in the West Indies is,
however, noteworthy. The family with the widest distribution is the
_Soricidae_, the _Talpidae_ being unknown in Africa. The tree-shrews
(_Tupaiidae_) are exclusively Asiatic, whereas the jumping-shrews
(_Macroscelididae_) are equally characteristic of the African
continent. Madagascar is the sole habitat of the tenrecs
(_Centetidae_), as is Southern Africa of the golden moles
(_Chrysochloridae_). It is, however, important to mention that an
extinct South American insectivore, _Necrolestes_, has been referred
to the family last mentioned; and even if this reference should not be
confirmed in the future, the occurrence of a representative of the
order in Patagonia is a fact of considerable importance in
distribution.

The Rodentia have a wider geographical range than any other order of
terrestrial mammals, being, as already mentioned, represented by
numerous members of the mouse-tribe (_Muridae_) even in Australasia.
With the remarkable exception of Madagascar, where it is represented
by the _Nesomyidae_, that family has thus a cosmopolitan distribution.
Very noteworthy is the fact that, with the exception of Madagascar
(and of course Australia) the squirrel family (_Sciuridae_) is also
found in all parts of the world. Precisely the same may be said of the
hares, which, however, become scarce in South America. On the other
hand, the scaly-tailed squirrels (_Anomaluridae_), the jumping-hares
(_Pedetidae_), and the strand-moles (_Bathyergidae_) are exclusively
African; while the sewellels (_Haplodontidae_) and the pocket-gophers
(_Geomyidae_) are as characteristically North American, although a few
members of the latter have reached Central America. The beavers
(_Castoridae_) are restricted to the northern hemisphere, whereas the
dormice (_Gliridae_) and the mole-rats (_Spalacidae_) are exclusively
Old World forms, the latter only entering the north of Africa, in
which continent the former are largely developed. The jerboa group
(_Dipodidae_, or _Jaculidae_) is also mainly an Old World type,
although its aberrant representatives the jumping-mice (_Zapus_) have
effected an entrance into Arctic North America. Porcupines enjoy a
very wide range, being represented throughout the warmer parts of the
Old World, with the exception of Madagascar (and of course
Australasia), by the _Hystricidae_, and in the New World by the
_Erethizontidae_. Of the remaining families of the Simplicidentata,
all are southern, the cavies (_Caviidae_), chinchillas
(_Chinchillidae_), and degus (_Octodontidae_) being Central and South
American, while the _Capromyidae_ are common to southern America and
Africa, and the _Ctenodactylidae_ are exclusively African. The near
alliance of all these southern families, and the absence of so many
Old World families from Madagascar form two of the most striking
features in the distribution of the order. Lastly, among the
Duplicidentata, the picas (_Ochotonidae_ or _Lagomyidae_) form a group
confined to the colder or mountainous regions of the northern
hemisphere.

Among the existing land Carnivora (of which no representatives except
the introduced dingo are found in Australasia) the cat-tribe
(_Felidae_) has now an almost cosmopolitan range, although it only
reached South America at a comparatively recent date. Its original
home was probably in the northern hemisphere; and it has no
representatives in Madagascar. The civet-tribe (_Viverridae_), on the
other hand, which is exclusively an Old World group, is abundant in
Madagascar, where it is represented by peculiar and aberrant types.
The hyenas (_Hyaenidae_), at any rate at the present day, to which
consideration is mainly limited, are likewise Old World. The dog-tribe
(_Canidae_), on the other hand, are, with the exception of Madagascar,
an almost cosmopolitan group. Their place of origin was, however,
almost entirely in the northern hemisphere, and not improbably in some
part of the Old World, where they gave rise to the bears (_Ursidae_).
The latter are abundant throughout the northern hemisphere, and have
even succeeded in penetrating into South America, but, with the
exception of the Mediterranean zone, have never succeeded in entering
Africa, and are therefore of course unknown in Madagascar. The raccoon
group (_Procyonidae_) is mainly American, being represented in the Old
World only by the pandas (_Aelurus_ and _Aeluropus_), of which the
latter apparently exhibits some affinity to the bears. The birthplace
of the group was evidently in the northern hemisphere--possibly in
east Central Asia. The weasel-tribe (_Mustelidae_) is clearly a
northern group, which has, however, succeeded in penetrating into
South America and Africa, although it has never reached Madagascar.

The extinct creodonts, especially if they be the direct descendants of
the anomodont reptiles, may have originated in Africa, although they
are at present known in that continent only from the Fayum district.
Elsewhere they occur in South America and throughout a large part of
the northern hemisphere, where they appear to have survived in India
to the later Oligocene or Miocene.

In the case of the great order, or assemblage, of Ungulata it is
necessary to pay somewhat more attention to fossil forms, since a
considerable number of groups are either altogether extinct or largely
on the wane.

So far as is at present known, the earliest and most primitive group,
the Condylarthra, is a northern one, but whether first developed in
the eastern or the western hemisphere there is no sufficient evidence.
The more or less specialized Litopterna and Toxodontia, as severally
typified by the macrauchenia and the toxodon, are, on the other hand,
exclusively South American. With the primitive five-toed Amblypoda, as
represented by the coryphodon, we again reach a northern group, common
to the two hemispheres; but there is not improbably some connexion
between this group and the much more specialized Barypoda, as
represented by _Arsinöitherium_, of Africa. The Ancylopoda, again,
typified by _Chalicotherium_, and characterized by the claw-like
character of the digits, are probably another northern group, common
to the eastern and western hemispheres.

Recent discoveries have demonstrated the African origin of the
elephants (Proboscidea) and hyraxes (Hyracoidea), the latter group
being still indeed mainly African, and in past times also limited to
Africa and the Mediterranean countries. As regards the elephants (now
restricted to Africa and tropical Asia), there appears to be evidence
that the ancestral mastodons, after having developed from African
forms probably not very far removed from the Amblypoda, migrated into
Asia, where they gave rise to the true elephants. Thence both
elephants and mastodons reached North America by the Bering Sea route;
while the former, which arrived earlier than the latter, eventually
penetrated into South America.

The now waning group of Perissodactyla would appear to have originally
been a northern one, as all the three existing families, rhinoceroses
(_Rhinocerotidae_), tapirs (_Tapiridae_), and horses (_Equidae_), are
well represented in the Tertiaries of both halves of the northern
hemisphere. If eastern Central Asia were tentatively given as the
centre of radiation of the group, this might perhaps best accord with
the nature of the case. Rhinoceroses disappeared comparatively early
from the New World, and never reached South America. In Siberia and
northern Europe species of an African type survived till a
comparatively late epoch, so that the present relegation of the group
to tropical Asia and Africa may be regarded as a modern feature in
distribution. Horses, now unknown in a wild state in the New World,
although still widely spread in the Old, attained a more extensive
range in past times, having successfully invaded South America. On the
other hand, in common with the rest of the Perissodactyla, they never
reached Madagascar. In addition to the occurrence of their fossil
remains almost throughout the world, the former wide range of the
tapirs is attested by the fact of their living representatives being
confined to such widely sundered areas as Malaysia and tropical
America.

The Artiodactyla are the only group of ungulates known to have been
represented in Madagascar; but since both these Malagasy forms--namely
two hippopotamuses (now extinct) and a river-hog--are capable of
swimming, it is most probable that they reached the island by crossing
the Mozambique Channel. As regards the deer-family (_Cervidae_), which
is unknown in Africa south of the Sahara, it is quite evident that it
originated in the northern half of the Old World, whence it reached
North America by the Bering Sea route, and eventually travelled into
South America. More light is required with regard to the past history
of the giraffe-family (_Giraffidae_), which includes the African okapi
and the extinct Indian _Sivatherium_, and is unknown in the New World.
Possibly, however, its birthplace may prove to be Africa; if so, we
shall have a case analogous to that of the African elephant, namely
that while giraffes flourished during the Pliocene in Asia (where
they may have originated), they survive only in Africa. An African
origin has also been suggested for the hollow-horned ruminants
(_Bovidae_); and if this were substantiated it would explain the
abundance of that family in Africa and the absence from the heart of
that continent of the deer-tribe. Some confirmation of this theory is
afforded by the fact that whereas we can recognize ancestral deer in
the Tertiaries of Europe we cannot point with certainty to the
forerunners of the _Bovidae_. Whether its birthplace was in Africa or
to the north, it is, however, clear that the hollow-horned ruminants
are essentially an Old World group, which only effected an entrance
into North America at a comparatively recent date, and never succeeded
in reaching South America. So far as it goes, this fact is also in
favour of the African ancestry of the group.

The _Antilocapridae_ (prongbuck), whose relationships appear to be
rather with the _Cervidae_ than with the _Bovidae_, are on the other
hand apparently a North American group. The chevrotains
(_Tragulidae_), now surviving only in West and Central Africa and
tropical Asia, are conversely a purely Old World group.

The camels (_Tylopoda_) certainly originated in the northern
hemisphere, but although their birthplace has been confidently claimed
for North America, an equal, if not stronger, claim may be made on the
part of Central Asia. From the latter area, where wild camels still
exist, the group may be assumed to have made its way at an early
period into North America; whence, at a much later date, it finally
penetrated into South America. In the Old World it seems to have
reached the fringe of the African continent, where its wanderings in a
wild state were stayed.

The pigs (_Suidae_) and the hippopotamuses (_Hippopotamidae_) are
essentially Old World groups, the former of which has alone succeeded
in reaching America, where it is represented by the collateral branch
of the peccaries (_Dicotylinae_). An African origin would well explain
the present distribution of both groups, but further evidence on this
point is required before anything decisive can be affirmed, although
it is noteworthy that the earliest known pig (_Geniohyus_) is African.
The Suinae are at present spread all over the Old World, although the
African forms (other than the one from the north) are markedly
distinct from those inhabiting Europe and Asia. Hippopotamuses, on the
contrary, are now exclusively African, although they were represented
in tropical Asia during the Pliocene and over the greater part of
Europe at a later epoch.

A brief notice with regard to the distribution of the Primates must
suffice, as their past history is too imperfectly known to admit of
generalizations being drawn. The main facts at the present day are,
firstly, the restriction of the Prosimiae, or lemurs, to the warmer
parts of the Old World, and their special abundance in Madagascar
(where other Primates are wanting); and, secondly, the wide structural
distinction between the monkeys of tropical America (Platyrrhina), and
the Old World monkeys and apes, or Catarrhina. It is, however,
noteworthy that extinct lemurs occur in the Tertiary deposits of both
halves of the northern hemisphere--a fact which has induced Dr J. L.
Wortman to suggest a polar origin for the entire group--a view we are
not yet prepared to endorse. For the distribution of the various
families and genera the reader may be referred to the article
PRIMATES; and it will suffice to mention here that while chimpanzees
and baboons are now restricted to Africa and (in the case of the
latter group) Arabia, they formerly occurred in India.

As regards aquatic mammals, the greater number of the Cetacea, or
whales and dolphins, have, as might be expected, a very wide
distribution in the ocean. A few, on the other hand, have a very
restricted range, the Greenland right whale (_Balaena mysticetus_)
being, for instance, limited to the zone of the northern circumpolar
ice, while no corresponding species occurs in the southern hemisphere.
In this case, not only temperature, but also the peculiar mode of
feeding, may be the cause. The narwhal and the beluga have a very
similar distribution, though the latter occasionally ranges farther
south. The bottle-noses (_Hyperöodon_) are restricted to the North
Atlantic, never entering, so far as known, the tropical seas. Other
species are exclusively tropical or austral in their range. The pigmy
whale (_Neobalaena marginata_), for instance, has only been met with
in the seas round Australia, New Zealand and South America, while a
beaked whale (_Berardius arnouxi_) appears to be confined to the New
Zealand seas.

The Cetacea, however, are by no means limited to the ocean, or even to
salt water, some entering large rivers for considerable distances, and
others being exclusively fluviatile. The susu (_Platanista_) is, for
instance, extensively distributed throughout nearly the whole of the
river systems of the Ganges, Brahmaputra and Indus, ascending as high
as there is water enough to swim in, but apparently never passing out
to sea. The individuals inhabiting the Indus and the Ganges must
therefore have been for long ages isolated without developing any
distinctive anatomical characters, those by which _P. indi_ was
separated from _P. gangetica_ having been shown to be of no constant
value. _Orcella fluminalis_, again, appears to be limited to the
Irrawaddy; and at least two distinct species of dolphin, belonging to
different genera, are found in the Amazon. It is remarkable that none
of the great lakes or inland seas of the world is inhabited by
cetaceans.

The great difference in the manner of life of the sea-cows, or
Sirenia, as compared with that of the Cetacea, causes a corresponding
difference in their geographical distribution. Slow in their
movements, and feeding on vegetable substances, they are confined to
the neighbourhood of rivers, estuaries or coasts, although there is a
possibility of accidental transport by currents across considerable
distances. Of the three genera existing within historic times, one
(_Manatus_) is exclusively confined to the shores of the tropical
Atlantic and the rivers entering into it, individuals scarcely
specifically distinguishable being found both on the American and the
African. The dugong (_Halicore_) is distributed in different colonies,
at present isolated, throughout the Indian Ocean from Arabia to North
Australia; while the _Rhytina_ or northern sea-cow was, for some time
before its extinction, limited to a single island in the extreme north
of the Pacific Ocean.

The seals (_Pinnipedia_) although capable of traversing long reaches
of ocean, are less truly aquatic than the last two groups, always
resorting to the land or to ice-floes for breeding. The geographical
range of each species is generally more or less restricted, usually
according to climate, as they are mostly inhabitants either of the
Arctic or Antarctic seas and adjacent temperate regions, few being
found within the tropics. For this reason the northern and the
southern species are for the most part quite distinct. In fact, the
only known exception is the case of a colony of elephant-seals
(_Macrorhinus leoninus_), whose general range is in the southern
hemisphere, inhabiting the coast of California. In this case a
different specific name has been given to the northern form, but the
characters by which it is distinguished are of little importance, and
probably, except for the abnormal geographical distribution, would
never have been discovered. The most remarkable circumstance connected
with the distribution of seals is the presence of members of the order
in the three isolated great lakes or inland seas of Central Asia--the
Caspian, Aral and Baikal--which, notwithstanding their long isolation,
have varied but slightly from species now inhabiting the Polar Ocean.

AUTHORITIES.--The above article is partly based on that of Sir W. H.
Flower in the 9th edition of this work. The literature connected with
mammals is so extensive that all that can be attempted here is to
refer the reader to a few textbooks, with the aid of which, combined
with that of the annual volumes of the _Zoological Record_, he may
obtain such information on the subject as he may require: F. E.
Beddard, "Mammals," _The Cambridge Natural History_, vol. x. (1902);
W. H. Flower and R. Lydekker, _The Study of Mammals_ (London, 1891);
Max Weber, _Die Säugethiere_ (Jena, 1904); W. T. Blanford, _The Fauna
of British India--Mammalia_ (1888-1891); D. G. Elliot, _Synopsis of
the Mammals of North America_ (Chicago, 1901) and _The Mammals of
Middle America and the West Indies_ (Chicago, 1904); W. L. Sclater,
_The Fauna of South Africa--Mammals_ (Cape Town, 1901-1902); W. K.
Parker, _Mammalian Descent_ (London, 1885); E. Trouessart, _Catalogus
mammalium, tam viventium quam fossilium_ (Paris, 1898-1899); and
supplement, 1904-1905; T. S. Palmer, _Index generum mammalium_
(Washington, 1904); W. L. and P. L. Sclater, _The Geography of
Mammals_ (London, 1899); R. Lydekker, _A Geographical History of
Mammals_ (Cambridge, 1896). (W. H. F.; R. L.)

MAMMARY GLAND (Lat. _mamma_), or female breast, the organ by means of which the young are suckled, and the possession of which, in some region of the trunk, entitles the animal bearing it to a place in the order of Mammalia.

_Anatomy._--In the human female the gland extends vertically from the second to the sixth rib, and transversely from the edge of the sternum to the mid axillary line; it is embedded in the fat superficial to the pectoralis major muscle, and a process which extends toward the arm-pit is sometimes called the axillary tail. A little below the centre of the glandular swelling is the _nipple_, surrounding which is a pigmented circular patch called the areola; this is studded with slight nodules, which are the openings of areolar glands secreting an oily fluid to protect the skin during suckling. During the second or third month of pregnancy the areola becomes more or less deeply pigmented, but this to a large extent passes off after lactation ceases. In structure the gland consists of some fifteen to twenty lobules, each of which has a _lactiferous duct_ opening at the summit of the nipple, and branching in the substance of the gland to form secondary lobules, the walls of which are lined by cubical epithelium in which the milk is secreted. These secondary lobules project into the surrounding fat, so that it is difficult to dissect out the gland cleanly. Before opening at the nipple each lactiferous duct has a fusiform dilatation called the _ampulla_.

After the child-bearing period of life the breasts atrophy and tend to
become pendulous, while in some African races they are pendulous
throughout life. Variations in the mammary glands are common; often
the left breast is larger than the right, and in those rare cases in
which one breast is suppressed it is usually the right, though
suppression of the breast does not necessarily include absence of the
nipple.

FIG. 1.--Dissection of the Mammary Gland.]

_Supernumerary nipples and glands_ are not uncommon, and, when they
occur, are usually situated in the mammary line which extends from the
anterior axillary fold to the spine of the pubis; hence, when an extra
nipple appears above the normal one, it is external to it, but, when
below, it is nearer the middle line. The condition of extra breasts is
known as _polymasty_, that of extra nipples as _polythely_, and it is
interesting to notice that the latter is commoner in males than in
females. O. Ammon (quoted by Wiedersheim) records the case of a German
soldier who had four nipples on each side. These nipples in the human
subject are seldom found below the costal margin. In normal males the
breast structure is present, but rudimentary, though it is not very
rare to find instances of boys about puberty in whom a small amount of
milk is secreted, and one case at least is recorded of a man who
suckled a child. A functional condition of the mammary glands in men
is known as _gynaekomasty_. (For further details see _The Structure of
Man_, by R. Wiedersheim, translated by H. and M. Bernard, and edited
by G. B. Howes, London, 1895.)

_Embryology._--There is every probability that the mammary glands are
modified and hypertrophied sebaceous glands, and transitional stages
are seen in the areolar glands, which sometimes secrete milk. At an
early stage of foetal life a raised patch of ectoderm is seen, which
later on becomes a saucer-like depression; from the bottom of this
fifteen or twenty solid processes of cells, each presumably
representing a sebaceous gland, grow into the mesoderm which forms the
connective-tissue stroma of the mamma. Later on these processes
branch. The last stage is that the centre of the _mammary pit_ or
saucer-like depression once more grows up to form the nipple, and at
birth the processes become tubular, thus forming lactiferous ducts.
The glands grow little until the age of puberty, but their full
development is not reached until the birth of the first child.

_Comparative Anatomy._--In the lower Mammals the mammary line, already
mentioned, appears in the embryo as a ridge, and in those which have
many young at a birth patches of this develop in the thoracic and
abdominal regions to form the mammae, while the intervening parts of
the ridge disappear. The number of mammae is not constant in animals
of the same species; as an instance of this it will be found that in
the dog the number of nipples varies from seven to ten, though animals
with many nipples are more liable to variation than those with few.
When only a few young are produced at a time the mammae are few, and
it seems to depend on the convenience of suckling in which part of the
mammary line the glands are developed. In the pouched Mammals
(Monotremes and Marsupials) inguinal mammae are found, and so they are
in most Ungulates as well as in the Cetacea. In the elephants,
Sirenia, Chiroptera and most of the Primates, on the other hand, they
are confined to the pectoral region, and this is also the case in some
Rodents, e.g. the jumping hare (_Pedetes caffer_). In the monotremes
the mammary pit remains throughout life, and the milk is conducted
along the hairs to the young, but in other Mammals nipples are formed
in one of two ways. One is that already described in Man, which is
common to the Marsupials and Primates, while in the other the margin
or _vallum_ of the mammary pit grows up, and so forms a nipple with a
very deep pit, into the bottom of which the lactiferous ducts open.
The latter is regarded as the primary arrangement. In the monotremes
the mammae are looked upon, not as modified sebaceous glands, as in
other Mammals, but as altered sweat glands. It is further of interest
to notice that in these primitive Mammals the glands are equally
developed in both sexes, and it is thought that among the bats the
male often assists in suckling the young (see G. Dobson, _Brit. Museum
Cat. of the Chiroptera_, London, 1878). These facts, together with the
occasional occurrence of gynaekomasty in man, make it probable that
the ancestral Mammal was an animal in which both sexes helped in the
process of lactation.

For further details and literature up to 1906 see _Comparative Anatomy
of Vertebrates_, by R. Wiedersheim, adapted by W. N. Parker (1907),
and Bronn's _Classen und Ordnungen des Thierreichs_. (F. G. P.)

_Diseases of the Mammary Gland._--Inflammation of the breast
(_mastitis_) is apt to occur in a woman who is suckling, and is due to
the presence of septic micro-organisms, which, as a rule, have found
their way into the milk-ducts, the lymphatics or the veins, through a
crack, or other wound, in a nipple which has been made sore by the
infant's vigorous attempts to obtain food. Especially is this septic
inflammation apt to occur if the nipple is depressed, or so badly
formed that the infant has difficulty in feeding from it. The inflamed
breast is enlarged, tender and painful, and the skin over it is hot,
and perhaps too reddened. The woman feels ill and feverish, and she
may shiver, or have a definite rigor--which suggests that the
inflammation is running on to the formation of an abscess. The abscess
may be superficial to, or beneath, the breast, but it is usually
within the breast itself. The infant should at once be weaned, the
milk-tension being relieved by the breast-pump. Fomentations should be
applied under waterproof jaconette, and the breast should be evenly
supported by a bandage or by the corsets. Belladonna and glycerine
should be smeared over the breast, with the view of checking the
secretion of milk, as well as of easing pain. But before this is done
six or eight leeches may be applied. On the first indication that
matter is collecting, an incision should be made, for if the matter is
allowed to remain locked up in the breast tissue the abscess will
rapidly increase in size, and the whole of the breast may become
infected and destroyed. Supposing that, in making the incision, no pus
is discovered, the relief to the vascular tension thus afforded will
be nevertheless highly beneficial. The operation had better be done
under a general anaesthetic, so that the surgeon can introduce a
probe, or his finger, into the wound, breaking down the partitions
which are likely to exist between separate abscesses, and thus enable
them to be drained through the one opening. As the discharge begins to
cease, the tenderness subsides, and gentle massage, or firm strapping
of the breast, will prove useful. The general treatment will consist
in the administration of an aperient, and, the tongue being clean, in
prescribing such drugs as quinine, strychnia and iron. The diet should
be liberal, but not carried to such excess that the power of digestion
and absorption is overtaxed. During the early acute stage of the
disease small doses of morphia may be necessary. When the tongue has
cleaned, a little wine may be given with advantage.

_Chronic Eczema_ around the nipple of a woman late in life, with,
perhaps, localized ulceration, is known as _Paget's Disease_. The
importance of it is that cancerous infiltration is apt to pass from it
along the milk-ducts and to involve the breast in malignant disease.
Hence, when eczema about the nipple refuses to clear up under the
influence of soothing treatment, it is well to insist on the removal
of the entire breast. Sometimes this eczema is malignant from the
beginning, being associated with the active prolifization of the
epithelial cells of the milk-ducts, and with their escape into the
surrounding tissues. The nipple is retracted in most of these cases,
which, however, are not often met with.

_Chronic Mastitis_ is of frequent occurrence in women who are past
middle age. The part of the breast involved is enlarged, hard, and
more or less tender and painful. It is sometimes impossible clinically
to distinguish this disease from cancer. True, the tumour is not so
definite or so hard as a cancer, nor is it attached to the skin, nor
to the muscles of the chest wall, and if there are any glands
secondarily enlarged in the arm-pit they are not so hard as they may
be in cancer. But all these are questions of degree. It is, of course,
highly inadvisable to leave it to time to clear up the diagnosis, for
a chronic mastitis, innocent at first, may eventually become
cancerous. If in any case the difficulty of distinguishing a chronic
mastitis from a malignant tumour of the breast is insuperable, the
safest course is to remove the breast and have it examined by the
microscope. The suggestion, sometimes made, as to the preliminary
removal of a small piece of the tumour for examination is not to be
recommended.

A simple glandular tumour, _fibro-adenoma_, is apt to be found in the
breasts of youngish women, who may possibly give an account of some
blow or other injury; there may, however, be no history of injury. The
tumour is smooth, rounded or oval, and lies loose in the midst of the
breast; as a rule it is not tender. It is not associated with enlarged
glands in the arm-pit. The tumour had best be removed, though there is
no urgency about the operation, as the growth is absolutely innocent.
There is, however, no telling as to what course an innocent tumour of
the breast may take as middle age comes on.

_Cysts of the Breast._--A _galactocele_ is a tumour due to the locking
up of milk in a greatly dilated duct. Other forms of cystic disease
may be due to serous or hydatid fluid, or to thin pus, being
surrounded by fibrous walls. Such cysts are best treated by free
incision, and by passing a gauze dressing into their depths. If the
tissue is occupied by many cysts, the whole breast had better be
removed.