The Cambridge Natural History Part 5

URINARY ORGANS.--The kidneys in the Mammalia have a compact form, which contrasts with the somewhat diffuse and vaguely-outlined kidneys of the Sauropsida. In mammals the organ is as a rule of that peculiar shape which is called "kidney-shaped"; a depression termed the hilum, which receives the ducts of the glands, indenting the border of an otherwise oval-shaped gland. In some few mammals the kidney is broken up {69} into lobules; this is the case with the Whales, the Bears, the Oxen, and a few other forms. A curious fact about the kidneys of the Mammalia is their very general asymmetry of position. One of them usually lies in a more advanced position than the other. The ureters lead from the kidneys to the urinary bladder, which in its form and relations is quite distinctive of the Mammalia. The bladder is formed out of the remains of the allantois, and is therefore not the exact h.o.m.ologue of the bladder of the frog, which is the equivalent of the entire sac which grows out of the cloaca in the mammal, and is the foetal allantois. The ureters open into the bladder in the higher Mammalia, but lower down in the urino-genital pa.s.sage in the more primitive mammals.

THE BODY CAVITY.--The Mammalia differ from all other living vertebrates by the arrangement of the body cavity in which lie the viscera. That cavity is divided into two by a partly muscular and partly tendinous part.i.tion, the diaphragm. No other vertebrate has this precise disposition of the coelom.

The diaphragm lies usually transversely to the longitudinal axis of the body, but gets a much more oblique arrangement in the Cetacea and the Sirenia, whose needs demand a more expanded chamber for the lungs. For in front of the diaphragm lie the lungs and heart; behind it the stomach, liver, intestines, and the organs of reproduction and excretion. The diaphragm is used in respiration; when its muscles contract, the surface directed toward the pleural cavity becomes less convex, and the cavity of the lungs is thus increased, allowing them to expand under the pressure of the entering air.

THE LUNGS.--The lungs of the Mammalia differ from those of animals lying lower in the series by the fact, just referred to, that they occupy a pleural cavity completely shut off from the abdomen by the diaphragm. As a rule the lungs of the Mammalia are to be distinguished by their more or less extensive lobation. In the Whales, however, and in the Sirenia, they are not much divided, but present the appearance of the simple sac-like lungs of the reptiles. In some mammals there is a median and posterior unpaired lobe of the lung, which lies in the post-pericardial cavity behind the pericardium. This is not universally present. The lungs are very frequently not symmetrical in their lobation, the number of separate lobes on the right side {70} and on the left being different. The lungs of mammals agree with those of the lower reptiles in being freely suspended within their coelomic cavity, and in not being, as in birds, crocodiles, and the Varanidae among lizards, tied down to the dorsal surface of that cavity by a sheet of peritoneum covering them.

[Ill.u.s.tration]

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

_bl.v_, Blood-vessels; _foll_, strings and groups of cells derived from the germinal epithelium becoming developed into follicles; _g.ep_, germinal epithelium; _in_, ingrowing cord of cells from the germinal epithelium; _pr.ov_, primitive ova. (From Hertwig, after Waldeyer.)

THE GONADS (OVARIES AND TESTES).--The ovary in the Mammalia is always paired; there is never a partial or complete abortion of one gonad as in birds--except of course in pathological cases. The ovaries are small, and lie in the abdominal cavity behind the kidneys. In the immense majority of the Mammalia the ova which are produced within the ovaries are of minute size; those of even the colossal Rorqual are, so far as we know, not markedly larger than the ova of a Mouse. The smallness of size of these reproductive elements implies necessarily an absence of much nutritive yolk; and as a consequence the developing embryo, since it is not hatched in an early stage as a free living larva, has to be nourished by the mother, to whose tissues it is attached through the intermediary of the placenta, a structure partly composed of foetal structures derived from the embryo, and partly of portions of the lining membranes of the uterus of the mother. The ova of the {71} Eutherian mammals, including the Marsupials, are very small as compared with those of any other vertebrates, excepting only _Amphioxus_, where the young are hatched early as free swimming larvae. They also differ in a highly characteristic way in the mode of their development within the ovary. These processes are to some extent ill.u.s.trated in Fig. 44. The main framework of the ovary is formed of the so-called "stroma," which is a ma.s.s of tissue formed of more or less connective-tissue-like cells. Within this are numerous cavities, the Graafian follicles. The very young follicles consist of but a single layer of follicular cells surrounding the ovum, which lies centrally. The follicular cells gradually increase in number until the ovum lies in the midst of several layers of cells. At this period a vacuity is formed between some of these cells, and grows into a large cell-free cavity; the ovum does not lie loosely in this s.p.a.ce, but is connected at one side with the follicular cells, which still line the interior of the Graafian follicle by the so-called discus or c.u.mulus proligerus. The egg or ovum has, moreover, a layer of cells immediately surrounding itself. All these facts can be gathered by an inspection of Fig. 45. It has been shown that, as in lower vertebrates, the cells immediately surrounding the ovum are connected with it directly by delicate processes which penetrate the actual membrane of the egg.

[Ill.u.s.tration]

FIG. 45.--Two stages in the development of the Graafian follicle. A, With the follicular fluid beginning to appear; B, after the s.p.a.ce has largely increased. _caps_, Capsule; _disc_, c.u.mulus proligerus; _memb_, membrana granulosa; _ov_, ovum; _sp_, s.p.a.ce containing fluid. (After Hertwig.)

[Ill.u.s.tration]

FIG. 46.--Ovarian egg of _Echidna_. _b_, Basilar membrane; _fe_, follicular epithelium; _o_, oil globules; _vm_, vitelline membrane; _y_^1, _y_^2, yolk. (Partly after Caldwell.)

The only ova which depart at all in structure from that above described are those of the Monotremata. The credit of this {72} discovery rests with Owen and with Professor Poulton, who pointed out in 1884,[37] that the ovum of _Ornithorhynchus_ is very large as compared with those of other Mammalia (6 mm. as against .2 mm.), that it is filled with yolk, and that it completely fills the follicle, being surrounded by two layers of follicular cells only. This latter fact was proved by Caldwell. Subsequently Gyldberg[38]

and I[39] described the ovarian ovum of _Echidna_, showing it to be identical with that of _Ornithorhynchus_. Later still a more elaborate and beautifully ill.u.s.trated paper was published by Caldwell[40] upon the early stages of development in the Monotremata and Marsupials, in which the ovum of the former was accurately described (see Fig. 46). In the particulars mentioned above, the ovum of the Monotremata is practically identical with that of the large-yolked ova of the Sauropsida. {73}

[Ill.u.s.tration]

FIG. 47.--_Lepus cuniculus._ The anterior end of the v.a.g.i.n.a, with the right uterus, Fallopian tube, and ovary. (Nat. size.) Part of the ventral wall of the v.a.g.i.n.a is removed, and the proximal end of the left uterus is shown in longitudinal section, _fl.t_, Fallopian tube; _fl.t_', its peritoneal aperture; _l.ut_, left uterus; _l.ut_', left os uteri; _ov_, ovary; _r.ut_, right uterus; _r.ut_', right os uteri; _s_, v.a.g.i.n.al septum; _va_, v.a.g.i.n.a.

(From Parker's _Zootomy_.)

It is the general rule among vertebrate animals that the ovaries are completely independent of the ducts which convey their products to the exterior. In certain fishes, however, there is an absolute continuity between the two structures, which is believed to be due to a simple concrescence between the originally distinct ovary and oviduct. The latter has grown round the former, an obvious advantage in preventing the eggs from wandering into the abdominal cavity and becoming lost. In the Mammalia we find discontinuity as a general rule. But in quite a number of forms folds of the lining membrane of the abdominal cavity are developed, which practically ensure the pa.s.sage of the ova into the oviduct when they are extruded from the ovaries. The oviduct, moreover, has a large and fimbriated mouth, called in human anatomy--which is provided with a number of fanciful names--the morsus diaboli. This almost wraps round the ovary, and thus prevents the ova from straying in the wrong direction. Moreover, the ovary itself is often so arranged that it can easily be withdrawn into a pocket of the peritoneum, from which the obvious exit is by the gaping mouth of the oviduct. This disposition of the generative parts is still further modified in a few animals, such as the Rat[41] and the Kinkajou.[42] In these animals the mouth of the oviduct actually opens into the interior of a closed chamber which contains the ovary. In this case there is but one route for the {74} extruded ova to follow. This series of steps in the perfecting of the mode of safe extrusion of the ova is highly interesting, and is a piece of evidence in favour of the high position of the mammals.

[Ill.u.s.tration]

FIG. 48.--Female urino-genital apparatus of various Marsupials. A, _Didelphys dorsigera_ (young); B, _Trichosurus_; C, _Phascolomys wombat_.

_B_, Urinary bladder; _Cl_, "cloaca"; _Fim_, fimbriae; _g_, c.l.i.toris; _N_, kidney; _Od_, Fallopian tube; _Ot_, aperture of Fallopian tube; _Ov_, ovary; _r_, r.e.c.t.u.m; _Sp_, septum dividing v.a.g.i.n.a; _Sug_, urino-genital sinus; _Ur_, ureter; _Ut_, uterus; _Ut_', opening of the uterus into the median v.a.g.i.n.a (_VgB_); _Vg_, lateral v.a.g.i.n.a; _Vg_', its opening into the urino-genital sinus; + (in B), point of approximation of uteri; + (in C) and *, rectal glands. (From Wiedersheim's _Comparative Anatomy_.)

{75}

The oviducal apparatus of the mammal is more specialised than that of lower vertebrates. It is most simple, as might be imagined, in the egg-laying Monotremes, where, indeed, it is on the same level as that of reptiles. But in the Eutheria the fimbriated mouth of the oviduct pa.s.ses into a narrow and winding tube, the Fallopian tube; this widens into a uterus, and the two uteri combine into a single tube in the higher forms. They are called the Monodelphia on this account. In the Marsupials the uteri are distinct though they often join above, and from this junction depends a median "uterus." After the uterus or the uteri follows in every case a single v.a.g.i.n.a.

The testes of the Mammalia, like those of other vertebrates, occupy primitively a position within the body cavity precisely corresponding to that of the ovaries. And in the lowly-organised Monotremata, and some other forms, such as the Whales, they retain that primitive position within the body. It is, however, distinctive of the Mammalia as opposed to lower vertebrates that the testes descend later into a s.c.r.o.t.u.m, which is simply a protrusion of the skin of the body surrounded by muscles, and, of course, containing a section of the body cavity in which lie the testes. The p.e.n.i.s of the Mammalia, represented by the c.l.i.toris and a.s.sociated structures in the female, is of a structure entirely peculiar to this group.

[Ill.u.s.tration]

FIG. 49.--Brain of Dog. A, ventral; B, dorsal; C, lateral aspect. _B.ol_, Olfactory lobe; _Cr.ce_, crura cerebri; _Fi.p_, great longitudinal fissure; _HH_, _HH_^1, lateral lobes of cerebellum; _Hyp_, hypophysis; _Med_, spinal cord; _NH_, medulla oblongata; _Po_, pons Varolii; _VH_, cerebral hemispheres; _Wu_, middle lobe (vermis) of cerebellum; _I-XII_, cerebral nerves. (From Wiedersheim's _Comparative Anatomy_.)

THE BRAIN.--Inasmuch as Professor Wiedersheim has said with perfect truth that "the brain of the extinct Ungulate _Dinoceras_ shows so striking a likeness to that of a lizard that one would be compelled to explain it as that of a lizard without a knowledge of the skeleton," it is clear that to define the mammalian brain is a difficult matter. The existing Mammalia, however, all possess brains which can be readily distinguished from those of vertebrates lying lower in the scale. They are of relatively large size, brought about mainly by the dimensions of the cerebral hemispheres, which have an importance in this cla.s.s of vertebrates that they have not elsewhere. Coupled with this large size of the hemispheres is a more elaborate system of transverse commissures uniting the two; and this culminates in the higher Mammalia, where the corpus callosum attains a large size and great physiological importance. A {76} very marked feature, moreover, of the mammal's brain is the development of regular fissures upon its surface, which fissures are only absent from _Ornithorhynchus_, various small Rodents, Bats, and Insectivores, among living mammals. It is sometimes, but erroneously, said that the more complicated the fissures of the brain are, the higher in intelligence and "zoological position" is the possessor of that brain. Instances can undoubtedly be quoted to support such a view; but they are {77} merely selected cases, which do not indicate a wide applicability of such a generalisation. Thus it is true that the brain of a Man is more elaborate in its furrows and convolutions than is that of a Cat. The real fact of the matter is, that the complexity of the brain from this point of view increases with the size of the animal within the group.

[Ill.u.s.tration]

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

(Nat. size.) _a.co_, Anterior commissure; _b.fo_, body of the fornix; _cb_, cerebellum, showing arbor vitae; _c.c_, crus cerebri; _c.h_^1, parencephalon or cerebral hemisphere; _c.h_^2, temporal lobe; _c.ma_, corpus mammillare; _cp.cl_, corpus callosum; _f.m_, foramen of Monro; _inf_, infundibulum; _l.t_, lamina terminalis; _ly_, lyra; _m.co_, middle commissure; _m.o_, medulla oblongata; _o.ch_, optic chiasma; _o.l_^1, _o.l_^2, corpora quadrigemina or optic lobes; _olf_, olfactory lobe; _p.co_, posterior commissure; _pd.pn_, peduncle of the pineal "gland,"

_pn_; _p.fo_, anterior pillar of the fornix; _pty_, pituitary body; _pv.a_, pons Varolii; _sp.lu_, septum lucidum; _v_^4, fourth ventricle; _vl.ip_, velum interpositum; _v.vn_, valve of Vicussens; _II_, optic nerve. (From Parker's _Zootomy_.)

The Gorilla and the Chimpanzee have a more furrowed brain than has the little Marmoset; the Bear a more complicated brain than the Weasel, etc.

The most highly-convoluted brains of all mammals are those of the Elephants, and there does not seem in the Ungulates to be so marked a relation between size and abundance of fissures as there is among other mammals. A regular plan of the fissures can be detected with certainty for each group considered by itself; but it is not so easy to h.o.m.ologise the details of arrangement from group to group. This is so far in accord with the view that the existing groups of mammals have diverged from each other _ab initio_.

Another marked characteristic of the mammalian as opposed to other brains is the relatively small importance in size and yet the fourfold nature of the optic lobes. What was the case with the optic lobes of the early Ungulates is difficult to understand, on account of the fact that the casts are necessarily imperfect. {78} Altogether the enormous progress in the complexity of the brain from the early Tertiary mammals down to the present, is one of the most remarkable revelations of palaeontology. It goes perhaps some way in explaining the remarkable diversity in mode of life exhibited by the mammals as compared, for example, with the birds, whose brains have not diverged so much or in so many directions from the primitive form.

THE PRESENT DISTRIBUTION OF THE MAMMALIA.--In the following pages some of the princ.i.p.al facts in the geographical range of the orders, families, and many of the genera of Mammalia will be given. It has been justly observed by Mr. Sclater that the habitat of an animal is as much a part of its definition as is its structure or external form. No systematic account of the Mammalia would therefore be complete without such geographical facts.

But that branch of zoology which is concerned with the past and present distribution of animals is wider in scope than this. Zoogeography deals not only with the actual facts in the range of animals, but with the inferences as to past changes in the relations of land and sea which the facts seem to indicate, and with speculations as to the place of origin of the different groups, of which more than hints are sometimes given by their past and present distribution. In addition to this, the earth can be mapped out into provinces and regions which are definable by their animal inhabitants. In the present volume, dealing only with the Mammalia, it will be obviously impossible to enter fully into the entire subject of zoogeography. All that will be attempted is a brief general survey of the science so far as it can be ill.u.s.trated by the Mammalia. For fuller knowledge the reader is referred to the treatises mentioned below.[43]

There are certain facts in the distribution of animals which are commonplaces of knowledge, but which may be set forth with definiteness.

Everybody knows that an animal has a given range: Elephants, for example, are found in India and certain adjacent parts of Asia, and again in Africa; the Rhinoceroses have roughly the same range; the Tiger is limited to Asia; the {79} Jaguar to America, and so forth. The entire expanse of country which is inhabited by an animal is called its area of distribution. Such areas are larger or smaller. The Lion ranges over the whole of Africa, a small part of India, and some neighbouring countries; on the other hand, the Insectivore _Solenodon_ is limited to Cuba and Hayti, a separate species to each. Among other groups of animals are instances of an even more restricted range. There are humming-birds confined to the slopes of a single mountain, and fishes limited in their range to a single small lake.

A species may be found everywhere within the area of its distribution, or it may be confined to a number of limited tracts within that area. In this case it is usual to speak of "stations." In such cases the species in question is generally suited to some particular kind of environment. Thus the Otter and other aquatic mammals will only be found where there is water; and intervening tracts of waterless country will contain no Otters.

Goats and Chamois live only upon mountains; the intervening plains are dest.i.tute of them. This discontinuity of distribution within the area is very general. But a discontinuity of area is also seen--not so commonly however; and, indeed, when it does occur, it is a matter of a genus and not of a species. Thus the Tapir is found in the East Indies on the one hand and in South and Central America on the other, being absent in the intermediate tracts.

It is clear that tracts of country eminently suitable for the housing of a particular mammal do not always possess that kind, or even an allied form.

Africa, for example, possesses no arboreal Anteaters; there are no Anteaters at all (of the order Edentata) in Australia, though there are plenty of ants for them to feed upon, and tropical conditions of climate prevail. But as in these cases the inference may be denied on the grounds that no experiments exist to prove or to disprove the a.s.sertion, the matter may be better emphasised by such cases as the introduction of the Rabbit into Australia, and various mammals, such as Goats, into oceanic islands.

The plague caused by the former is a matter of notoriety. But although climate and conditions and animal inhabitants do not march accurately together, there is certainly some connexion between temperature and the range of animals. Mr. Lydekker writes on this point as follows: "The llama-like animals, respectively known as vicunas and guanacos, are met with in {80} company on the highlands of the Cordillera in Peru and Ecuador, but as we go farther south the latter are found on the plains of southern Argentina and Patagonia, as well as on the island of Tierra del Fuego at the sea level. Here then is a clear proof of the intimate connexion existing between temperature and station; the guanaco being an animal which can only live in cold or temperate climates, finds suitable conditions for its existence in tropical lat.i.tudes solely at a height of so many thousands of feet, although farther south it is able to thrive at the sea level." This, however, cannot be pushed too far--the world cannot be mapped out into areas bounded by parallels of temperature as was once attempted--since there are plenty of cases like that of the Tiger, which is as much at home in a tropical jungle as on the icy plains of Northern Asia.

Seeing that there are in many cases no climatic barriers to the spreading of a given race of animals over a larger area of distribution than it actually occupies, it becomes important to inquire why there are so many cases of restriction in range.

It is possible to see, at any rate, three causes which are responsible for a large number of such cases. In the first place, a given species of animal must have originated at a certain spot; its multiplication in individuals must always be a slow matter, since enemies, and untoward events generally, would conspire to check the natural multiplication by geometrical progression. A long time might therefore elapse before the species greatly extended its range. A restricted distribution may therefore, in some cases, mean a modern race. In the second place, there are definite physical barriers which check the migration of species. The terrestrial Mammalia cannot cross wide arms of the sea; that they can and do swim for considerable distances has been proved in several instances; but, as has been pointed out, it is unlikely that a purely terrestrial mammal would voluntarily swim out into an unknown sea. And then if it did, and successfully reached the opposite side, nothing would happen unless it were a pregnant female; or, if not pregnant, till a male swam very soon afterwards in exactly the same direction. Many travellers have told of floating islands, formed of torn-up trees and brushwood, which have been seen at the mouths of large rivers, with animal pa.s.sengers upon them. These are, however, so much at the mercy of currents and storms, {81} that but little reliance can be placed on them as a means of transit; besides, here again, two individuals, or a pregnant female, would be required to effect a settlement on a foreign sh.o.r.e. The existence of oceanic islands is often urged as a proof of this inability to cross tracts of sea; even those which are comparatively near an extensive continent, such as, for example, Fernando Noronha in the Atlantic, are dest.i.tute of mammals (except, indeed, the ubiquitous Mouse, which is believed to have been carried there, often in company with the equally widely-spread Rat, in s.h.i.+ps). This argument, however, is not so conclusive as might appear; it doubtless is in the case of far-distant islands. But the size of the islands has to be taken into account. For there are islands, such as the Galapagos, or, to take a less contested instance, some of the islands of the Malagasy Archipelago, undoubtedly continental, which have an exceedingly reduced number of mammals. An area of a certain size seems to be a necessity.

The converse of this is in many cases easy to show, that is, the wide range of animals when there are no marine barriers to stop their spreading. John Hunter, the celebrated anatomist and surgeon (not often quoted, however, as an authority upon geographical distribution), observes: "It is a curious circ.u.mstance in the natural history of animals to find most of the northern animals the same both on the continent of America and what is called the Old World, while those of the warmer parts of both continents are not so.

Thus we find the bear, fox, wolf, elk, reindeer, ptarmigan, etc., in the northern parts of both.... The reason why the same animals are to be found in the northern parts is the nearness of the two continents. They are so near as to be within the power of accident to bring the animals, especially the large ones, from one continent to the other either on the ice or even by water. But the continents diverging from each other southward, so as to be at a very considerable distance from each other even beyond the flight of birds, is the reason why the quadrupeds are not the same."

There is no doubt, in fact, that the ocean is the most insuperable of all barriers to the dispersal of mammals. In a less degree mountain ranges and deserts are also barriers. The Desert of Sahara is a striking instance to the point; it separates two exceedingly different faunas. {82}

A third cause of more or less limited range is the barrier due to compet.i.tion. If the ground is already taken up, there is no room for new immigrants. There is obviously a limit to the number of Antelopes or Deer that can graze upon a given tract of gra.s.sy plain. These two groups of Ungulates ill.u.s.trate the matter well: the Antelopes are African and Indian, especially the former, while Africa has no Deer at all; America, on the other hand, has plenty of Deer but no Antelopes, save the p.r.o.ng-horn. The more nearly akin the two species or groups of species are, the fiercer will be the compet.i.tion; for a near kins.h.i.+p will at least often imply similar habits, the need for similar food, and other likenesses which will prevent both from successfully occupying the same tract of country. The remarkable fauna of Australia is believed to afford an example of this. In that country the prevalent inhabitants are the Marsupials. The Monotremes are found there also, and nowhere else save in New Guinea and Tasmania. The remaining mammals are inconspicuous; they embrace a few Rodents and Bats, and the doubtfully indigenous Dingo-dog. Now the Marsupials are fitted to every variety of life. We have the grazing Kangaroos and Wallabies, the burrowing Wombats, the arboreal Phalangers, and the carnivorous Dasyures.

In the second place, it is an unquestioned fact that the Marsupials are an older race than are the existing Eutherian mammals; they were the dominant mammals during the Secondary epoch. At that time they were more widely distributed than at present. In most parts of the world they are now absent, since they have been successfully ousted by the more highly organised groups of Eutheria. But at that period, when the higher Eutheria were in the ascendant, Australia and the islands to the north became cut off from Asia, and thus became freed from inroads of Eutheria, which were partly prevented by the physical barrier of the sea from effecting a settlement, and partly perhaps prevented owing to the ground being already taken up by the Marsupials. Likeness of habit gave the older inhabitants victory in the struggle for existence.

The general statements that have been here made are in accord with current opinion upon the factors of geographical distribution. But the past range of animals appears to be less consonant with the received views. In the Tertiary {83} period, groups of animals had often a far wider range than at present. To-day the Rhinoceroses are limited to Asia and Africa, and to quite limited parts of the former continent. In the past, these animals were abundant in Europe and North America. Wild Horses now have a range which is not widely different from that of the Rhinoceroses, save that they extend into the more northern regions of Asia. Their remains are abundant both in North and South America. The Hippopotamus, now confined to Africa, once ranged over Europe, Madagascar, and India. There were plenty of American and European Lemurs. Elephants were nearly world-wide in their range; and, in short, restricted distribution seems to be on the whole a characteristic of animals of the present day.

These statements, however, though perfectly true, must not lead to erroneous inferences. It is rather impressed upon the reader, in books which contain sections dealing with geographical distribution, that animals on the whole occupy more restricted areas at present than in the past.

There are, however, plenty of examples of groups of extinct creatures which had, so far as we know, quite a restricted range. Thus the Toxodonts were purely South American, as were the Glyptodonts and some other forms. And, on the other hand, the Cervidae of to-day are as widely, if not more widely, distributed than at any other time. The Hares and Rabbits are now nearly universal in range; the Cats almost so. We meet with Bovidae, even excluding the Sheep and Goats, in all the four quarters of the globe, excluding only South America and, of course, Australia. The Camelidae are still common to both the Old and the New Worlds.

During certain periods of the Tertiary epoch it is true that there was more similarity between Europe and North America than there is at present. It would have been quite necessary to unite both into a Holarctic area, such as is now insisted upon by many; but the reasons for this union would then have been stronger. The fact is, however, that the closer resemblances were due to the larger number of families of animals which existed then than now; these have decayed away from both continents, and allowed the unlikenesses between the mammalian fauna of both to become evident. But the likenesses which still survive have led many to a.s.sociate the two regions closely together.

So far as the history of a genus or family or larger division {84} can be traced, it results as a conclusion that from a given area of origin the group in question migrated in all directions where possible to a varying degree; it then died out in intervening tracts, or was left only in a certain part of its former and more extensive area of range.

ZOOLOGICAL REGIONS.--Seeing that each species of animal has its own definite range, it is clear that the earth's surface can be apportioned into divisions which are characterised by their animal inhabitants. We shall divide the earth into realms, which are the largest divisions; then into regions; and finally into subregions. It must be borne in mind that the various groups of the animal kingdom are of different ages, geologically speaking, and have therefore had less or more time, as the case may be, to settle down into their present distribution, and that different animals differ greatly in their rate of multiplication, their power of migration, and their susceptibility to the effectiveness of various natural and other barriers to distribution. It is not, therefore, possible to divide the world into realms and regions which shall express the facts of distribution of the entire animal kingdom. Such divisions, which are common in text-books of zoology having but a small section devoted to zoogeography, are at best mere approximations and averages; no good is gained by taking such a comprehensive view of the matter, as the essential object of subdividing the earth's surface is thereby lost sight of. The zoogeographical division of the earth which will be adopted here is that originally recommended by Dr. Blanford, and now accepted by a number of authorities. There are three "realms," to which a fourth may perhaps be added--though on negative grounds, and merely for the purpose of emphasising the parts of the world to which mammals have not gained access.

The realms are again divisible into regions, at least in the case of one of them, and the regions may be again separated into more or less distinct subregions or provinces. The three primary divisions or realms which contain mammals are the Notogaean, including Australia and certain islands to the north of it; the Neogaean, or the South American continent and Central America; the Arctogaean, including the continents of North America, Europe, Asia, and Africa, together with the adjacent islands, such as the West Indies, East Indies (exclusive of those which fall within {85} the realm of Notogaea), and Madagascar; and finally, the realm of Antarctogaea or Atheriogaea, which embraces New Zealand, the Antarctic continent, and a series of islands such as South Georgia and Kerguelen, and possibly even the extreme south of Patagonia. This latter quarter of the globe will need no further reference, as it has no truly indigenous terrestrial mammalian inhabitants. We cannot include the Bats in this statement, as their distribution is due to different powers of extending their range, and to different barriers from those which govern the range of other groups of mammals.

(1) Notogaea.[44] This realm is characterised by the exclusive possession of the Monotremes:--that is to say, one of the two primary divisions of the Mammalia is absolutely restricted to this area. It contains, moreover, the vast majority of the Marsupials. Further, the realm of Notogaea is to be distinguished by the entire absence of the higher mammals, with the exception of a few small Rodents. (The Bats are ignored for the reasons stated, and the Dingo is believed to have been an importation.) It cannot be disputed that this is a very distinctly-marked area of the earth's surface.

(2) Neogaea. The continent of South America has no Monotremes and only a few Marsupials, all of which, with the exception of _Caenolestes_, belong to the Polyprotodont division of that order, and to a peculiar family, Didelphyidae. The recent discovery of other fossil Marsupials, however, to some extent favours Huxley's view that Neogaea and Notogaea form one realm as opposed to the rest of the world. Besides this, Neogaea possesses the Edentata, which are found nowhere else;--that is, the division of the Edentata to which the name is now restricted by some authorities. It is also characterised by the nearly entire absence of the important order of Insectivora; and, as minor marks of distinction, by the absence of Antelopes, Oxen and Sheep, of the Ichneumon tribe, of Horses, and of Lemurs. It has the exclusive possession of the Hapalidae and Cebidae, and of several families of Rodents.