The Cambridge Natural History Part 1

The Cambridge Natural History.

Vol X.

Mammalia.

by Frank Evers Beddard.

PREFACE

Inasmuch as Sir W. H. Flower and Mr. Lydekker could not profess to treat the Mammalia exhaustively within the limits of nearly 800 pages, in their _Introduction to the Study of Mammals_, it is obvious that the present volume, which appears ten years later and is of rather less size, can contain but a selection of the enormous ma.s.s of facts at the disposal of the student of this group. Thus the chief question for myself was what to select and what to leave aside. It will be observed that I have reduced the pages of this book to conformity with those of other volumes of the series by treating some groups more briefly than others. It has appeared to me to be desirable to treat fully such groups as the Edentata and the Marsupialia, and permissible to be more brief in dealing with such huge Orders as those of the Rodentia and Chiroptera. Lengthy disquisitions upon such familiar and comparatively uninteresting animals as the Lion and Leopard have been curtailed, and the s.p.a.ce thus saved has been devoted to shorter and more numerous accounts of other creatures. As there are nearly six hundred genera of living Mammals known to science, omission as well as compression became an absolute necessity. I have given, I hope, adequate treatment from the standpoint of a necessarily limited treatise to the majority of the more important genera of Mammals both living and extinct; but the length of this part of the book had to be increased by the discoveries, which give me at once an advantage and a disadvantage as compared with the two authors whose names I have quoted, of a considerable number of important new types in the last ten years. {iv} Such forms as _Notoryctes_, _Romerolagus_, _Caenolestes_, "_Neomylodon_," and _Ocapia_ could not possibly have been omitted.

In preparing my accounts of both living and extinct forms I have nearly invariably consulted the original authorities, and have often supplemented or verified these accounts by my own dissections at the Zoological Society's Gardens. My rule has not, however, been invariable in this matter, inasmuch as there exist two recent and trustworthy text-books of Mammalian Palaeontology--Professor Zittel's _Handbuch der Palaeontologie_, and Dr. A. Smith Woodward's manual, _Outlines of Vertebrate Palaeontology_, in the Cambridge Biological Series. Where the name of a genus only or its range, or merely one or two facts about it, are mentioned, I have not thought it necessary to go further than these two works. But a good deal has been done even since the appearance of these two volumes which it will be found that I have not ignored.

I have to thank my editors for the trouble which they have taken in the revision of the proofs and for many suggestions. To Professor Osborn, of Columbia University, New York, I am indebted for some kind suggestions. My daughter Iris has a.s.sisted me in various ways. Finally, I desire to express my indebtedness to Mr. Dixon and to Mr. M. P. Parker for the care which they have taken in the preparation of the figures which were drawn by them especially for this work.

FRANK E. BEDDARD.

LONDON, _February 28, 1902_.

CHAPTER I

INTRODUCTORY

The Mammalia form a group of vertebrated animals which roughly correspond with what are termed in popular language "quadrupeds," or with the still more vernacular terms of "beasts" or "animals." The name "Mammal" is derived from the most salient characteristic of the group, _i.e._ the possession of teats; but if the term were used in an absolutely strict etymological sense, it could not include the Monotremes, which, though they have mammary glands, have not fully-differentiated teats (see p. 16). There are, however, as will be seen shortly, other characters which necessitate the inclusion of these egg-laying quadrupeds within the cla.s.s Mammalia.

The Mammalia are unquestionably the highest of the Vertebrata. This statement, however, though generally acceptable, needs some explanation and justification. "Highest" implies perfection, or, at any rate, relative perfection. It might be said with perfect truth that a serpent is in its way an example of perfection of structure: not incommoded with limbs it can slip rapidly through the gra.s.s, swim like a fish, climb like a monkey, and dart upon its prey with rapidity and accuracy. It is an example of an extremely specialised reptile, the loss of the limbs being the most obvious way in which it is specialised from more generalised reptilian types.

Specialisation in fact is often synonymous with degradation, and, this being the case, implies a restricted life. On the other hand, simplification is not always to be read as degeneration. The lower jaw, for instance, of mammals has fewer bones in it than that of reptiles, and is more concisely articulated to the skull; this implies greater efficiency {2} as a biting organ. The term highest, however, includes increased complexity as well as simplification, the two series of modifications being interwoven to form a more efficient organism. It cannot be doubted that the increased complexity of the brain of mammals raises them in the scale, as does also the complex and delicately adjusted series of bonelets which form the organ for the transmission of sound to the internal ear. The separation of the cavity containing the lungs, and the investment of the part.i.tion so formed with muscular fibres, renders the action of the lungs more effective; and there are other instances among the Mammalia of greater complexity of the various parts and organs of the body when compared with lower forms, which help to justify the term "highest" generally applied to these creatures.

Complexity and finish of structure are often accompanied by large size; and the Mammalia are, on the whole, larger than any other Vertebrates, and also contain the most colossal species. The huge Dinosaurs of the Mesozoic epoch, though among the largest of animals, are exceeded by the Whales; and the latter group includes the mightiest creature that exists or has ever existed, the eighty-five-feet-long Sibbald's Rorqual. Confining ourselves rigidly to facts, and avoiding all theorising on the possible relation between complexity and nicety of build and the capacity for increase in bulk, it is plain from the history of more than one group of mammals that increase in bulk accompanies specialisation of structure. The huge Dinocerata when compared with the ancestral _Pantolambda_ teach us this, as do many similar examples. Within the mammalian group, as in the case of other Vertebrates, difference of size has a certain rough correspondence with difference of habitat. The Whales not only contain the largest of animals, but their average size is great; so too with the equally aquatic Sirenia and very aquatic Pinnipedia. Here the support offered by the water and the consequent decreased need for muscular power to neutralise the effects of gravity permit of an increase in bulk. Purely terrestrial animals come next; and finally arboreal, and, still more, "flying" mammals are of small size, since the maintenance of the position when moving and feeding needs enormous muscular effort.

The Mammals are more easily to be separated from the Vertebrates lying lower in the series than any of the latter are from each other in ascending order. A large number of {3} characters might be used in addition to those which will be made use of in the following brief catalogue of essential mammalian features, were it not for the low-placed Monotremata on the one hand and the highly specialised Whales on the other. Including those forms, the Mammalia are to be distinguished from all other Vertebrates by the following series of structural features, which will be expanded later into a short disquisition upon the general structure of the Mammalia. The cla.s.s Mammalia may, in fact, be thus defined:--

Hair-clad Vertebrates, with cutaneous glands in the female, secreting milk for the nourishment of the young. Skull without prefrontal, postfrontal, quadrato-jugal, and some other bones, and with two occipital condyles formed entirely by the exoccipitals. Lower jaw composed of dentary bone only, articulating only with the squamosal. Ear bones a chain of three or four separate bonelets. Cervical vertebrae sharply distinguished from the dorsals, and if with free ribs, showing no transition between these and the thoracic ribs. Brain with four optic lobes. Lungs and heart separated from abdominal cavity by a muscular diaphragm. Heart with a single left aortic arch. Red blood-corpuscles non-nucleate.

The following characters are also very nearly universal, and in any case absolutely distinctive:--Cervical vertebrae, seven; vertebrae with epiphyses. Ankle-joint "cruro-tarsal," _i.e._ between the leg and the ankle, and not in the middle of the ankle.[1] Attachment of the pelvis to the vertebral column pre-acetabular in position.

The Mammalia since they are hot-blooded creatures are more independent of temperature than reptiles; they are thus found spread over a wider area of the earth's surface. As however, though hot-blooded, they have not the powers of locomotion possessed by birds, they are not quite so widely distributed as are those animals. The Mammalia range up into the extreme north, but, excepting only forms mainly aquatic, such as the Sea Lions, are not known to occur on the Antarctic continent. With the exception of the flying Bats, indigenous mammals are totally absent from New Zealand; and it seems to be doubtful whether those supposed oceanic islands which have a mammalian fauna are really {4} oceanic in origin. The continents and oceans are peopled by rather over three thousand species of Mammalia, a number which is considerably less than that of either birds or reptiles. It seems clear that, so far at any rate as concerns the numbers of families and genera, the mammalian fauna of to-day is less varied than it was during the Mid-tertiary period, the heyday of mammalian life. It is rather remarkable to contrast in this way the mammals and the birds. The two cla.s.ses of the animal kingdom seem to have come into being at about the same period; but the birds either have reached their culminating point to-day, or have not yet reached it. The Mammalia, on the other hand, multiplied to an extraordinary extent during the Eocene and the Miocene periods, and have since dwindled. The break is most marked at the close of the Pleistocene, and may be in part due to the direct influence of man. At present man exercises so enormous an effect, both directly and indirectly, that the future history of the Mammalia is probably foreshadowed by the instances of the White Rhinoceros and the Quagga. On the other hand, the economic usefulness of the Mammalia is greater than that of any other animals; and the next most important era in their history will be probably that of domesticity and "preservation."

{5}

CHAPTER II

STRUCTURE AND PRESENT DISTRIBUTION OF THE MAMMALIA

EXTERNAL FORM.--It would be quite impossible for any one to confuse any other quadrupedal animal with a mammal. The body of a reptile is, as it were, slung between its limbs, like the body of an eighteenth century chariot between its four wheels; in the mammal the body is raised entirely above, and is supported by, the four limbs. The axes of these limbs too, as a general rule, are parallel with the vertical axis of the body of their possessor. There is thus a greater perfection of the relations of the limbs to the trunk from the point of view of a terrestrial creature, which has to use those limbs for rapid movement. The same perfection in these relations is to be seen, it should be observed, in such running forms among the lower Vertebrata as the Birds and the Dinosaurs, where the actual angulation of the limbs is as in the purely running Mammalia. These relations are of course absolutely lost in the aquatic Cetacea, and not marked in various burrowing creatures. The way in which the fore- and hind-limbs are angulated is considerably different in the two cases. In the latter, which are most used and, as it were, push on the anterior part of the body, the femur has its lower end directed forwards, the tibia and the fibula project backwards at the lower end, while the ankle and foot are again inclined in the same direction as the femur. With the fore-limbs there is not this regular alternation. The humerus is directed backwards, the fore-arm forwards, and the hand still more forwards. This angulation seems to facilitate movement, inasmuch as it is seen in even the Amphibia and the lower Reptiles, in which, however, the differences between the fore- and hind-limbs are less marked, {6} indicating therefore a less specialised condition of the limbs. It is an interesting fact that the angulation of the limbs is to some extent obliterated in very bulky creatures, and almost entirely so in the elephants (see p. 217), which seem to need strong and straight pillars for the due support of their huge bodies.

The alertness and general intellectual superiority of mammals to all animals lying below them in the series (with the exception of the birds, which are in their way almost on a level with the Mammalia) are seen by their active and continuous movements. The lengthy periods of absolute motionlessness, so familiar to everybody in such a creature as the Crocodile, are unknown among the more typical Mammalia except indeed during sleep. This mental condition is clearly shown by the proportionate development of the external parts of all the organs of the higher senses.

The Mammalia as a rule have well-developed, often extremely large, flaps of skin surrounding the entrance to the organ of hearing, often called "ears,"

but better termed "pinnae." These are provided with special muscles, and can be often moved and in many directions. The nose is always, or nearly always, very conspicuous by its naked character; by the large surface, often moist, which surrounds the nostrils; and again by the muscles, which enable this tract of the integument to be moved at will. The eyes, perhaps, are less marked in their predominance over the eyes of lower Vertebrates than are the ears and nose; but they are provided as a rule with upper and lower eyelids, as well as by a nict.i.tating membrane as in lower Vertebrates. The apparent predominance of the senses of smell and hearing over that of sight appears to be marked in the Mammalia, and may account for their diversity of voice as well as of odour, and for the general sameness of coloration which distinguishes this group from the brilliantly-coloured birds and reptiles. The head, too, which bears these organs of special sense, is more obviously marked out from the neck and body than is the case with the duller creatures occupying the lower branches of the Vertebrate stem.

[Ill.u.s.tration]

FIG. 1.--A, Section of human skin. _Co_, Dermis; _D_, sebaceous glands; _F_, fat in dermis; _G_, vessels in dermis; _GP_, vascular papillae; _H_, hair; _N_, nerves in dermis; _NP_, nervous papillae; _Sc_, h.o.r.n.y layer of epidermis; _SD_, sweat gland; _SD_^1, duct of sweat gland; _SM_, Malpighian layer. B, Longitudinal section through a hair (diagrammatic). _Ap_, Band of muscular fibres inserted into the hair-follicle; _Co_, corium (dermis); _F_, external longitudinal; _F_^1, internal circular, fibrous layer of follicle; _Ft_, fatty tissue in the dermis; _GH_, hyaline membrane between the root-sheath and the follicle; _HBD_, sebaceous gland; _HP_, hair-papilla with vessels in its interior; _M_, medullary substance (pith) of the hair; _O_, cuticle of root-sheath; _R_, cortical layer; _Sc_, h.o.r.n.y layer of epidermis; _Sch_, Hair shaft; _SM_, Malpighian layer of epidermis; _WS_, _WS_^1, outer and inner layers of root-sheath. (From Wiedersheim's _Comparative Anatomy_.)

[Ill.u.s.tration]

THE HAIR.--The Mammalia are absolutely distinguished from all other Vertebrates (or, for the matter of that, Invertebrates) by the possession of hair. To define a mammal as a Vertebrate with hair would be an entirely exclusive definition; even in the smooth Whales a few hairs at least are present, which may be {7} reduced to as few as two bristles on the lips.

The term "hair," however, is apt to be somewhat loosely applied; it has been made use of to describe, for example, the slender processes of the chitinous skin of the Crustacea. It will be necessary, therefore, to enter into the microscopical structure and development of the mammalian hair.

Hair is found in every mammal. The first appearance of a hair is a slight thickening of the stratum Malpighii of the epidermis, the cells taking part in this being {8} elongated and converging slightly above and below. Dr.

Maurer has called attention to the remarkable likeness between the embryonic hair when at this stage and the simple sense-organs of lower Vertebrates. Later there is formed below this a denser aggregation of the corium, which ultimately becomes the papilla of the hair. This is the apparent h.o.m.ologue of the first formed part of a feather, which projects as a papilla before the epidermis has undergone any modification. Hence there is from the very first a difference between feathers and hairs--a difference which must be carefully borne in mind, especially when we consider the strong superficial resemblance between hairs and the simple barbless feathers. Still later the k.n.o.b of epidermic cells becomes depressed into a tubular structure, which is lined with cells also derived from the stratum Malpighii, but is filled with a continuation of the more superficial cells of the epidermis. This is the hair-follicle, and from the epidermic cells arises the hair by direct metamorphosis of those cells; there is no excretion of the hair by the cells, but the cells become the hair. From the hair-follicle also grows out a pair of sebaceous glands, which serve to keep the fully-formed hair moist.

[Ill.u.s.tration]

FIG. 2.--Four diagrams of stages in the development of a hair. A, Earliest stage in one of those mammals in which the dermal papilla appears first; B, C, D, three stages in the development of the hair in the human embryo.

_blb_, Hair-bulb; _crn_, h.o.r.n.y layer of the epidermis; _foll_, hair-follicle; _grm_, hair-germ; _h_, hair, in D, projecting on the surface; _muc_, Malpighian layer of epidermis; _pp_, dermal papilla; _seb_, developing sebaceous glands; _sh.1_, _sh.2_, inner and outer root-sheaths.

(After Hertwig.)

Dr. Meijerle[2] has lately described in some detail the {9} particular arrangement of the individual hairs among mammals; they are not by any manner of means scattered without order, but show a definite and regular arrangement, which varies with the animal. For instance, in an American Monkey (_Midas_), the hairs arise in threes--three hairs of equal size springing from the epidermis close together; in the Paca (_Coelogenys_) there are in each group three stout hairs alternating with three slender hairs. In some forms a number of hairs spring from a common point: in the Jerboa (_Dipus_) twelve or thirteen arise from a single hole; in _Ursus arctos_ there is the same general plan, but there is one stout hair and four or five slender ones. There are numerous other complications and modifications, but the facts, although interesting, do not appear to throw any light upon the mutual affinities of the animals. Allied forms may have a very different arrangement, while in forms which have no near relationship the plan may be very similar, as is shown by the examples cited from Dr. Meijerle's paper. The groups of hairs, moreover, have themselves a definite placing, which the same anatomist has compared with the disposition of the bundles of hairs behind and between the scales of the Armadillo, and which has led him to the view that the ancestors of mammals were scaly creatures--a view also supported by Professor Max Weber,[3]and not in itself unreasonable when we consider the numerous points of affinity between the primitive Mammalia and certain extinct forms of reptiles.[4]

The hairs are greatly modified in form in different mammals and in different parts of their bodies. It is very commonly the case that a soft under-fur can be distinguished from the longer and coa.r.s.er hairs, which to some extent hide the latter. Thus the "sealskin" of commerce is the under-fur of the _Otaria ursina_ of the North. The coa.r.s.er hairs may be further differentiated into bristles; these again into spines, such as those of the Hedgehog and of the Porcupine. Again, the flattening and agglutination of hairs seems to be responsible for the scales of the _Manis_ {10} and for the horns of the _Rhinoceros_. It is a matter of common knowledge that upon the head of various animals, _e.g._ the Domestic Cat, long and sensitive hairs are developed, which are connected with the terminations of nerves, and perform a sensory, probably tactile function.

These occur on the snout, above the eyes, and in the neighbourhood of the ears. It is an interesting fact that a tuft of quite similar hairs occurs on the hand of many mammals close to the wrist, which, at least in the case of _Ba.s.saricyon_, are connected with a strong branch from the arm-nerve.

These tufts also occur in Lemurs, in the Cat, various Rodents and Marsupials, and are probably quite general in mammals who "feel" with their fore-limbs;--in which, in fact, the fore-limbs are not exclusively running organs. That the last remaining hairs of the Cetacea are found upon the muzzle, is perhaps significant of the importance of these sensory bristles.

The entire absence of hairs is quite common in this order, although traces of them are sometimes found in the embryo. The Sirenia, too, are comparatively hairless, as are also many Ungulates. Whether the presence of blubber in the former case and the existence of a very thick skin in the latter animals are facts which have had anything to do with the disappearance of hair or not, is a matter for further inquiry.

The intimate structure of the hair varies considerably. The variations concern the form of the hair, which may be round in transverse section, or so oval as to appear quite flat when the hair is examined in its entirety.

The substance of the hair is made up of a central medulla or pith with a peripheral cortex; the latter is scaled, and the scales are often imbricated and with prominent edges. The amount of the two const.i.tuents also differs, and the cortex may be reduced to a series of bands surrounding only tracts of the enclosed pith. In the hair is contained the pigment to which the colour of mammals is chiefly due. Tracts of brightly-coloured skin may exist, as in the Apes of certain genera; but such structures are not general. The pigment of the hair seems to consist of those pigmentary substances known as melanins. It is remarkable to find such a uniform cause of coloration, when we consider the great variety of feather-pigments found in birds. The variations of colour of the hair of mammals are due to the unequal distribution of these brown pigments. There are very few mammals which can {11} be called brightly coloured. The Bats of the genus _Kerivoula_ have been compared to large b.u.t.terflies, and some of the Flying Squirrels have strongly-marked contrasts of reddish brown, white, and yellow. The same may be said of the spines of certain Porcupines. But we find in the hair no bright blues, greens, and reds such as are common among birds.

There are certain general facts about the coloration of mammals which require some notice here. Next to the usually sombre hues of these animals the general absence of secondary s.e.xual coloration is noteworthy. In but a few cases among the Lemurs and Bats do we find any marked divergences in hues between males and females. Secondary s.e.xual characters in mammals are, it is true, often exhibited by the great length of certain hair-tracts in the male, such as the mane of the Lion, the throat- and leg-tufts of the Barbary Sheep, and so forth; but apart from these, the secondary s.e.xual characters of mammals are chiefly shown in size, _e.g._ the Gorilla, or in the presence of tusks, _e.g._ various Boars, or of horns, as in the Deer, etc. The coloration of mammals frequently exhibits conspicuous patterns of marking. These are in the form of longitudinal stripes, of cross-stripes, or of spots; the latter may be "solid" spots, or broken up, as in the Leopard and Jaguar, into groups of smaller spots arranged in a rosette-fashion. We never find in mammals the complicated "eyes" and other markings which occur in so many birds and in other lower Vertebrates. It is important to note that in the Mammalia whose sense of sight is quite keen there should be a practical absence of secondary s.e.xual colours. As to the relationship of the various forms of marking that do occur, it seems clear that there has been a progression from a striped or spotted condition to uniform coloration. For we find that many Deer have spotted young; that the young Tapir of the New World is spotted, while its parents are uniform blackish brown; the strongly-marked spotting of the young Puma contrasts with the uniform brown of the adult; and the Lion cub, as every one knows, is also spotted, the adult lioness showing considerable traces of the spots.

The seasonal change in the colours of certain mammals is a subject upon which much has been written. The extreme of this is seen in those creatures, such as the Polar Hare and the Arctic Fox, which become entirely blanched in the winter, recovering {12} their darker coat in the spring.

This is, however, only an extreme case of a change which is general. Most animals get a thicker fur in winter and exchange it for a lighter one in summer. And the hues of the coat change in correspondence.

GLANDS OF THE SKIN.--The great variety of integumental glands possessed by the Mammalia distinguishes them from any group of lower Vertebrates. This variability, however, only concerns the anatomical structure of the glands in question. Histologically they are all of them apparently to be referred to one of two types, the sudoriparous or sweat gland and the sebaceous gland. Simple sweat and sebaceous glands are abundant in mammals, with but a few exceptions. The structures that we are now concerned with are agglomerations of these glands. The mammary glands will be treated of in connexion with the marsupium; they are either ma.s.ses of sweat glands, or of sebaceous glands whose secretion has been converted into milk.

Many Carnivora possess glands opening to the exterior, near the a.n.u.s, by a large orifice. These secrete various odoriferous substances, of which the well-known "civet" is an example. Other odoriferous glands are the musk glands of the Musk-deer and of the Beaver; the suborbital gland of many Antelopes; the dorsal gland of the Peccary, which has given the name of _Dicotyles_ to the genus on account of its resemblance in form to a navel.

This gland may be seen to secrete a clear watery fluid. The Elephant has a gland situated on the temple, which is said to secrete during certain periods only, and to be a warning to leave the animal alone. Very remarkable are the foot glands of certain species of _Rhinoceros_; they are not universally present in those animals, and are therefore useful as specific distinctions. On the back of the root of the tail in many Dogs are similar glands. The Gentle Lemur (_Hapalemur_) has a peculiar gland upon the arm, about the size of an almond, which in the male underlies a patch of spiny outgrowths. In _Lemur varius_ is a hard patch of black skin which may be the remnants of such a gland. It is thought that the callosities on the legs of Horses and a.s.ses are remnants of glands.

One of the most complex of these structures which has been examined microscopically exists in the Marsupial _Myrmecobius_.[5] On the skin of the anterior part of the chest, just in front of the {13} sternum, is a naked patch of skin which is seen to be perforated by numerous pores.

Besides the ordinary sebaceous and sweat glands there are a series of ma.s.ses of glands, opening by larger orifices, which present the appearance of groups of sebaceous glands, and are of a racemose character; but the existence of muscular fibres in their coats seems to show that they should be referred rather to the sudoriparous series. Beneath the integument is a large compound tubular gland quite half an inch in diameter.

In _Didelphys dimidiata_ there is a precisely similar glandular area and large underlying gland, the correspondence being remarkable in two Marsupials so distant in geographical position and affinities. Even among the Diprotodont genera there is something of the kind; for in _Dorcopsis luctuosa_ and _D. muelleri_ is a collection of four unusually large sebaceous follicles upon the throat, and in the Tree Kangaroo (_Dendrolagus bennettii_) there is the same collection of enlarged hair-follicles, though they are apparently somewhat reduced as compared with those of _Dorcopsis_.

These are of course a few examples out of many.