The Cambridge natural history, Vol. 10 (of 10)

THE

CAMBRIDGE NATURAL HISTORY

EDITED BY

S. F. Harmer and A. E. Shipley

VOLUME X

MAMMALIA

by F. BEDDARD

Reprint Edition

1958

© 1902, by Macmillan & Co., Limited

Authorized reprint by
Wheldon & Wesley, Ltd. and H. R. Engelmann (J. Cramer)

Printed in Germany

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 mass 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 space 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. 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 assisted 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.

CONTENTS

Scheme of the Classification Adopted In This Book

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 class 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 grass, 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 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 partition 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 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 class 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 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 classes 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."

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, 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 nictitating 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.

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 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 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 homologue 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 knob 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.