III. ORIGIN AND CLASSIFICATION OF THE MAMMALIA
Origin.—Although, as stated in the first chapter, the mammalian class, as at present known either by existing or extinct forms, is completely isolated from all other groups of the animal kingdom, yet it is impossible to refrain from speculating as to its origin and nearest affinities. In arranging the classes of vertebrates in a linear series it is customary to place them in the following order—Pisces, Amphibia, Reptilia, Aves, Mammalia,—an order which probably indicates the relative degree of elevation to which the most highly developed members of each class has attained. Such an arrangement appears to express the true relationship of the first four classes to one another, but it is quite clear that the Mammalia have no sort of affinity with the Aves. Writing in 1879, Professor Huxley[22] came to the conclusion that, in looking among vertebrates for the progenitors of the Mammalia, we must pass over all known forms of birds and reptiles, and go straight down to the Amphibia. In addition to the characters derived from the conformation of the pelvis upon which the argument was primarily based, the following reasons were given for this conclusion: “The Amphibia are the only air-breathing Vertebrata which, like mammals, have a dicondylian skull. It is only in them that the articular element of the mandibular arch remains cartilaginous, while the quadrate ossification is small, and the squamosal extends down over it to the osseous elements of the mandible, thus affording an easy transition to the mammalian condition of those parts. The pectoral arch [girdle] of the Monotremes is as much amphibian as it is sauropsidian; the carpus and the tarsus of all Sauropsida, except the Chelonia, are modified away from the Urodele type, while those of the mammal are directly reducible to it. Finally, the fact that in all Sauropsida it is a right aortic arch which is the main conduit of arterial blood leaving the heart, while in mammals it is a left aortic arch which performs this office, is a great stumbling-block in the way of the derivation of the Mammalia from any of the Sauropsida. But, if we suppose the earliest forms of both the Mammalia and the Sauropsida to have had a common Amphibian origin, there is no difficulty in the supposition that, from the first, it was a left aortic arch in the one series, and the corresponding right aortic arch in the other, which became the predominant feeder of the arterial system.” Subsequently Professor E. D. Cope[23] in a suggestive paper called attention to the remarkable resemblances to the Monotremes presented by the skeleton of that group of early secondary reptiles which he then designated the Theromorpha, but which may be included in the Anomodontia of Sir R. Owen, and came to the conclusion that in that group we have the true ancestors of the Mammalia. This conclusion was, however, disputed by Dr. Baur,[24] who considered that the Anomodontia were too specialised to have been the actual progenitors of the Mammalia, and that they should rather be regarded as a divergent branch of the stem which had given origin to the Mammalia. Since that date observations made on the structure of the South African Anomodonts have shown such an intimate connection between that group and the Labyrinthodont Amphibians, that there can be no hesitation in regarding the one as the direct descendant of the other; and we may probably regard the Mammalia as having originated from the same ancestral stock at the time the Amphibian type was passing into the Reptilian. From this point of view, some of the mammalian features found in the more specialised Anomodonts may probably be regarded as having been acquired during a parallel line of development.
Both the Anomodontia and the Mammalia differ from the Amphibians in the loss of the splint-like parasphenoid which underlies the basisphenoid axis of the skull, and by the ossification of that axis; but while the former have become monocondylic by the participation of the basioccipital in the support of the cranium, the latter retain the Amphibian dicondylic plan. The skull of the Anomodonts presents mammalian resemblances not found in any other Reptiles, this being especially noticeable in the region of the squamosal; and it is only in this group and mammals that the temporal or zygomatic arch is a squamoso-maxillary one (see p. 37). The resemblance between the pectoral and pelvic girdles of the Anomodonts and those of the Monotreme Mammals is noticed under the head of the latter, where reference is also made to the similarity in the structure of the humerus in the two groups. The pes of the Amphibia and Anomodontia agree in having a distinct intermedium, tibiale, fibulare, and centrale, whereas in other Reptiles these bones are not generally distinct; in Mammals the intermedium, fibulare, and centrale are distinct, and according to Cope’s interpretation there may be a distinct tibiale.
Classification.—In the present condition of the world, mammals have become so broken up into distinct groups by the extinction of intermediate forms, that a systematic classification is perfectly practicable. Most of the associations of species, which we call “orders,” and even the “suborders” and “families,” are natural groups. In isolating, defining, and naming them, we are really dealing with facts of nature of a totally different order from the artificial and fanciful divisions formed in the infancy of zoological science.
When, however, we pass to the extinct world, all is changed. In many cases the boundaries of our groups become enlarged until they touch those of others. New forms are discovered which cannot be placed within any of the existing divisions. As the horizon of our vision is thus expanded, the principles upon which a scheme of classification is constructed must be altogether changed. Our present divisions and terminology are no longer sufficient for the purpose; and some other method will have to be invented to show the complex relationships existing between different animal forms when viewed as a whole. The present time, pre-eminently distinguished by the rapidly changing and advancing knowledge of extinct forms, is scarcely one in which this can be done with any satisfactory result; so that all attempts to form a classification embracing even the already known extinct species must be only of a provisional and temporary nature.
In systematic descriptions in books, in lists, and catalogues, and in arranging collections, the objects dealt with must be placed in a single linear series. But by no means whatever can such a series be made to coincide with natural affinities. The artificial character of such an arrangement, the constant violation of all true relationships, are the more painfully evident the greater the knowledge of the real structure and affinities. But the necessity is obvious; and all that can be done is to make such an arrangement as little as possible discordant with facts.
The following table contains a list of the orders, suborders, and families of existing mammals as recognised by the authors, and placed in the order in which they will be treated of in this work. The more important of the groups containing only extinct forms are added in a different type, being interpolated, as near as may be, among those that appear to be their existing relatives.
A few explanatory remarks upon the mutual relations of some of the principal groups mentioned in the table may be useful here, but the subject will be more fully developed in treating separately of each division.
One of the most certain and fundamental points in the classification of the Mammalia is, that all the animals now composing the class can be grouped primarily into three natural divisions, which, presenting very marked differential characters, and having no existing, or yet certainly demonstrated extinct, intermediate, or transitional forms, may be considered as subclasses of equal value, taxonomically speaking, though very different in the numbers and importance of the animals at present composing them. These three groups are often called by the names originally proposed for them by Blainville—(1) Ornithodelphia, (2) Didelphia, (3) Monodelphia—the first being equivalent to the order Monotremata, the second to the Marsupialia, and the third including all the remaining members of the class. Although actual palæontological proof is wanting, there is much reason to believe that each of these, as now existing, are survivors of distinct branches to which the earliest forms of mammals have successively given rise, and for which hypothetical branches Professor Huxley has proposed the names of Prototheria, Metatheria, and Eutheria, names which, being far less open to objection than those of Blainville, are here used as equivalents of the latter.
The only known existing PROTOTHERIA, although agreeing in many important characters, evidently represent two very divergent stocks, perhaps as far removed as are the members of some of the accepted orders of the Eutheria. It would, however, be merely encumbering zoological science with new names to give them any other than the ordinarily known family designations of Ornithorhynchidæ and Echidnidæ.
Similarly with regard to the METATHERIA, although the great diversity in external form, in anatomical characters, and in mode of life of the various animals of this section might lead to their division into groups equivalent to the orders of the Eutheria, we do not think it advisable to depart from the usual custom of treating them all as forming one order, called Marsupialia, the limits of which are equivalent to those of the subclass. The characters of the six families which compose the group are extremely well marked and easily defined; and since they form a regular gradation between two extreme types, they can be satisfactorily arranged in a serial order. A marked distinction in the dentition enables us to divide them into primary groups or suborders.
The remaining mammals are included in the EUTHERIA, PLACENTALIA, or MONODELPHIA. Their affinities with one another are so complex that it is impossible to arrange them serially with any regard to natural affinities. Indeed each order is now so isolated that it is almost impossible to say what its affinities are; and none of the hitherto proposed associations of the orders into larger groups stand the test of critical investigation. All serial arrangements of the orders are therefore perfectly arbitrary; and although it would be of very great convenience for reference in books and museums if some general sequence, such as that here proposed, were generally adopted, such a result can scarcely be expected, since equally good reasons might be given for almost any other combination of the various elements of which the series is composed. In fact, we have already seen reason to depart in some respects from that used in the “Encyclopædia.”
The Edentata, Sirenia, and Cetacea stand apart from all the rest in the fact that their dentition does not conform to the general heterodont, diphyodont type to which that of all other Eutheria can be reduced, and which is such a close bond of union between them. In all three orders, however, some indications may be traced of relationship, however distant, with the general type.
With regard to the Edentata, reasons will be given for believing that both the Sloths and Anteaters are nearly related, and that the Armadillos, though much modified, belong to the same stock, but that the Pangolins and the Aard-varks represent very isolated forms.
There is no difficulty about the limits of the order Sirenia, comprising aquatic, vegetable-eating animals, with complete absence of hind limbs, and low cerebral organisation, represented in our present state of knowledge only by two existing genera, Halicore and Manatus, and a few extinct forms, which, though approaching a more generalised mammalian type, show no special characters allying them to any of the other orders. The few facts as yet collected relating to the former history of the Sirenia leave us as much in the dark as to the origin and affinities of this peculiar group of animals as we were when we only knew the living members. They lend no countenance to their association with the Cetacea; and, on the other hand, their supposed affinity with the Ungulata receives no very material support from them.
Another equally well-marked and equally isolated, though far more numerously represented and diversified order, is that of the Cetacea, placed simply for convenience next to the Sirenia; with which, except in their fish-like adaptation to aquatic life, they have little in common. The old association of these orders in one group can only be maintained either in ignorance of their structure or in an avowedly artificial system. Among the existing members of the order, there are two very distinct types, the toothed Whales or Odontoceti, and the Baleen Whales or Mystacoceti, which present as many marked distinguishing structural characters as are found between many other divisions of the Mammalia usually reckoned as orders. Since the extinct Zeuglodonts, so far as their characters are known, do not fall into either of these groups, but are in some respects annectant forms, we have placed them provisionally, at least, in a third group by themselves, named Archæoceti. There is nothing known at present to connect the Cetacea with any other order of Mammals; but it is quite as likely that they are offsets of a primitive Ungulate as of a Carnivorous type, or perhaps of a still more generalised mammalian stock.
The remaining Eutherian mammals are clearly united by the characters of their teeth, being all heterodont and diphyodont, with their dental system reducible to a common formula.
Although older views of the relationship of Ungulate mammals expressed by the terms Pachydermata, Ruminantia, and so forth, still linger in some corners of zoological literature, no single point in zoological classification can be considered so firmly established as the distinction between the Perissodactyle and Artiodactyle Ungulates; both being in the existing fauna of the world perfectly natural and distinctly circumscribed groups. The breaking-up of the latter into four equivalent sections, the Pecora, Tylopoda, Tragulina, and Suina, is equally in accordance with all known facts. Less certain, however, is the association of the Proboscidea and the Hyracoidea with the true Ungulates. By many zoologists they are each, although containing so very few existing species, made into distinct orders; and much is to be said in favour of this view. The discovery, however, of a vast number of extinct species of Ungulates which cannot be brought under the definition of either Perissodactyla or Artiodactyla, and yet are evidently allied to both, and to a certain extent bridge over the interval between them and the isolated groups just mentioned, make it necessary either to introduce a number of new and ill-defined ordinal divisions, or so to widen the scope of the original order as to embrace them all, considering the Elephants and the Hyraces as representing suborders equivalent to the great Perissodactyle and Artiodactyle groups. It is the latter alternative that we have adopted.
The Rodentia, although generally presenting a low grade of development, are a very specialised and distinct group. The position here assigned to them would accord with apparent relationships with the Ungulates, through the Elephant on the one hand and the extinct Typotherium on the other.
In the present state of the fauna of the earth, the Carnivora form a very distinct order, though naturally subdivided into two groups, the members of the one being more typical, while those of the other (the Pinnipedia) are aberrant, having the whole of their organisation specially modified for living habitually in the water.
The Insectivora comprise various lowly organised and generalised forms, exhibiting considerable divergence of character, and apparently connected through transitional extinct species with the Carnivora. As no other order can claim the family Galeopithecidæ, it is placed here, but rather for convenience than for any other consideration, since it has but little if any relationship with any of the other members. Its isolated position is indicated by assigning it a distinct subordinal rank.
The Chiroptera have always been placed near the Insectivora; but they are really a highly specialised group, as much isolated from all other mammals by the modification of their anterior limbs in adaptation to aerial locomotion, as the Cetacea and the Sirenia, by the absence of hind limbs, are specially adapted for an aquatic life.
Lastly, the Primates, which in any natural system must be placed at the head of the series, are divisible into two very distinct groups—one containing the various forms of Lemurs (Lemuroidea), and the other the Monkeys and Man (Anthropoidea). Whether the Lemuroidea should form part of the Primates (according to the traditional view), or a distinct order altogether removed from it, is as yet an undetermined question, for both sides of which there is much to be said. There can, however, be no doubt that the Anthropoidea form a perfectly natural group, presenting a series of tolerably regular gradations from the Marmosets (Hapale) to Man. Certain breaks in the series, however, enable us to divide it into five distinct families:—Hapalidæ or Marmosets: Cebidæ or American Monkeys, with three premolar teeth on each side of each jaw; Cercopithecidæ, containing the majority of Old-world Monkeys; Simiidæ, consisting of the genera Hylobates, Simia, Gorilla, and Anthropopithecus, the true Man-like Apes; and, lastly, Hominidæ, containing the genus Homo alone.
Subclass I. PROTOTHERIA.
Order i. MONOTREMATA—Monotremes.
Fam. 1. Ornithorhynchidæ—Duck-bill. 2. Echidnidæ—Spiny Anteater.
Group. =MULTITUBERCULATA.=[25]
Fam. 1. =Plagiaulacidæ=—Plagiaulax. 2. =Polymastodontidæ=—Polymastodon. 3. =Tritylodontidæ=—Tritylodon.
Subclass II. METATHERIA.
Order ii. MARSUPIALIA—Marsupials.
Suborder 1. POLYPROTODONTIA—Polyprotodonts.
Fam. 1. =Dromatheriidæ=—Dromatherium. 2. =Amphitheriidæ=—Amphitherium, etc. 3. =Spalacotheriidæ=—Spalacotherium. 4. =Tritylodontidæ=—Tritylodon. 5. Didelphyidæ—Opossums. 6. Dasyuridæ—Thylacine and Dasyures. 7. Peramelidæ—Bandicoots.
Suborder 2. DIPROTODONTIA—Diprotodonts.
Fam. 8. Phascolomyidæ—Wombats. 9. Phalangeridæ—Phalangers. 10. =Diprotodontidæ=—Diprotodon. 11. =Nototheriidæ=—Notothere. 12. Macropodidæ—Kangaroos.
Subclass III. EUTHERIA.
Order iii. EDENTATA—Edentates.
Fam. 1. Bradypodidæ—Sloths. 2. =Megatheriidæ=—Ground Sloths. 3. Myrmecophagidæ—Anteaters. 4. Dasypodidæ—Armadillos. 5. =Glyptodontidæ=—Glyptodonts. 6. Manidæ—Pangolins. 7. Orycteropodidæ—Aard-varks.
Order iv. SIRENIA—Sirenians.
Fam. 1. Manatidæ—Manatees. 2. =Rhytinidæ=—Rhytina. 3. Halicoridæ—Dugongs. 4. =Halitheriidæ=—Halithere.
Order v. CETACEA—Cetaceans.
Suborder 1. MYSTACOCETI—Baleen Whales.
Fam. 1. Balænidæ—Greenland Whale, etc.
Suborder 2. =ARCHÆOCETI.=
Fam. 2. =Zeuglodontidæ=—Zeuglodonts.
Suborder 3. ODONTOCETI—Toothed Whales.
Fam. 3. Physeteridæ—Sperm Whale. 4. Platanistidæ—Freshwater Dolphins. 5. Delphinidæ—Dolphins, Porpoises, etc.
Order vi. UNGULATA—Hoofed Mammals.
Suborder 1. ARTIODACTYLA—Artiodactyles.
Section A. SUINA—Pig-like Artiodactyles.
Fam 1. Hippopotamidæ—Hippopotamus. 2. Suidæ—Pigs and Peccaries.
Annectant types.
{ 3. =Chœropotamidæ=—Chœropotamus. { 4. =Anthracotheriidæ=—Anthracothere. { 5. =Merycopotamidæ=—Merycopotamus. { 6. =Cotylopidæ=—Oreodonts. { 7. =Anoplotheriidæ=—Anoplothere. { 8. =Dichodontidæ=—Dichodon.
Section B. TRAGULINA—Chevrotains.
9. Tragulidæ—Chevrotains.
Section C. TYLOPODA—Camels.
10. Camelidæ—Camels and Llamas. 11. =Poebrotheriidæ=—Poëbrotherium.
Section D. PECORA—True Ruminants.
12. Cervidæ—Deer. 13. Giraffidæ—Giraffe. 14. Antilocapridæ—Prong-buck. 15. Bovidæ—Sheep, Cattle, etc.
Suborder 2. PERISSODACTYLA—Perissodactyles.
Fam. 16. Tapiridæ—Tapirs. 17. =Lophiodontidæ=—Lophiodonts. 18. =Palæotheriidæ=—Palæotheres. 19. Equidæ—Horses. 20. Rhinocerotidæ—Rhinoceroses. 21. =Lambdotheriidæ=—Palæosyops. 22. =Chalicotheriidæ=—Chalicothere. 23. =Titanotheriidæ=—Titanothere. 24. =Macraucheniidæ=—Macrauchenia.
Suborder 3. =TOXODONTIA=—Toxodonts.
Fam. 25. =Toxodontidæ=—Toxodon. 26. =Typotheriidæ=—Typothere.
Suborder 4. =CONDYLARTHRA.=
Fam. 27. =Periptychidæ=—Periptychus. 28. =Phenacodontidæ=—Phenacodus. 29. =Meniscotheriidæ=—Meniscothere.
Suborder 5. HYRACOIDEA—Hyraces.
Fam. 30. Hyracidæ—Hyrax.
Suborder 6. =AMBLYPODA.=
Fam. 31. =Pantolambdidæ=—Pantolambda. 32. =Coryphodontidæ=—Coryphodon. 33. =Uintatheriidæ=—Uintathere.
Suborder 7. PROBOSCIDEA—Proboscideans.
Fam. 34. =Dinotheriidæ=—Dinothere. 35. Elephantidæ—Elephants.
Group. =TILLODONTIA=—Tillodonts.
Fam. =Anchippodontidæ=—Anchippodus. =Calamodontidæ=—Calamodon.
Order vii. RODENTIA—Rodents.
Suborder 1. SIMPLICIDENTATA.
Fam. 1. Anomaluridæ—Anomalurus. 2. Sciuridæ—Squirrels and Marmots. 3. Haplodontidæ—Haplodon. 4. =Ischyromyidæ=—Ischyromys. 5. Castoridæ—Beavers. 6. Myoxidæ—Dormice. 7. Lophiomyidæ—Lophiomys. 8. Muridæ—Rats, Mice, and Voles. 9. Spalacidæ—Mole-rats. 10. Geomyidæ—Pouched Rats. 11. Dipodidæ—Jerboas. 12. =Theridomyidæ=—Theridomys. 13. Octodontidæ—Spiny Mice. 14. =Castoroididæ=—Castoroides. 15. Hystricidæ—Porcupines. 16. Chinchillidæ—Chinchillas. 17. Dinomyidæ—Dinomys. 18. Caviidæ—Cavies. 19. Dasyproctidæ—Agouties.
Suborder 2. DUPLICIDENTATA.
Fam. 20. Lagomyidæ—Picas. 21. Leporidæ—Hares and Rabbits.
Order viii. CARNIVORA—Carnivores.
Suborder 1. CARNIVORA VERA—Fissipedes.
Fam. 1. Felidæ—Cats. 2. Hyænidæ—Hyænas. 3. Proteleidæ—Earth-wolf. 4. Viverridæ—Civets and Ichneumons. 5. Canidæ—Wolves and Foxes. 6. Ursidæ—Bears. 7. Mustelidæ—Weasels and Otters. 8. Procyonidæ—Raccoons and Cat-bear.
Suborder 2. PINNIPEDIA—Pinnipedes.
Fam. 9. Otariidæ—Eared Seals. 10. Trichechidæ—Walrus. 11. Phocidæ—Seals.
Suborder 3. =CREODONTA=—Creodonts.
Fam. 12. =Hyænodontidæ=—Hyænodon. 13. =Proviverridæ=—Proviverra. 14. =Arctocyonidæ=—Arctocyon. 15. =Mesonychidæ=—Mesonyx.
Order ix. INSECTIVORA—Insectivores.
Suborder 1. INSECTIVORA VERA.
Fam. 1. Tupaiidæ—Tupaias. 2. Macroscelididæ—Elephant-Shrews. 3. Erinaceidæ—Hedgehogs. 4. Soricidæ—Shrews. 5. Talpidæ—Moles. 6. Potamogalidæ—Potamogale. 7. Solenodontidæ—Solenodon. 8. Centetidæ—Centetes. 9. Chrysochloridæ—Golden Moles.
Suborder 2. DERMOPTERA.
Fam. 10. Galeopithecidæ—Galeopithecus.
Order x. CHIROPTERA—Bats.
Suborder 1. MEGACHIROPTERA—Frugivorous Bats.
Fam. 1. Pteropodidæ—Flying Foxes.
Suborder 2. MICROCHIROPTERA—Insectivorous Bats.
Fam. 2. Vespertilionidæ—Common Bats. 3. Nycteridæ—Nycteris. 4. Rhinolophidæ—Leaf-nosed Bats. 5. Emballonuridæ—Emballonura. 6. Phyllostomatidæ—Vampyres.
Order xi. PRIMATES.
Suborder 1. LEMUROIDEA—Lemuroids.
Fam. 1. =Hyopsodontidæ=—Hyopsodus. 2. Chiromyidæ—Aye-Aye. 3. Tarsiidæ—Tarsier. 4. Lemuridæ—Lemurs.
Suborder 2. ANTHROPOIDEA—Anthropoids.
Fam. 5. Hapalidæ—Marmosets. 6. Cebidæ—American Monkeys. 7. Cercopithecidæ—Old World Monkeys. 8. Simiidæ—Gibbons and Man-like Apes. 9. Hominidæ—Man.
The distinctive character of these subclasses and orders, with an account of their subdivisions and the principal forms contained in each, will be given in subsequent chapters.
IV. GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION
I. GEOGRAPHICAL DISTRIBUTION.[26]
In considering the present distribution of mammals over the globe, we may, in the first place, direct our attention to terrestrial or land types, reserving the consideration of aerial types, like the Bats, and aquatic forms, as exemplified by the Cetaceans, Sirenians, and Seals, to separate sections.
Among terrestrial forms each species has a certain definite area of distribution in space, which may be of very wide extent, or may be confined to a restricted region. This distributional area is, however, always connected, or continuous; that is to say, that although we may have a single species inhabiting two continents, like the Lion in Asia and Africa, or dwelling both on a continent and adjacent continental islands, like the Javan Rhinoceros of India, Java, and Borneo, yet we shall always find that such areas, if not still connected, show evident signs of having been so connected in comparatively late geological epochs; and we never find instances of the same species inhabiting totally disconnected areas, such as India and South America. As examples of mammals with a wide distribution we may mention the Lion and the Leopard, which are now found throughout Africa, and also occur in India, as well as in the intervening areas of Arabia and Persia. In the case of the former species, palæontology further teaches us that its distribution in the last geological epoch was even more extensive, since we have good evidence to show that it formerly ranged over the greater part of Europe, including the British Isles. The Jackal affords another well-known instance of a species common to India and Africa. The American Puma, again, may be cited as an example of a mammal having a very wide range in latitude, since it is found from Patagonia in the south to Canada in the north. As instances of wide range in the opposite direction we have only to mention the Reindeer and the Elk or Moose, found in the northern regions of both the Old and New Worlds, which are only separated from one another by the narrow channel of Behring Strait.
Of mammals with extremely restricted distributional areas, we may mention many of the Insectivora, such as the Desman of the Pyrenees, and some of the Madagascar types of this order, the Lemurs from the same island, some of the species of Marmots, the remarkable bear-like Æluropus of Eastern Tibet, one species of Zebra, and other Ungulates from Africa.
The distribution of a genus (except of course when the genus is represented only by a single form) is very generally more extensive than that of a species; and this may be markedly the case when there are only some two or three species in a genus. In genera, moreover, we meet with what is known as discontinuous distribution, that is, where the distributional area of one or more species is totally separated from that of others. The best instance of this occurs in the case of the Tapirs, where we find one species inhabiting the Malayan Peninsula, and no others anywhere in the world, with the exception of South America. The explanation of such an apparently anomalous feature in distribution is to be found in the past history of the globe, which shows us that Tapirs once existed in China, Europe, and North America, and, therefore, indicates that the existing isolated species are the sole survivors of a group once spread over a large portion of the earth’s surface. In regard to generic distribution it must, however, be mentioned that this depends to a great extent on the limits which we are disposed to assign to genera themselves.
As the distributional area of a genus generally exceeds that of a species, so that of a family, or group of genera, is larger than that of a single genus; and similarly the distribution of an order, or assemblage of families, usually occupies a larger area than that of a single family. Thus, for instance, the genus Thylacinus, represented only by the so-called Tasmanian Wolf or Thylacine, is now entirely restricted to Tasmania; but the family Dasyuridæ, to which that genus belongs, ranges all over Australia, while the order Marsupialia, which includes the Dasyuridæ, is found both in Australia and America, and in past epochs was probably spread over the entire globe.
A remarkable feature in connection with the distribution of the terrestrial Mammalia is the circumstance that, with the exception of certain species introduced by human agency, and small forms which can easily have been transported on floating timber or other similar means, they are totally absent from what are known as oceanic islands—that is islands arising from great depths in the ocean, mainly composed of coral or volcanic rocks, and showing no signs of having ever been connected with the existing continents, or the larger and so-called continental islands. The obvious explanation of this feature is, that from their total isolation these islands have never been able to receive a mammalian fauna from the great continental areas on which mammalian life was probably first developed.
As an intermediate step between these islands which are practically void of mammalian life and the continents which teem with such a variety of forms, are certain larger islands and portions of continents containing a mammalian fauna more or less markedly distinct from that of the whole of the other regions of the globe. The best instance of this is Australia, which, with the exception of one dog—the Dingo—and certain Muridæ and Bats, has no mammals except Monotremes and Marsupials. The latter are, moreover, perfectly distinct from those of America, which, if we exclude the islands in the neighbourhood of Australia, is the only other region which now possesses any Marsupials at all. Here also we have a ready and full explanation which accords with all the facts; since it is evident that Australia has been isolated from the Asiatic continent from some very remote geological epoch, at which period it is probable that Monotremes and Marsupials were the dominant if not the sole representatives of the Mammalia then existing. Consequently Australia has never been able to receive an influx of the Eutherian orders, which have probably swept away all the Marsupials except the small American Opossums from the rest of the globe. Again, the large island of Madagascar, which has a fauna of an African type, but still very markedly different from that of the mainland, may be considered to have been connected with the latter at a time when the Eutheria had become the dominant forms, but has been separated for a sufficiently long period to have enabled a large number of its species and genera to have become distinct from those of the adjacent continent. Similarly, there is evidence to show that South America was probably cut off for a considerable period from the northern half of the American continent, in consequence of which its lowly organised fauna of Edentates were enabled to attain such a remarkable development in the later geological periods.
In contrast to the mammalian fauna of islands of the preceding type is, or rather was, that of the British Islands, which in the early historic and prehistoric periods was identical with that of the Continent. This leads to the inference that at a comparatively late epoch there was a direct land communication between Britain and the Continent, which is shown by geological evidence to have actually been the case.
The above instances are sufficient to show what an important influence the date of separation of islands from the adjacent continents has had upon their existing mammalian fauna, and how largely the present distribution of mammalian life is bound up with the past history of our globe. We must, however, not omit to mention another very important agency of past times which has likewise had great influence on the present distribution of the various faunas of the northern hemisphere. This is the so-called glacial epoch, which took place immediately before the establishment of the present condition of things, and appears to have been the cause of the extinction of many of the larger mammalian types which formerly inhabited Europe, and whose retreat to the warmer regions of the south was apparently cut off by the Mediterranean.
Zoological Regions.—Zoologists are now generally agreed in dividing the land surfaces of the globe into a number of zoological regions or provinces, characterised by a more or less distinctly marked general facies of their fauna as a whole. Some of these regions are much more distinctly defined than the others; and in the majority of cases there is a kind of neutral ground or No-man’s-land at the junction between any two of these regions. It must also be remembered that in the Old World proper as we go back in time we find a gradual assimilation in the mammalian faunas of the different regions, indicating that originally there was one large fauna of a generally similar type occupying the greater portion of this area. Thus we find that Hippopotami, Giraffes, Kudus, Elands, and other types of Antelopes now restricted to Africa, formerly extended to Europe and India, while there is also evidence to show that the group of large anthropoid Apes, now found only in Africa and the Bornean region, were likewise spread over a large part of the south-western half of the Old World. Moreover, while at the present day there is a marked connection between the mammals of the northern regions of both the Old and New Worlds, in the Tertiary period it appears that the fauna of the whole of North America was much more nearly allied to that of the central regions of the Old World than is now the case. Thus in the Tertiary rocks of America we meet with remains of what we are accustomed to regard as such essentially Old World genera as Horses and Rhinoceroses. On the other hand there are no traces in America of the existence at any period of Apes, Giraffes, Hippopotami, or Hyænas, while that continent has yielded evidence of groups of Ungulates totally unrepresented in the eastern hemisphere.
The chief zoological regions of the globe, proposed by Mr. Sclater in 1857, and now recognised by the majority of authorities, are six in number, and are named as follows. Firstly, the Palæarctic region, embracing the whole of Europe, Persia, Northern Arabia, and all of Asia northward of the line of the Himalaya proper, Japan, that part of Africa lying northward of the Sahara Desert, and the oceanic islands of the North Atlantic. Secondly, the Ethiopian region, which comprises all Africa lying to the south of the Sahara, the southern part of Arabia, Madagascar, and the Mascarene Islands. Thirdly, the Oriental or Indian region, which is taken to include India south of the Himalaya, and to the north-west as far as Beluchistan, the Malay peninsula, southern China, Sumatra, Java, Borneo, and the Philippines. Fourthly, the Australasian region, which is usually defined as being bounded to the north-west by the deep sea channel lying between Borneo and Celebes known as Wallace’s line, and is taken to include Celebes, Lumbok, New Guinea, Australia, Tasmania, New Zealand, and the host of oceanic islands in the South Pacific. Several writers, however, prefer to regard Celebes and some of the adjacent islands as representing a transitional Austro-Malayan region. Fifthly, the Nearctic region, comprising Greenland and North America as far south as the north of Mexico. And sixthly, the Neotropical region, which embraces the remaining portion of the American continent and the West Indies.
Various minor modifications of this scheme have been proposed. Thus some writers are disposed to raise India to the rank of a distinct primary region, while others propose the same for New Zealand. The Palæarctic and Nearctic regions have a large number of common types, more especially among the mammals, and Dr. A. Heilprin[27] has expressed his opinion that they should be regarded as a single primary region under the name of the Holarctic. The same writer would also separate the South Pacific Islands as constituting a Polynesian region.
Minor divisions or sub-regions have also been marked out, but it will be unnecessary to indicate their limits in the present work. We may, however, mention the Mediterranean sub-region of the Palæarctic, which includes the peninsular portion of southern Europe, North Africa, Asia Minor, Persia, Afghanistan, Beluchistan, and Northern Arabia, as a good instance of the transition from one region to another, since its fauna has a mingling of Palæarctic, Ethiopian, and Oriental types, the former being, however, the predominant ones.
Of the chief mammalian types characteristic of these various regions only a brief sketch can be given in this work.
Palæarctic Region.—The Palæarctic region is of enormous extent, and includes countries varying greatly in their flora, climate, and elevation. Thus it embraces the Arctic plains of Siberia, the warm regions of Italy, Southern France, and Northern Africa, the forest-clad slopes of the outer Himalaya, and the lofty arid plains of Turkestan and Tibet, scorched by a burning sun in summer and chilled by a still more terrible cold in winter. Its extreme limits in the west are marked by the Canaries and Azores, and in the east by distant Japan; and yet throughout this vast expanse we find a great uniformity of life, as exemplified by the large number of British genera which occur also in Japan. The mammals which are on the whole the most characteristic of this region are the Sheep and Goats, forming a section of the great family of Bovidæ; nearly all the species of which are Palæarctic, although we meet with one Goat (Capra) in the Nilgherries of Southern India, and a Sheep (Ovis) in the Nearctic region. The Musk Ox (Ovibos) is characteristic of the Palæarctic and Nearctic regions. At least one species of Camel is characteristic of this region, and it is not improbable that the second may also have originated in it. There are a few characteristic types of Antelopes, such as the Alpine Chamois (Rupicapra), the Saiga of Tartary, and the Chiru (Pantholops) of Tibet, each of which is represented by only a single species: and we miss the host of Antelopes so characteristic of the Ethiopian region. Deer (Cervus) are abundant, although by no means confined to this region; and the Musk Deer (Moschus), the sole representative of the subfamily Moschinæ, is exclusively Palæarctic. Monkeys, as a rule, are absent, although we meet with one species of Macacus in Northern Africa and at Gibraltar, and some other types on the southern border of Tibet. The Moles (Talpa) are mainly Palæarctic, although one species enters Northern India, while the Desmans (Myogale) of the Pyrenees and Southern Russia are unknown beyond the limits of this region. The Water-shrew (Nectogale) is likewise a peculiar eastern Palæarctic type. Among the Rodents, the Picas or Tailless Hares (Lagomys) and the Dormice (Myoxus) are essentially Palæarctic forms, only one species of each being found beyond the limits of the region, and the one extra-Palæarctic species of Lagomys occurring in the cognate Nearctic region. The Mice and Rats are represented by the typical genus Mus and other types, and Hares (Lepus) and one species of Squirrel (Sciurus) are common. The Carnivora include two species of Bears (Ursus), Wolves and Foxes (Canis), a Lynx and a few species of Cats (Felis), as well as numerous weasels (Mustela), and some other types.
Ethiopian Region.—The Ethiopian region is of great interest to the student of mammals, since it is inhabited by a number of forms remarkable for their large size. A considerable portion of the area consists of desert, especially in the north; but there is also a wide extent of grassy plains (veltd), as well as vast tracts of equatorial forests of great density. Perhaps the most striking feature in the Ethiopian fauna is the number of Ungulates, both of the Artiodactyle and Perissodactyle sections. In the former section we have the Giraffes (Giraffa) represented by one species, which is the type of a family, and is unknown elsewhere. Equally characteristic are the Hippopotami, which likewise form the type of a family, while the Pigs are represented by the Wart-hogs (Phacochœrus) and the River-hogs, forming an aberrant group of the genus Sus. The Oxen (Bos) are represented by Buffaloes, but there are no species of true Oxen or Bison. The Antelopes attain an extraordinary development, the number of species being estimated at from eighty to ninety, which are referred to a large number of genera, although several of these are more or less ill-defined. Most of these genera are peculiar to this region, but the Gazelles (Gazella) are also found in the desert regions of other parts of the Old World, and Oryx ranges into Arabia and Persia. In contrast to this abundance of Antelopes is the total absence of the Deer family, or Cervidæ, which are so characteristic of the Palæarctic and Oriental regions. The Chevrotains or Tragulidæ are, however, represented by Dorcatherium.[28] In the Perissodactyle section we may notice the presence of two species of Rhinoceros, both furnished with two horns, and distinguished from those of the Oriental region by the absence of incisor and canine teeth. The Horse family (Equidæ) is also represented by several species, and includes the peculiar group of Zebras, characterised by their beautifully striped skins. Of other Ungulates the Elephants, which, like the Rhinoceroses, are now peculiar to the Ethiopian and Oriental regions, have one species, which is widely different from its Indian congener. The Hyraces are mainly characteristic of this region, although one species occurs in Syria and Palestine. The Carnivora include some forms like the Lion, Leopard, and Jackal, common to the Oriental region, but likewise include certain peculiar types like the Earth-wolf (Proteles), which may be regarded as the type of a distinct family, and two species Hyænas, which are referred by some authorities to a distinct genus (Crocuta). There is also the Hunting dog (Lycaon), and the peculiar group of Foxes known as the Fennecs, together with Otocyon. Bears, Wolves, and true Foxes are absent; but Civets, etc., are abundant, although not characteristic of the region. The Primates yield several very characteristic types, such as the Gorilla and the Chimpanzee (Anthropopithecus) among the Simiidæ, which, with the exception of the Orangs of Borneo, are the only existing large man-like Apes, and the group of Dog-faced Baboons (Cynocephalus) in the Cercopithecidæ. The genus Colobus is also a group of the latter family, absolutely characteristic of the region. Lemurs, again, occur on the continent of Africa, but the great development of this group is in the adjacent island of Madagascar, where several peculiar genera occur, and where the larger Carnivora and Ungulata are absent. These peculiarities of the fauna of Madagascar apparently point, as previously mentioned, to its separation from the mainland before the latter was overrun by the larger types, and at a time when its chief mammals were Lemurs and Insectivores. There are two genera of Edentates, the Pangolins (Manis), and the Aard-vark (Orycteropus), the latter being peculiar.
Although the foregoing groups of mammals are now so characteristic of the Ethiopian region, it cannot be too strongly insisted that their restriction to this region is, so to speak, merely a feature of the present day, and that at a late geological epoch nearly all the peculiar genera were represented in India, and many of them also in Europe.
Oriental Region.—The third or Oriental region is likewise of very considerable extent, and is the only one, in addition to the Ethiopian, which is the home of huge Ungulates, like Elephants and Rhinoceroses, and the large man-like Apes. A large proportion of this extensive area is occupied by tropical and subtropical forests and swamps; these being especially abundant in Burma, Southern China, Siam, and the southern ridges of the Himalaya, collectively constituting the Indo-Chinese sub-region, and also in the Indo-Malayan sub-region of the Malay peninsula and adjacent islands. In the third or Indian sub-region, comprising peninsular India, with the exception of the Carnatic, there are large tracts of open country, including some of the hottest regions in the world, parts of which form plains more or less covered with vegetation during the cooler and rainy seasons, while others are barren rocky table-lands, as in the Deccan, or arid deserts like those of parts of the Punjab and Sind. Finally, in the fourth or Cingalese sub-region, represented by the Carnatic and the island of Ceylon, we find vast areas of luxuriant forest and jungle. In the north-western desert area of the Indian sub-region the fauna includes a mixture of Palæarctic and Ethiopian forms, with those characteristic of the Oriental region.
Among the chief features of the mammalian fauna of this region we may notice the absence of Hippopotami and Giraffes, the greatly diminished number of Antelopes, as compared with those of Africa, and the abundance of Deer and true Pigs. The Antelopes comprise the two peculiar genera Boselaphus (Nilghai) and the typical Antilope (Black-buck), each of which is represented by only a single species, while the Deer belong to the so-called Rusine group, which is markedly different from that to which the Palæarctic Red Deer belongs. True Chevrotains (Tragulus) are peculiar to this region. The Oxen include the true Buffalo, differing in many respects from the African species of the same group, and also certain species of true Oxen, such as the Gaour and Banting, belonging to the Bibovine group, which is confined to this region. In the Perissodactyla Horses (Equus) are represented only by a single species in the desert area of the Indian sub-region, while the two species of Rhinoceros differ from those of Africa in being furnished with canines and incisors. The Malayan Tapir is the only Old World species of its genus. The Indian Elephant differs, moreover, so markedly from its African ally that some writers regard the two as types of distinct genera. The Carnivora include the Lion, Leopard, Jackal, and Hunting-Leopard, which are common to Africa; but the Tiger is very characteristic of this region, although extending northwards into the Palæarctic. Civets are abundant, comprising some peculiar genera, of which it will suffice to mention the well known Paradoxurus. Wolves closely allied to the Palæarctic species occur in Northern India, and there are also Foxes related to the typical species. The Dog-like animals which hunt in packs, and are separated by some writers from Canis under the name of Cyon, occur in the present and the Palæarctic region. The striped Hyæna is the Indian representative of its genus. Ratels are common to this and the Ethiopian region, and constitute the genus Mellivora. The most striking feature in the Carnivorous fauna of this region, as distinguished from the Ethiopian, is, however, the presence of Bears, some of which belong to the typical genus Ursus, while one species is usually generically separated under the name of Melursus. Among the Rodents we may especially notice the abundance of the Muridæ and Sciuridæ. In the former family we have numbers of true Mice (Mus), and also the peculiar genus Nesocia (Bandicoot-Rat), while in the latter both the true Squirrels (Sciurus) and the Flying-Squirrels (Pteromys) attain great development. The genus (Pteromys) is, indeed, mainly characteristic of this region, although in Kashmir and Japan it enters the Palæarctic. The Bats are very numerous, being represented by all the families, with the exception of the Phyllostomatidæ, or Vampyres, of South America. Among the Insectivora the genera Tupaia and Galeopithecus (Flying Lemur) are peculiar to this region, although not found in India. Finally, in the Primates we have the genera Macacus and Semnopithecus very abundantly represented, although both also enter the Palæarctic region; but the Anthropoid types are confined to the south-eastern half of the region, and include the Orangs (Simia) of Borneo, and the smaller long-armed Gibbons (Hylobates), which are abundant in the Malay peninsula, both genera not being found beyond this region. The Lemurs are much less abundant than in the Ethiopian region, but they include the peculiar Tarsier of Sumatra, Borneo, and Celebes (Austro-Malayan region), which differs so markedly in dentition and structure of the feet from all other forms that it has been made the type of a separate family. The Edentates, so poorly represented in the Old World, include only Pangolins (Manis), which, as we have already seen, also occur in the Ethiopian region.
Australasian Region.—With the fourth or Australasian region we come to a mammalian fauna so peculiar that we have no difficulty whatever in defining it from all the other regions of the globe, although it should be observed that in the Austro-Malayan islands we have a partial mingling of the Australasian and Malayan faunas. If we exclude Celebes from this region we find that, with the exception of a Pig in New Guinea, of the Dingo in Australia, of numerous Mice and Rats (Muridæ), and Bats, there are no Eutherian mammals throughout the area. The mammals of this region are restricted to the Australian mainland, the island of Tasmania, New Guinea, and the Aru islands, the whole area of New Zealand having been totally devoid of mammalian life until introduced by man. The whole of the Monotremata, constituting the subclass Prototheria, and all the Marsupials, exclusive of the few outlying forms ranging into the transitional Austro-Malayan area, and with the exception of the American family of the Opossums (Didelphyidæ), are absolutely confined to this region.
Celebes.—The mammals of Celebes—the typical representative of the Austro-Malayan transitional region or sub-region—include the peculiar Ape known as Cynopithecus, Tarsius (also Oriental), the Anoa, and the single species of Babirusa. Several other types of placental mammals are found in this transitional area, while the Marsupials are represented by Phalanger and Petaurus.
Nearctic Region.—The two remaining regions we have to consider are comprised in the New World. The first of these is the Nearctic, which, as already mentioned, has a fauna showing such a strongly marked relationship to that of the Palæarctic region, that it has been proposed to unite the two regions. Among types common to these two regions we may mention closely allied species of true Deer (Cervus) as exemplified by the Red Deer and the Wapiti; the allied Bisons of the two regions; the Reindeer and Elk common to both; as well as nearly related, and in some cases identical, species of Cats, Lynxes, Bears, Wolves, Foxes, Beavers, Squirrels, Marmots, and Hares. The Glutton or Wolverene, and the Musk Ox is also common to the Arctic portions of the two regions. The Ungulates are very poorly represented, but we have, in addition to the forms already mentioned, one species of the Palæarctic genus Ovis, namely the Bighorn, and the Prong-buck (Antilocapra), which is quite peculiar. There are, however, no Perissodactyla. The Raccoons and Coatis (Procyonidæ) constitute a family represented out of the New World only by the aberrant Cat-Bear (Ælurus) of Nipal. The characteristic American feline known as the Puma extends over this region; but there are no Edentates, and the Marsupials are represented only by a single species of Opossum. Rodents are extremely numerous, and comprise several characteristic types, which alone would tell us what part of the globe we were visiting. The most distinctive are the Pouched Rats (Geomyidæ), and the Beaver-like rodents known as the Haplodontidæ. True Rats and Mice (Mus), which are represented throughout the Old World, are totally wanting in the New, where they are replaced by the Vesper-mice, which may be included in the European genus Cricetus, although often separated as Hesperomys. This feature alone would seem to justify the distinction of the Nearctic from the Palæarctic region. The Musquash (Fiber) is a genus of Nearctic rodents unknown in the Old World. Among other characteristic genera we may mention, in the Carnivora, the Skunk (Mephitis) and the American Badger (Taxidea). Primates are absent from the entire region.
Neotropical Region.—The last of the six main regions is the Neotropical, including Mexico, South America, and the West Indies. A very large extent of this area is occupied by forests, which are described as being denser and more luxuriant than those of any other part of the globe. Alternating with these forest areas are the vast grassy plains known in different regions as llanos, savannas, and pampas. The back-bone of the region is formed by the great chain of the Andes. Next to the Australasian, this region is perhaps better characterised by its mammalian fauna than any of the others. Commencing with the Ungulates, we find a total absence of Antelopes, Sheep, and Oxen, and also of all Perissodactyles except Tapirs. Deer are, however, represented, although by peculiar forms (Cariacus) unknown beyond the New World. The Peccaries (Dicotyles), which are often made the type of a distinct family, take the place of the Old World Pigs, while the Llamas and Alpacas (Auchenia) are the substitutes for the Palæarctic Camels. The Carnivora include several Cats (Felis), among which the Puma and the Jaguar are the most noticeable; and there are also Raccoons, Coatis, Foxes, and one species of Bear. Insectivora are totally wanting; but the Bats are characterised by the presence of the Vampyres (Phyllostomatidæ), which are almost restricted to this region. The Rodents likewise include three families unknown elsewhere, namely the Chinchillas and Viscacha (Chinchillidæ), the Agouties (Dasyproctidæ), and the Cavies (Caviidæ); while a large number of the Octodontidæ are Neotropical, all the other forms being Ethiopian. In the Primates, again, we have all the forms quite peculiar to this region, and constituting two families, viz. the Cebidæ or Prehensile-tailed Monkeys, and the Hapalidæ, or Marmosets, both of which differ decidedly in their dentition, as well as in other features, from the Old World Monkeys. Lemuroids are unknown. Perhaps, however, the mammals which may be considered as most characteristic of the Nearctic region are the numerous Edentates, which form three families, mostly confined to it. These comprise the Bradypodidæ or Sloths, which solely inhabit the forest region; the Myrmecophagidæ or Anteaters; and the Dasypodidæ or Armadillos, of which one species has crept northward as far as Texas. Almost equally characteristic are the numerous Opossums, the majority of which belong to the genus Didelphys. Finally, it should be observed that the West Indies are distinguished from the rest of the region by the absence of Primates, Carnivora, and Edentates.
Aquatic Mammals.—Many mammals grouped for the present purpose as terrestrial pass a great portion of their lives in brooks, lakes, or rivers, and, being dependent upon such waters for obtaining their subsistence, are necessarily confined to their vicinity; but the truly aquatic mammals, or those living constantly in the water, and unable to move their quarters from place to place by land, are the orders Cetacea and Sirenia, with which may also be grouped the Seals, forming the Pinniped division of the order Carnivora.
For the marine Cetacea, animals mostly of large size and endowed with powers of rapid locomotion, there are obviously no barriers to universal distribution over the surface of the earth covered by sea, except such as are interposed by uncongenial temperature or absence of suitable food. Nevertheless it was thought some years ago that the fact of a Whale or a Dolphin occurring in a sea distant from that in which it had usually been found was sufficient justification for considering it as a distinct species and imposing a new name upon it. There are now, however, so many cases known in which Cetaceans from the northern and southern seas, from the Atlantic and Pacific Oceans, present absolutely no distinguishing external or anatomical characters upon which specific determination can be based that the opposite view is gaining ground; and, since some species are undoubtedly very widely distributed, being in fact almost cosmopolitan, there seems little reason why many others should not be included in the same category. The evidence is satisfactory enough in those instances in which the intermediate regions are inhabited by the same forms;—the cases of “continuous areas” of distribution. In those in which the areas of distribution are apparently discontinuous, there may be more room for doubt; but it must not be forgotten that the negative evidence is here of much less value than in the case of land animals, since the existence of Cetaceans in any particular part of the ocean may be easily overlooked. The great Sperm Whale (Physeter macrocephalus) is known to be almost cosmopolitan, inhabiting or passing through all the tropical and temperate seas, although not found near either pole. At least three of the well-known species of Rorqual (Balænoptera) of the British coasts are represented in the North Pacific, on the South American shores, and near New Zealand, by species so closely allied that it is difficult to point out any valid distinctive characters, though it may perhaps be desirable to wait for a more exhaustive examination of a large series of individuals before absolutely pronouncing them to be specifically identical. There is nothing yet known by which we can separate the “Humpback Whales” (Megaptera) of Greenland, the Cape of Good Hope, and Japan. The same may be said of the common Dolphin of the European seas (Delphinus delphis) and the so-called D. bairdi of the North Pacific and D. forsteri of the Australian seas. The Pilot Whale (Globicephalus melas) and the Pseudorca of the North Atlantic and of New Zealand are also, so far as present knowledge enables us to judge, respectively alike. Many other similar cases might be given. Captain Maury collected much valuable evidence about the distribution of the larger Cetacea, and, finding Right Whales (Balæna) common in both northern and southern temperate seas, and absent in the intermediate region, laid down the axiom that “the torrid zone is to the Right Whale as a sea of fire, through which he cannot pass.” Hence all cetologists have assumed that the Right Whale of the North Atlantic (B. biscayensis), that of the South Seas (B. australis), and that of the North Pacific (B. japonica), are necessarily distinct species. The anatomical structure and external appearance of all are, however, so far as yet known, marvellously alike, and, unless some distinguishing characters can be pointed out, it seems scarcely justifiable to separate them from geographical position alone; as, though the tropical seas may be usually avoided by them, it does not seem impossible, or even improbable, that some individuals of animals of such size and rapid powers of swimming may have at some time traversed so small a space of ocean as that which divides the present habitual localities of these supposed distinct species. If identity or diversity of structural characters is not to be allowed as a test of species in these cases, as it is usually admitted to be in others, the study of their geographical distribution becomes an impossibility.
Although many species are thus apparently of such wide distribution, others are certainly restricted; thus the Arctic Right Whale (Balæna mysticetus) has been conclusively shown to be limited in its range to the region of the northern circumpolar ice, and no corresponding species has been met with in the southern hemisphere. In this case, not only temperature, but also the peculiarity of its 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 common Hyperoödon is restricted to the North Atlantic, never entering, so far as is yet known, the tropical seas. Other species are exclusively tropical or austral in their range. One of the true Whalebone Whales (Neobalæna marginata) has only been met with hitherto in the seas round Australia and New Zealand; and a large Ziphioid (Berardius arnouxi) only near the last-named islands.
The Cetacea are not limited to the ocean, or even to salt water, some entering large rivers for considerable distances, and others being exclusively fluviatile. One species of Platanista is 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 definite distinguishing anatomical characters; for those by which the supposed P. indi was formerly separated from P. gangetica have been shown by Anderson to be of no constant value. Orcella fluminalis appears to be limited to the Irawaddy river, and at least two distinct species of Dolphin belonging to different genera are found in the waters of the upper Amazon. A Neomeris has been found in the great Chinese river, the Yang-tsi-Kiang, nearly a thousand miles from the sea. It is remarkable, however, that none of the great lakes or inland seas of the world are, according to our present knowledge, inhabited by Cetaceans. A regular seasonal migration has been observed in many of the oceanic Cetacea, especially those inhabiting the North Atlantic, but further observations upon this subject are still much needed.
The great difference in the manner of life of the Sirenia, as compared with that of the Cetacea, causes a corresponding difference in their geographical distribution. Slow in their movements, and feeding exclusively upon vegetable substances, water-grasses, or fuci, the Sirenia are confined to rivers, estuaries, or coasts where these grow, and are not denizens of the open sea, although of course there is a possibility of accidental transport by the assistance of oceanic 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 side of the ocean. The Dugong (Halicore) is distributed in different colonies, at present isolated, throughout the Indian Ocean from Arabia to North Australia. 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 Pinnipeds, although capable of traversing long reaches of ocean, are less truly aquatic than the last two groups, always resorting to the land or to extensive ice-floes for the purpose of breeding. The geographical range of the various 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, very 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 the Sea-Elephant (Macrorhinus leoninus), the general range of which is in the southern hemisphere, inhabiting the coast of California. Even in this case a different specific name has been given to the northern form; but the characters by which it is distinguished are not of great importance, and probably, except for the abnormal geographical distribution, would never have been noticed. The most remarkable circumstance connected with the distribution of the Pinnipeds is the presence of members of the suborder in the three isolated great lakes or inland seas of Central Asia—the Caspian, Aral, and Baikal; these forms, notwithstanding their long isolation, having varied but slightly from species now inhabiting the Polar Seas.
II. GEOLOGICAL DISTRIBUTION.
Geological Sequence.—In order to understand the geological distribution, or in other words the distribution in time of mammals, it is necessary to be acquainted with the chief divisions, or time-periods, of the strata constituting the crust of the globe. These are shown in the following table, which commences with the uppermost or most recent beds and ends with the lowest and oldest.
I. CAINOZOIC OR TERTIARY— 1. Pleistocene—River alluvia, etc. 2. Pliocene—Suffolk Crag. 3. Miocene—Hempstead Beds of Hampshire. 4. Eocene—Paris Gypsum and London Clay.
II. MESOZOIC OR SECONDARY— 1. Cretaceous—Chalk, Greensands, etc. 2. Jurassic—Oolites and Lias. 3. Triassic—Red Marls, Dolomites, etc.
III. PALÆOZOIC OR PRIMARY— 1. Permian—Beds overlying the Coal. 2. Carboniferous—Coal-measures, etc. 3. Devonian—Old Red Sandstone. 4. Silurian—Wenlock Limestone, etc. 5. Cambrian—Llanberis Slate, etc. 6. Archæan—Gneiss and other schists.
The names in the first column indicate the primary divisions or life-periods, while those in the second column are the great systems, each of which is again divided into minor groups, the popular names of a few of these minor groups being given in the third column. There are at present no means of arriving at any satisfactory conclusion as to the absolute length of time indicated by either the primary or secondary divisions; but there is little doubt that the whole of the Tertiary period is only equal to a fraction of the Mesozoic as regards its duration, while it is probable that the duration of the Mesozoic epoch was largely exceeded by that of the Palæozoic.
Mesozoic Mammals.—The earliest date at which mammals are at present known is in the upper part of the Triassic period, which forms the base of the great Mesozoic epoch; and from this date they are represented more or less abundantly in various horizons of the Jurassic and Cretaceous.
The very rapid advances in our knowledge of these forms which have been made in the last few years, especially in consequence of the explorations of rich fossiliferous beds in North America, have not only completely changed the present aspect of the science, but give such promise for the future, that any sketch which we may now attempt of this branch of the subject can only be regarded as representing a transient phase of knowledge. It will be well, however, to gather together in this place the leading facts now ascertained with regard to the most ancient forms, as, owing to the uncertainty of their relationship with any of the existing orders, they will be most conveniently treated of separately, while the ascertained facts relating to the geological history of the forms more nearly allied to those now living will be more appropriately described under the account of the different groups into which the class may now be divided.
The remains of mammals which existed anterior to the Tertiary period hitherto discovered nearly all belong to creatures of very small size, many of the largest scarcely exceeding the common Polecat or Squirrel. Some are known only by a few isolated teeth, others by nearly complete sets of these organs, and the majority by more or less nearly perfect specimens of the rami of the lower jaw. It is a very curious circumstance that this part of the skeleton alone has been preserved in such a large number of instances. Only very rarely has a nearly complete cranium been found; and there is no satisfactory evidence of the structure of the vertebral column of any single individual, and only one known case of a complete limb.[29] The species already described from European strata are numerous, although the number of genera and species has lately been reduced. Of these by far the greater number have been found at a single spot near Swanage in Dorsetshire, in a bed of calcareous mud only forty feet long, ten feet wide, and averaging five inches in depth. The marvellous results obtained by the exploration by Mr. S. H. Beckles of this small fragment of the earth’s surface show by what accidents, as it were, our knowledge of the past history of life has been gained, and what may still remain in store where little thought of at present. A bed, apparently equally rich, has been discovered in the Jurassic of Wyoming, North America, the contents of which have been made known by Professor Marsh, while another fertile source of these remains occurs in the Laramie beds of the Upper Cretaceous of the United States.[30]
The whole of the Mesozoic mammals at present known may be divided into two great groups, the one characterised by a type of dentition more or less clearly resembling that found among the existing Polyprotodont Marsupials, while the other presents an altogether peculiar modification, recalling in some respects that of the Diprotodont Marsupials, although differing so decidedly as to show that the owners of this form of dentition cannot be included in that group.
Multituberculata.—The name Multituberculata has been proposed for the group exhibiting the type of dentition last mentioned, and is generally adopted, although the term Allotheria has been also suggested. The essential characteristic of the dentition of this group is the presence of a single scalpriform incisor on each side of the lower jaw (Fig. 25) and of one larger incisor, and in some instances of one or two smaller ones in each premaxilla (Fig. 24). These incisors are separated by an interval or diastema from the first of the premolars. The true molars, and in some instances the premolars (Fig. 24), are characterised by having longitudinal rows of tubercles separated by one or more grooves; there being either two or three of these rows in the upper molars of those forms in which these teeth are known, while there are, at least usually, only two in those of the lower jaw. In other cases the premolars are of a secant type, with a highly convex cutting-edge, and usually either serrated or obliquely grooved (Figs. 25, 26). From a certain resemblance between these secant premolars and those of some of the smaller Macropodidæ it was at one time considered that we had in these mammals representatives of Diprotodont Marsupials. The great difference in the structure of the molar teeth of these forms, coupled with the circumstance that when the number of upper incisors is reduced below three it is the second in place of the first which becomes enlarged and opposed to the incisor of the lower jaw, seems to prevent the acceptation of this view. Moreover, in their peculiar structure the molars seem, on the whole, to make a nearer approximation to the teeth of Ornithorhynchus than to any other known mammal; and it has accordingly been suggested that the Multituberculata may really represent an order of Prototheria. Some support is afforded to this suggestion by certain fragmentary bones from the Cretaceous of the United States, which are regarded by Marsh as parts of a coracoid and interclavicle. The peculiar character of the whole dentition of these forms indicates that if they are really Prototherians they cannot be regarded as primitive and ancestral types.
It would be beyond the scope of the present work to describe in detail, or even to mention the names of all the members of this group, and it will therefore suffice to refer to a few of the principal types. Of the forms with tubercular premolars the best known is the genus Tritylodon (Fig. 24), which occurs typically in beds of Lower Mesozoic in South Africa, but is also known from the Trias of Stuttgart. In the Stonesfield Slate, near Oxford, which belongs to the lower part of the Jurassic system, and is separated from the Trias by the intervening Lias, a fragmentary jaw with three teeth (Fig. 27) appears to indicate an allied type, the teeth having three longitudinal ridges separated by grooves. In the Purbeck beds of Dorsetshire, forming the top of the Jurassic system, we find another member of this group, which has been described as Bolodon, closely allied to which is Allodon of the Upper Jurassic of the United States.
The first discovery of the remains of Mesozoic mammals was made in the Keuper or Upper Trias of the Rhætian Alps in Bavaria. In 1847 Professor Pleininger of Stuttgart, while sifting some sand from the Keuper of Diegerloch and Steinenbronn, found, among an immense mass of teeth, scales, and unrecognisable fragments of skeletons of fish and saurians, two minute teeth, each with well-defined, enamelled, tuberculated crowns and distinct roots, plainly showing their mammalian character. These were considered by their discoverer to indicate a predaceous and carnivorous animal of very small size, to which he gave the name of Microlestes antiquus. Subsequently Mr. C. Moore discovered in a bone bed of Rhætic (topmost Trias) age, filling a fissure in the Mountain Limestone at Holwell, near Frome in Somersetshire, various isolated teeth with their crowns much worn, but apparently including both upper and lower molars and a canine, which are assigned by Sir R. Owen to Pleininger’s genus Microlestes, and described specifically as M. moorei. Under the name of Hypsiprymnopsis rhæticus, Professor Boyd Dawkins described a single tooth with two roots discovered in the Rhætic Marlstone at Watchet in Somersetshire. Sir R. Owen referred the latter tooth to Microlestes, and if its describer is right in regarding it as a much worn premolar of the type of those of Plagiaulax (Fig. 25) there would be evidence that Microlestes was closely allied to the latter, from the molars of which those of Microlestes are scarcely distinguishable.
Plagiaulax, of the Dorsetshire Purbeck (Figs. 24, 25), is at once distinguished from Tritylodon by its secant premolars, which, as already mentioned, recall those of some of the Macropodidæ, although readily distinguished by the convexity of the cutting edge and their oblique grooving. This remarkable and highly specialised type has been the occasion of one of the most interesting discussions on the inferences which may be drawn as to the affinities and habits of an otherwise unknown animal from the structure of a small portion of its organisation which occurs in the annals of natural history—a discussion carried on with great ability, ingenuity, and wealth of illustration on both sides. Dr. Falconer maintained that it was more nearly allied to the Rat-Kangaroo (Potorous or Hypsiprymnus) than to any other existing form, and that, as it is known that these animals feed upon grass and roots, “it may be inferred of Plagiaulax that the species were herbivorous or frugivorous. I can see nothing in the character of their teeth,” he adds, “to indicate that they were either insectivorous or omnivorous.” Sir R. Owen, on the other hand, from the same materials came to the conclusion that “the physiological deductions from the above-described characteristics of the lower jaw and teeth of Plagiaulax are that it was a carnivorous Marsupial. It probably found its prey in the contemporary small insectivorous mammals and Lizards, supposing no herbivorous form like Stereognathus to have co-existed during the Upper Oolitic period.”
It is impossible here to give at any length the arguments by which these opposing views are respectively supported, but it may be indicated that the first-mentioned is strongly countenanced by the consideration of the following facts: (1) all existing Marsupials may be divided, so far as their dentition is concerned, into two groups—(a) those which have a pair of large more or less procumbent incisors close to the symphysis of the lower jaw, and rudimentary or no canines (diprotodont dentition), and (b) those which have numerous small incisors and large pointed canines (polyprotodont dentition); (2) the vast majority of the former group are purely vegetable feeders, and almost all of the latter are carnivorous or insectivorous; and (3) Plagiaulax, so far as its structure is known, shows an analogy with the former group; and, as we have no sure basis for inferences as to the habits of an unknown animal, but the knowledge of the habits of such as are known, we have no grounds for supposing that its habits differed from those forms having an analogous type of dental structure.[31]
Allied types, such as Ctenacodon, are also met with in the Upper Jurassic of North America; and the Plagiaulacidæ also persisted into the lower part of the Eocene division of the Tertiary period; Neoplagiaulax being a Tertiary form common to Europe and the United States, while Liotomus and Ptilodus are at present known only from the latter country.
The present group is also represented in the upper Cretaceous of the United States by Selenacodon (Meniscoëssus in part), Cimoliomys, etc. Polymastodon, of the Lowest or Puerco Eocene of New Mexico is the largest known form, and is characterised by the presence of only one premolar and the elongated molars. The angle of the mandible is inflected after the Marsupial fashion.
Polyprotodont Types.—The second type of mammalian dentition found in the Mesozoic period resembles that occurring among recent Polyprotodont Marsupials—that is to say there are at least three lower incisors, the canines are well developed, and the premolars and molars are cuspidate, the number of the latter reaching in some cases to seven or eight. There has been much discussion as to the taxonomic position of these forms, and while the majority of writers admit the Marsupial affinities of at least a moiety, it has been contended that others indicate distinct ordinal groups more or less closely allied to the Insectivora. At present, however, there is no decisive evidence to support such a view. Important proof of the Marsupial affinity of one of these forms is afforded by the replacement of the teeth, which appears to be of the same nature as in the existing Marsupials, that is to say, the last premolar alone is preceded by a milk-tooth.
The most generalised forms appear to be Dromatherium and Microconodon, from Lower Mesozoic beds in the United States, of which enlarged views of the teeth are given in Fig. 4 (1, 2), p. 31. Professor Osborn points out the extremely simple character of these teeth, and it is quite possible that these forms may prove to be Prototheria. There are three premolars and seven molars in the lower jaw of Dromatherium.
A common form in the Purbeck of Dorsetshire is Triconodon (Triacanthodon), in which the formula of the lower teeth is i 3, c 1, p 4, m 3-4. A lower jaw is shown in Fig. 28, and an enlarged view of a molar tooth in Fig. 4 (5). The molar teeth consist of three flattened cones placed in the same antero-posterior line, those of the upper and lower jaw being alike. Priacodon, of the Jurassic of the United States, is probably inseparable from Triconodon. In the genus Phascolotherium (Fig. 29) of the Lower Jurassic Stonesfield Slate, the lower teeth may be classified as i 4, c 1, p 3, m 4, the premolars and molars being much alike. The molars approximate to the type of those of Triconodon, but the anterior and posterior cones are relatively smaller. Like that of the last-named genus, the mandible of Phascolotherium is remarkable for the extremely low position of its articular condyle. In Amphilestes (Fig. 30) of the Stonesfield Slate the molars appear to be of the same general type as those of Phascolotherium, but are more numerous, although their exact number cannot be determined. A somewhat different type of lower molar is displayed by the genus Amblotherium, of the Dorsetshire Purbeck, to which Amphitherium of the Stonesfield Slate was probably allied. This type of tooth is shown in Fig. 4 (8, 9, 12) p. 31, and, as there stated, represents that modification of the tritubercular type known as the tubercular sectorial. The three primitive tritubercular cusps form what is known as the blade of the tooth, behind which there is the talon or hypocone. A similar form of molar occurs in the existing Opossums and Bandicoots. The number of lower teeth in Amblotherium is i 4, c 1, p 4, m 7-8. Numerous allied types, such as Achyrodon and Dryolestes occur in the Upper Jurassic of Europe or the United States, while from only one side of the jaw being exposed in each case so-called genera like Stylodon and Stylacodon have been formed upon specimens showing the opposite side to that which is exposed in the types of Amblotherium and Amphitherium. The only parallel among existing forms to the excessive number of molar teeth found in these Mesozoic genera occurs in the Marsupial genus Myrmecobius, of which a description is given in a succeeding chapter. Jaws more or less closely resembling those described under the names mentioned above are also found in the uppermost Cretaceous of the United States. A feature common to these Mesozoic mammals and Myrmecobius and some other existing forms is the presence of a narrow channel on the inner side of the mandibular ramus known as the mylohyoid groove (Fig. 29).
The last type of molar dentition occurring among the Mesozoic Mammalia is that found in the lower jaws (Fig. 31), upon which the genus Spalacotherium was established, the upper jaws, described as Peralestes, being apparently referable to the same animal. Upper and lower teeth of this form are represented in Fig. 4 (6, 7), p. 31, where they are described as typical examples of the tritubercular type of molars, the upper teeth having one inner and two outer cusps, and the reverse condition obtaining in the lower ones. This type of molar presents a marked resemblance to that found in the existing Insectivorous genus Chrysochloris; the number of lower teeth in Spalacotherium is, however, i 3, c 1, p + m 10, by which it is widely distinguished from all the Insectivora. Menacodon, of the Upper Jurassic of the United States, appears to be allied to Spalacotherium.
Tertiary Mammals.—The more important types of Tertiary mammals will, as already mentioned, be noticed under the heads of the groups to which they are severally allied; but a few general remarks on this subject may be advantageously recorded in this chapter. In the first place, it may be observed that the comparatively scanty evidence of mammalian life hitherto yielded by the Cretaceous, coupled with the number and variety of forms approximating to the existing groups found even in the lowest Tertiary, indicates a great imperfection of the geological record. At present, indeed, we have no decisive evidence of the existence of any members of the Eutherian subclass previously to the Tertiary; but it can hardly be doubted that in some part of the world they had made their appearance before that epoch. The Eutherian mammals of the lowest Eocene, both in Europe and the United States, are of an extremely generalised type; and although many of them approximate to existing groups, they show such a combination of characters, now restricted to individual groups, as to indicate that several of the various orders into which the subclass is now divided were at that period very intimately connected. A marked feature of these early Eutherians is the prevalency of trituberculism in the dentition, not less noteworthy being the frequent occurrence of pentadactylism in the feet, while many of the individual bones were devoid of the grooves and ridges found in those of later types. By the time that we reach the upper division of the Eocene period, such as the horizon of the well-known gypsum of the Paris basin, nearly all the chief groups of mammals had become clearly differentiated from one another, although their representatives were usually more generalised than their existing allies. From this date to the later geological periods there is a gradual approximation to the types of mammalian life existing at the present day.
In addition to the features of trituberculism and pentadactylism so characteristic of the oldest known Eutherians, we may notice some other points in connection with the earlier types. Thus the older Tertiary mammals, as we have already stated, had relatively smaller and simpler brains than the later types, so that a gradual evolution in this respect may be traced from the Eocene to the Pleistocene. Again, there is a great tendency among the Eocene forms to a retention of the typical Eutherian dental formula noticed on page 25, and also to the absence of an interval, or diastema, in the dental series. Concomitantly with this feature we may notice the short crowns and simpler structure of the molar teeth of the earlier Ungulates as compared with those of to-day, of which details will be given in a later chapter. Another instance of the more generalised characters of the earlier mammals is afforded by the absence or slight development of horns, antlers, and tusks among the Ungulata. Thus the earlier Rhinoceroses were hornless, and the Deer either without antlers or with antlers of a very simple kind, while the male Swine were not furnished with the formidable tusks of the existing Wild Boars. Finally, all, or nearly all of the mammals, from the lowest Eocene of Rheims present the peculiarity of having a vertical perforation in the astragalus.
The intimate connection existing during the Middle Tertiary between many families of mammals now widely distinguished from one another may be more conveniently noted when we come to the consideration of the families in question.
V. THE SUBCLASS PROTOTHERIA OR ORNITHODELPHIA
General Characters.—The characters of the Prototheria can at present only be deduced from the two existing families, since hitherto no extinct animals which can be referred with certainty to other divisions of this remarkable and well-characterised group have been discovered. These two isolated forms, in many respects widely dissimilar, yet having numerous common characters which unite them together and distinguish them from the rest of the Mammalia, are the Ornithorhynchidæ and the Echidnidæ, both restricted in their geographical range to the Australian region of the globe. Taken altogether they represent the lowest type of evolution of the mammalian class, and most of the characters in which they differ from the other two subclasses tend to connect them with the inferior vertebrates, the Sauropsida and Amphibia; for, though the name Ornithodelphia owes its origin to the resemblance of the structure of the female reproductive organs to those of birds, there is nothing especially bird-like about them.
Their principal distinctive characters are these. The brain has a very large anterior commissure, and a very small corpus callosum, agreeing exactly in this respect with the Marsupials. The cerebral hemispheres, in Echidna at least, are well developed and convoluted on the surface. The auditory ossicles present a low grade of development, the malleus being very large, the incus small, and the stapes columelliform. The coracoid bone is complete, and articulates with the sternum, and there is a pre-coracoid (epi-coracoid) in advance of the coracoid, while there is also a large “interclavicle” or episternum in front of the sternum, and connecting it with the clavicles. There are also “epipubic” bones. The oviducts (not differentiated into uterine and Fallopian portions) are completely distinct, and open, as in oviparous vertebrates, separately into a cloacal chamber, and there is no distinct vagina. The testes of the male are abdominal in position throughout life, and the vasa deferentia open into the cloaca, not into a distinct urethral passage. The penis, attached to the ventral wall of the cloaca, is perforated by a canal in the greater part of its length, and not merely grooved, as in reptiles and those birds which have such an organ. The canal is open at the base and brought only temporarily in contact with the termination of the vasa deferentia, so as to form a seminal urethra when required; but it never transmits the urinary secretion. This condition is a distinct advance on that of the Sauropsida in the direction of the more complex development of these parts in most of the other Mammalia. The ureters do not open into the bladder, but behind it into the dorsal wall of the genito-urinary passage. The mammary glands have no distinct nipple, but pour out their secretion through numerous apertures situated in a cup-shaped depression of the abdominal skin, forming a mammary marsupium, especially developed in the females during lactation. It should be mentioned that, according to the observations of Professor Gegenbaur, the mammary glands of the Monotremes are the simplest found in the entire class. The region of the glands is, indeed, distinguished from the rest of the abdomen merely by its thicker layers of muscles. The glands themselves are closely connected with the hair-follicles, and belong to the sudoriparous type, whereas the glands of all other mammals are of sebaceous origin.
The young are produced from eggs laid by the female parent, which are meroblastic, like those of birds; that is to say only a portion of the yolk segments and forms the embryo, the remainder serving for the nourishment of the latter.
The above are the principal distinguishing characters of the group, and apply not only to the subclass, but of course equally to the one order Monotremata, in which the two existing genera are included. In addition to these more important characters, the following minor features may also be mentioned.
The scapula differs from that of all other mammals in that the ridge corresponding to the spine of other forms is situated on the anterior border instead of in the middle of the outer or dorsal surface. The humerus is much expanded at its two extremities, and has a very prominent deltoid crest, and a well-marked entepicondylar foramen.
The dorso-thoracic vertebræ are nineteen in number, and have no terminal epiphyses to their bodies. The transverse processes of the cervical vertebræ are of autogenous formation, and remain suturally connected with the remainder of the vertebra until the animal is full-grown. Though in this respect they present an approximation to the Sauropsida (Reptiles and Birds), they differ from these classes, inasmuch as there is not a gradual transition from these autogenous transverse processes of the neck (or cervical ribs, as they may be considered) into the thoracic ribs, for in the seventh vertebra the costal element is much smaller than in the others, indicative of a very marked separation of neck from thorax, not seen in the existing Sauropsida. The upper ends of the ribs are attached to the sides of the bodies of the dorsal vertebræ only, and not to the transverse processes. The sternal ribs are well ossified, and there are distinct partly ossified intermediate ribs. The cerebral cavity, unlike that of the lower Marsupials or the Reptiles, is large and hemispherical, flattened below and arched above, and about as broad as long. The cribriform plate of the ethmoid is nearly horizontal. The cranial walls are very thin, and smoothly rounded externally, and the sutures become completely obliterated in adult skulls, as in Birds. The broad occipital region slopes upwards and forwards, and the face is produced into a long and depressed rostrum. The bony palate is prolonged backwards, so that the posterior nares are nearly on a level with the glenoid fossæ. The mandible is without distinct ascending ramus; the coronoid process and angle are rudimentary, and the two halves are loosely connected at the symphysis. The fibula has a broad, flattened process, projecting upwards from its upper extremity above the articulation, like an olecranon. In the male there is an additional, flat, curved ossicle on the hinder and tibial side of the plantar aspect of the tarsus, articulating chiefly to the tibia, which supports in the adult a sharp-pointed perforated horny spur, with which is connected the duct of a gland situated beneath the skin of the back of the thigh, the function of which is not yet clearly understood. (A rudimentary spur is found in the young female Ornithorhynchus, but this disappears when the animal becomes adult.) The stomach is subglobular and simple; the alimentary canal has no ileo-cæcal valve, or marked distinction between large and small intestine, but has a small, slender vermiform cæcum with glandular walls. The liver is divided into the usual number of lobes characteristic of the Mammalia, and is provided with a gall-bladder.
In the presence of three distinct bones developed from cartilage in the shoulder-girdle (viz. scapula, coracoid, and pre- or epi-coracoid) the Monotremes agree with the Anomodont reptiles (see p. 83), and with no other representatives of that class. The pre-coracoid of the Anomodonts is, however, distinguished by extending upwards to articulate with the acromial process of the scapula. The Monotreme humerus is, moreover, strikingly like the corresponding bone of many of the Anomodonts and of some of the allied Labyrinthodont Amphibians.
Family ORNITHORHYNCHIDÆ.
Ornithorhynchus.[32]—Cerebral hemispheres smooth. Premaxillæ and mandible expanded anteriorly and supporting a horny beak something like that of a duck, bordered by a naked and very sensitive membranous expansion. The place of teeth in the adult is supplied functionally by horny structures, elongated, narrow, and sharp-edged, along the anterior part of the sides of the mouth, and broad, flat-topped or molariform behind. Functional molar teeth present in the young and adolescent condition. Legs short, fitted for swimming; feet webbed, each with five well-developed toes armed with large claws, beyond which in the fore feet the interdigital membrane is extended. Vertebræ: C 7, D 17, L 2, S 2, Ca 21. Acetabulum not perforated. Tongue not extensile. Mucous membrane of small intestine covered with delicate, close-set transverse folds or ridges. Tail rather short, broad, and depressed. Eyes very small. Fur close and soft.
The Duck-billed Platypus (Platypus anatinus) was the name assigned to one of the most remarkable of known animals by Shaw, who had the good fortune to introduce it to the notice of the scientific world in the Naturalist’s Miscellany (vol. x., 1799). In the following year it was independently described by Blumenbach (Voigts Magazin, ii. p. 205) under the name of Ornithorhynchus paradoxus. Shaw’s generic name, although having priority to that of Blumenbach, could not be retained, as it had been used at a still earlier time (1793) by Herbst for a genus of Coleoptera. Ornithorhynchus is therefore now universally adopted as the scientific designation, although Duck-billed Platypus or Duck-bill may be conveniently retained as a vernacular appellation. By the colonists it is called “Water-Mole,” but it need scarcely be said, its affinities with the true moles are of the slightest and most superficial description. Until the last few years the early stages of the development of the young were not fully known. It had, indeed, been repeatedly affirmed, in some cases by persons who have had actual opportunities of observation, that the Platypus lays eggs; but these statements were generally received with scepticism and even denial. This much-vexed question was, however, settled by the researches of Mr. W. H. Caldwell in 1884, who found that these animals, although undoubtedly mammals throughout the greater part of their structure, are oviparous, laying eggs, which in the manner of their development bear a close resemblance to the development of those of the Reptilia. Two eggs are produced at a time, each measuring about three-fourths of an inch in its long, and half an inch in its short, axis, and enclosed in a strong, flexible, white shell.
The Platypus is pretty generally distributed in situations suitable to its aquatic habits throughout the island of Tasmania and the southern and eastern portions of Australia. Slight variations in the colouring and size of different individuals have given rise to the idea that more than one species may exist; but all naturalists who have had the opportunity of investigating this question by the aid of a good series of specimens have come to the conclusion that there is but one, and no traces of any extinct allied forms have yet been discovered.
The length of the animal when full grown is from eighteen to twenty inches from the extremity of the beak to the end of the tail, the male being slightly larger than the female. The fur is short, dense, and rather soft to the touch, and composed of an extremely fine and close under-fur, and of longer hairs projecting beyond this, each of which is very slender at the base, and expanded, flattened, and glossy towards the free end. The general colour is deep brown, but paler on the under parts. The tail is short, broad, and depressed, and covered with coarse hairs, which in old animals generally become worn off from the under surface. The eyes are small and brown. There is no projecting pinna or ear-conch. The mouth, as is well known, bears a striking resemblance to the bill of a Duck. It is covered with a naked skin, a strong fold of which projects outwards around its base. The nostrils are situated near the extremity of the upper surface. There are no true teeth in the adult, but their purposes are served by horny prominences, or cornules, two on either side of each jaw—those in the front, narrow, longitudinal, sharp-edged ridges, and those behind broad, flattened, and molariform. The upper surface of the lateral edges of the mandible has also a number of parallel fine transverse ridges, like those on the bill of a Duck. Until 1888 it was thought that true teeth were totally wanting throughout the life of this animal; but in the spring of that year Mr. E. B. Poulton[33] announced the discovery in an embryo of teeth which were regarded as quite functionless. In the following year, however, Mr. O. Thomas[34] was fortunate enough to find some young skulls with functional teeth in situ, and was thus enabled to give a detailed account of their structure and of their relations to the cornules. From those specimens it appears that the teeth are functional for a considerable part of the life of the animal, cutting the gum in the usual manner, and, after being worn down by friction with food and sand, are shed from the mouth in the same manner as are the milk-teeth of other mammals. The cornules are developed from the epithelium of the mouth under and around the teeth, and the hollows found in the middle of them are the vestiges of the alveoli from which the teeth have been shed. One of the skulls showed on either side, both above and below, two completely calcified teeth; but in another example there were three teeth on either side of the lower jaw. According to Mr. Thomas’s account, “the teeth themselves are broad, flat, and low-crowned. The upper ones have each two high, conical, internal cusps, from which minute ridges run downwards and outwards to the outer borders of the crowns, where the edge is peculiarly crenulate rather than cuspidate, in the ordinary sense of the word. On the whole, the anterior and posterior upper teeth are essentially similar to one another, except that the former are narrower, and their outer edges are less markedly crenulated. In the lower jaw there is a greater difference between the two. The anterior is triangular in outline, its longest side is placed antero-externally, and its anterior and postero-external angles have each a high pointed cusp, ridged on its internal aspect, while the posterior and internal borders are indistinctly crenulated. The posterior tooth is broadly quadrangular in outline, with a projecting antero-internal angle. As in the corresponding tooth above, there are two cusps on one side, and a series of crenulations on the other, but they are of course reversed, the cusps being external and the crenulations internal. The cusps are high, and connected with transverse ridges running across towards the internal border.”
In trying to find any teeth like those of the Duck-bill among other known mammals Mr. Thomas considers, as was first suggested by Professor Cope, that those of the Mesozoic Multituberculata (p. 109) make the nearest approximation. He adds, however, that “it must be insisted that the resemblance between the Multituberculate and the Ornithorhynchus teeth is of the most general character, and that the two are certainly widely separated generically, even if we do admit that they appear to possess a relationship nearer to each other than to any other known groups of mammals.”
Reverting to the description of the Duck-bill, we find that in the cheeks are tolerably capacious pouches, which appear to be used as receptacles for food. The limbs are strong and very short, each with five well-developed toes provided with strong claws. In the fore feet the web not only fills the interspaces between the toes, but extends considerably beyond the ends of the long, broad, and somewhat flattened nails, giving great expanse to the foot when used for swimming, though capable of being folded back on the palm when the animal is burrowing or walking on the land. On the hind foot the nails are long, curved, and pointed, and the web extends only to their base. On the heel of the male is a strong, curved, sharply pointed, movable horny spur, directed upwards and backwards, attached by its expanded base to the accessory bone of the tarsus. This spur, which attains the length of nearly an inch, is traversed by a minute canal, terminating in a fine longitudinal slit near the point, and connected at its base with the duct of a large gland situated at the back part of the thigh. The whole apparatus is so exactly similar in structure to the poison-gland and tooth of a venomous snake as to suggest a similar function, but evidence that the Platypus ever employs its spur as an offensive weapon has, at all events until lately, been wanting. A case is, however, related by Mr. Spicer in the Proceedings of the Royal Society of Tasmania for 1876 (p. 162), of a captured Platypus inflicting a severe wound by a powerful lateral and inward movement of the hind legs, which wound was followed by symptoms of active local poisoning. It is not improbable that both the inclination to use the weapon and the activity of the secretion of the gland may be limited to the breeding season, and that their purpose may be, like that of the antlers of deer and many similar organs, for combat among the males. In the young female the spur is present in a rudimentary condition, but it disappears in the adult of that sex.
The Platypus is aquatic in its habits, passing most of its time in the water or close to the margin of lakes and streams, swimming and diving with the greatest ease, and forming for the purpose of sleeping and breeding deep burrows in the banks, which generally have two orifices—one just above the water level, concealed among long grasses and leaves, and the other below the surface. The passage at first runs obliquely upwards in the bank, sometimes to a distance of as much as fifty feet, and expands at its termination into a cavity, the floor of which is lined with dried grass and leaves, and in which the eggs are laid and the young brought up. The food consists of aquatic insects, small crustaceans, and worms, which are caught under water, the sand and small stones at the bottom being turned over with the bill. The creatures appear at first to deposit what they have thus collected in their cheek pouches, and when these are filled they rise to the surface and quietly triturate their meal with the horny plates before swallowing it. Swimming is effected chiefly by the action of the broad forepaws, the hind feet and tail taking little share in locomotion in the water. When asleep they roll themselves into a ball, as shown in the figure. In their native haunts they are extremely timid and wary, and very difficult to approach, being rarely seen out of their burrows in the daytime. Mr. A. B. Crowther, who has supplemented the often quoted observations of Dr. Bennett upon the habits of these animals in confinement, says, “They soon become very tame in captivity; in a few days the young ones appeared to recognise a call, swimming rapidly to the hand paddling the water; and it is curious to see their attempts to procure a worm enclosed in the hand, which they greedily take when offered to them. I have noticed that they appear to be able to smell whether or not a worm is contained in the closed hand to which they swim; for they desisted from their efforts if an empty fist was offered.” When irritated they utter a soft low growl, resembling that of a puppy.
Family ECHIDNIDÆ.
Cerebral hemispheres larger and well convoluted. Facial portion of skull produced into a long, tapering, tubular rostrum, at the end of which the anterior nares are situated. Rami of mandible slender, styliform. Opening of mouth small, and placed below the extremity of the rostrum. No teeth or laterally placed horny plates, though the palate and tongue are furnished with spines. Tongue very long, vermiform, slender, and protractile. Lining membrane of small intestine villous, but without transverse folds. Feet not webbed, but with long strong claws fitted for scratching and burrowing. The hinder feet with the ends of the toes turned outwards and backwards in the ordinary position of the animal when on the ground. Tail very short. Acetabulum with a large perforation, as in Birds. Calcaneal spur and gland of the male much smaller than in Ornithorhynchus. Fur intermixed with strong, sharp-pointed spines. Terrestrial and fossorial in habits, feeding exclusively on ants, and recalling in the structure of the mouth and various other parts, relating to their peculiar mode of life the true Anteaters of the order Edentata.
The Echidnas or Spiny Anteaters constitute a family which appears in some respects to be less specialised than the Ornithorhynchidæ. According to Mr. O. Thomas, all the living forms may be included in two species, which, with some hesitation, are referred to two genera—Echidna and Proechidna (Acanthoglossus).
Echidna.[35]—In Echidna there are five toes, all of which are provided with claws, those of the fore foot being broad, slightly curved, and directed forwards, while the posterior ones are slender, more curved, and inclined outwardly. The beak is about as long as the rest of the head, and either nearly straight, or slightly curved upwards, while the palate is comparatively wide, and but slightly vaulted. The number of the vertebræ is C 7, D 16, L 3, S 3, Ca 12. The one existing representative of the genus (E. aculeata) occurs in New Guinea, Tasmania, and Australia.
So much variation is displayed by this animal, that it has been divided into several species, but the latest researches tend to show that these variations cannot be regarded as indicating more than races, of which there are three well-marked types.
The first race, or variety, has been termed the Port Moresby Echidna, and is only known from that Papuan locality. It is distinguished from the typical form by its smaller size, by the shorter spines on the back, which admit of the fur being seen, and by the more spinous covering of the head, belly, and limbs, as well as by the lighter skull and relatively larger beak.
The typical variety is confined to the Australian mainland, and is of medium size. The spines of the back are very long and stout, often reaching a length of two inches, and almost completely concealing the hair. The colour of these spines varies from yellow at the roots to black at the tips, but some may be altogether yellow. The hair of the back is black or dark brown in colour, but it may be occasionally absent, or in the region of the loins may exceed the spines in length. The limbs and under surface of the body are covered with dark brown hair, thinly interspersed with short spines; and the hair of the face is of the same general hue as that of the body. The skull has a slender rostrum and a flat and narrow brain-case.
In the third or Tasmanian race, which is confined to Tasmania, the average size is somewhat larger than in the typical form. The most characteristic feature is, however, the shortness of the spines of the back, which in the greater part of that region are almost or quite concealed by the hairs. The hairs of the back are dark brown, those of the under surface and sides of the head being generally rather paler. There is often a white spot on the chest. Very frequently there is a difference in the proportionate lengths of the hinder claws from those of the typical race. In the skull the beak is comparatively short and stout, and the brain-case large and wide.
Echidnas are usually found in rocky districts, and more especially in the mountains. In a wild state they live mainly on ants. Specimens have been brought to this country and kept in the Zoological Society’s Gardens; and in captivity they will readily eat eggs, and bread-and-milk. They are able, however, to endure long fasts, an individual having been known to go without food for upwards of a month.
These animals seem to be mainly of nocturnal habits, and if brought out during the daytime appear to be sluggish and stupid, crouching to the ground with the head between the legs, and thus presenting a mass of spines to an enemy. They burrow rapidly in soft ground, sinking directly downwards, and not going head forwards. A specimen placed on a large chest of earth containing plants reached the bottom in less than two minutes; and it is said that the muzzle assists in the work of burrowing.
Proechidna.[36]—The one known representative of the genus Proechidna (Fig. 33) attains dimensions about equal to those of the largest race of Echidna aculeata. The skull is less depressed than in the latter, with the anterior portion of the palate very concave, and the deflected beak nearly twice the length of the remainder of the skull. As a rule, there are only three claws to each foot; but the first and fifth digits are represented by several phalanges, and one instance is known where there are five complete claws on the anterior and four on the posterior feet. There are two more vertebræ in the dorsal and lumbar region than in Echidna.
The head and body are covered with a thick coat of hair, among which there are a number of short spines in the region of the back, which are much less numerous than in the typical race of the last species. The colour of the fur is generally dark brown or black, but the head may be almost white; and the spines are usually entirely white, although in certain cases they may be brown at the root.
This species is known only from New Guinea, the recorded specimens being from the north-western regions of that country. It inhabits rocky ground, and dwells chiefly in the mountains, the specimens which were first described having been obtained at an elevation of about 3500 feet above the sea level. The Papuans capture it by digging trenches in the ground to a depth of about a yard, by which means they generally come upon its runs.
Fossil Species.—Remains of a species of Echidna of very much larger size than the existing forms have been obtained from the cave-deposits of New South Wales, which appear to be of Pleistocene age. This species was named Echidna oweni by the late Mr. Krefft, but was subsequently called E. ramsayi by Sir R. Owen. In referring this species to the genus Echidna, that term must be regarded as including Proechidna.
VI. THE SUBCLASS METATHERIA OR DIDELPHIA
General Characters.—The Metatheria or Didelphia are represented at present by numerous species, presenting great diversities of general appearance, structure, and habits, although all united by many essential anatomical and physiological characters, which, taken altogether, give them an intermediate position between the Prototheria and the Eutheria.
Although the striking differences in external form, in many anatomical characters, and in mode of life of various animals of this section might lead to their division into groups equivalent to the orders of the Eutheria, it is more convenient on the whole to adhere to the usual custom of treating them all as forming one order called MARSUPIALIA,[37] the limits of which are therefore equivalent to that of the subclass. The more essentially distinctive characters are as follows.
In the structure of the brain and the presence of epipubic bones they agree with the Prototheria, while in the structure of the ear-bones and the shoulder-girdle and the presence of teats on the mammary glands they resemble the Eutheria, the reproductive organs belonging to neither one nor the other type, but having a special character representing an intermediate grade of development. The ureters open into the base of the bladder. The oviducts are differentiated into uterine and Fallopian portions, and open into a long and distinct vagina, quite separate from the cystic urethra. The penis is large, but its crura are not directly attached to the ischia. The spongy body has a large bifurcated bulb. The young are born in an exceedingly rudimentary condition, and are never nourished by means of an allantoic placenta, but are transferred to the nipple of the mother, to which they remain firmly attached for a considerable time, nourished by the milk injected into the mouth by compression of the muscle covering the mammary gland. They are therefore the most typically mammalian of the whole class. The nipples are nearly always concealed in a fold of the abdominal integument or “pouch” (marsupium) which serves to support and protect the young in their early helpless condition.
Entering more fully into the characters of the subclass, which are also those of the order Marsupialia, it may be observed that the brain is generally small in proportion to the size of the animal, and the surface-folding of the cerebral hemispheres, though well marked in the larger species, is never very complex in character, and is absent in the medium-sized and smaller species. The arrangement of the folding of the inner wall of the cerebrum differs essentially from that of all known Eutheria, the hippocampal fissure being continued forward above the corpus callosum, which is of very small size. The anterior commissure is, on the other hand, greatly developed.
The teeth are always divisible, according to their position and form, into incisors, canines, premolars, and molars; but they vary much in number and character in the different families. Except in the genus Phascolomys, the number of incisors in the upper and lower jaws is never equal. The true molars are very generally four in number on either side of each jaw. The chief peculiarity in the dentition lies, however, in the mode of succession. Thus there is no vertical displacement and succession of the teeth, except in the case of a single tooth on either side of each jaw, which is always the hindermost of the premolar series, and is preceded by a tooth having more or less of the characters of a true molar (see Fig. 34); this deciduous tooth being the only one comparable to the “milk-teeth” of the diphyodont Eutheria. In some cases (as in Potorous) this tooth retains its place and function until the animal has nearly, if not quite, attained its full stature, and is not shed and replaced by its successor until after all the other teeth of the permanent series, including the posterior molars, are fully in place and use. In others, as the Thylacine, it is very rudimentary in form and size, being shed or absorbed before any of the other teeth have cut the gum, and therefore quite functionless. It must further be noted that there are some Marsupials, as the Wombat, Myrmecobius, and the Dasyures, in which no such milk-tooth, even in a rudimentary state, has yet been discovered, possibly in some cases from want of materials for observation at the right stage of development.
Epipubic or marsupial bones are present in both sexes of nearly all species. In one genus alone, Thylacinus, they are not ossified. The number of dorso-lumbar vertebræ is always nineteen, although there are some apparent exceptions caused by the last lumbar being modified into a sacral vertebra. The number of pairs of ribs is nearly always thirteen. The tympanic bone remains permanently distinct. The carotid canal perforates the basisphenoid. The lachrymal foramen is situated upon or external to the anterior margin of the orbit, and there are generally large vacuities in the bony palate. The angle of the mandible is (except in Tarsipes) more or less inflected. The hyoid bones have always a peculiar form, consisting of a small, more or less lozenge-shaped basihyal, broad ceratohyals, with the remainder of the anterior arch usually unossified, and stout, somewhat compressed thyrohyals. There are two anterior venæ cavæ,[38] into each of which a “vena azygos” enters. In the male the testes are always contained in a scrotum, which is suspended by a narrow pedicle to the abdomen in front of the penis. The vasa deferentia open into a complete and continuous urethra, which is also the passage by which the urine escapes from the bladder, and is perfectly distinct from the passage for the fæces, although the anus and the termination of the urethro-sexual canal are embraced by the same sphincter muscle. The glans is often bifurcated anteriorly. In the female the oviducts never unite to form a common cavity or uterus, but open separately into the vagina, which at least for part of its course is double. The mammæ vary much in number, but are always abdominal in position, having long teats, and in most of the species are more or less enclosed in a fold of the integument forming a pouch or marsupium, though in some this is entirely wanting, and the newly-born, blind, naked, and helpless young, attached by their mouths to the teat, are merely concealed and protected by the hairy covering of the mother’s abdomen. In this stage of their existence they are fed by milk injected into their stomach by the contraction of the muscles covering the mammary gland, the respiratory organs being modified temporarily, much as they are permanently in the Cetacea—the elongated upper part of the larynx projecting into the posterior nares, and so maintaining a free communication between the lungs and the external surface independently of the mouth and gullet, thus averting the danger of suffocation while the milk is passing down the latter passage.
Distribution.—The existing species of Marsupials are, with the exception of one family (the Didelphyidæ), limited in geographical distribution to the Australasian region,[39] forming the chief mammalian fauna of Australia, New Guinea, and some of the adjacent islands. The Didelphyidæ are almost purely Neotropical, one or two species ranging northwards into the Nearctic region. Fossil remains of members of this family have also been found in Europe and America in strata of the Eocene and early Miocene periods; and it is probable that at least many of the polyprotodont Mesozoic mammals noticed in Chapter IV. are referable to the Marsupialia.
Classification.—In dividing the Marsupials into minor groups, it may be observed that one of the most obvious distinctive characters among them is derived from the form and arrangement of the teeth. In certain species, as the Opossums, Dasyures, and Thylacine, the incisors are numerous, small, and subequal in size, and the canines large, as in the typical placental Carnivores (Fig. 35). To these the term “polyprotodont” is applied, and they are all more or less carnivorous in their habits. In others the central incisors are very prominent, and the lateral incisors and canines absent or subordinate in function (Fig. 36). These are called “diprotodont,” and they are all wholly or in great part vegetable feeders. In one group of these, the Wombats, there are but two incisors above and the same number below; but all the others, including the Kangaroos, Koalas, and Phalangers, have two functional incisors below and as many as six above, three on each side, but of these the first or central pair is the most fully developed.
Some hesitation has frequently been expressed as to whether the Polyprotodont and Diprotodont types are entitled to constitute distinct primary groups, owing to the presence of syndactylism among the Peramelidæ in the former, as well as in the latter; but if Mr. O. Thomas is right in regarding this feature as acquired independently in the two groups we may safely adopt such a division. Taking various combinations into consideration, the existing Marsupials readily group themselves into six very natural families, the leading characters of which may be summarised as follows:—
Order MARSUPIALIA.
A. POLYPROTODONTIA.—Incisors numerous, small, subequal. Canines larger than the incisors. Molars with sharp cusps.
α. Incisors ⁵⁄₄. Hind feet with the four outer toes subequal, distinct, and a well-developed opposable hallux. Didelphyidæ.
β. Incisors ⁴⁄₃. Hind feet with four outer toes distinct. Hallux small or rudimentary, rarely opposable. Dasyuridæ.
γ. Incisors ⁴⁻⁵⁄₃. Hind feet long and narrow. Fourth toe larger than the others. Hallux rudimentary or absent. Second and third toes very slender, and united in a common integument (syndactylous). Peramelidæ.
B. DIPROTODONTIA.—Incisors not exceeding ³⁄₃, usually ³⁄₁ but occasionally ¹⁄₁. Central (first) upper and lower incisors large and cutting. Upper canines generally, and lower invariably, absent or small. Molars with bluntly tuberculated or transversely ridged crowns.
α. Teeth with persistent pulps. Incisors ¹⁄₁, large, scalpriform, with enamel on the outer surface only. No canines. Hind feet with four subequal outer toes, partially syndactylous, and with rudimentary hallux. Phascolomyidæ.
β. Teeth rooted. Three upper incisors and a canine. Hind limbs not disproportionately large. Feet syndactylous, broad, with four subequal outer toes, and a large opposable hallux. Phalangeridæ.
γ. Teeth rooted. Three upper incisors, and frequently a canine. Hind limbs disproportionately large, with syndactylous feet as in Peramelidæ. Macropodidæ.
Suborder POLYPROTODONTIA.
The leading characters of this group are given in the foregoing schedule. This group is the only one represented at the present day, and so far as we know also in past epochs, beyond the confines of the Australasian region and adjacent islands.
Family DIDELPHYIDÆ.
Dentition: i ⁵⁄₄, c ¹⁄₁, p ³⁄₃, m ⁴⁄₄; total 50. Incisors very small and pointed. Canines large. Premolars with compressed pointed crowns. Molars with numerous sharp cusps. The last premolar preceded by a deciduous multicuspidate milk-molar, which remains in place until the animal is nearly adult (Fig. 34). Limbs of moderate development, each with five complete and distinct toes, all of which are provided with short, compressed, curved, sharp claws of nearly equal size, except the first toe of the hind foot or hallux (Fig. 37), which is large, widely separable from the others, to which it is opposed in climbing, and terminates in a dilated rounded extremity, without a nail. Tail generally long, partially naked and prehensile. Stomach simple. Cæcum of small or moderate size. Pouch generally absent, sometimes represented by two lateral folds of the abdominal integument, partially covering the teats, rarely complete. Vertebræ: C 7, D 13, L 6, S 2, C 19-35.
The Didelphyidæ, or true Opossums, differ from all other existing Marsupials in their habitat, being peculiar to the American continent. They are mostly carnivorous or insectivorous in their diet, and arboreal in habits.
Opossums occur throughout the greater part of the American continent, ranging from the United States to Patagonia, the greater number of species being found in the warmer regions. In South America the opossums take the place of the Eutherian Insectivora, and the sharp cusps on their teeth are admirably adapted for crushing the insects on which they mainly subsist.
Chironectes.[40]—The family comprises two genera only, namely Didelphys, containing all the species, with the exception of the curious Yapock, which forms by itself the genus Chironectes and is distinguished from all other Opossums by its webbed feet, non-tuberculated soles, and peculiar coloration. Its ground colour is light gray, with four or five sharply-contrasted brown bands passing across its head and back, and thus giving it a very peculiar mottled appearance. It is almost wholly aquatic in its habits, living on small fish, crustaceans, and water insects. Its range extends from Guatemala to southern Brazil.
Didelphys.[41]—The type genus Didelphys is a very large one, containing, according to Mr. O. Thomas, twenty-three existing species. It may be divided into five groups, or subgenera, all of which have received distinct names. The typical group is represented only by the common or Virginian Opossum (D. marsupialis), of which the numerous varieties have received separate specific names. This species is of large size, with a long, scaly, prehensile tail, and long bristle-like hairs mingled with the fur. The pouch is complete. It ranges over all temperate North America, and is also found in central and tropical South America, where it is commonly known as the Crab-eating Opossum. This animal is extremely common, being even found living in the towns, where it acts as a scavenger by night, retiring for shelter by day upon the roofs of the houses or into the sewers. The female produces in the spring from six to sixteen young ones, which are placed in her pouch immediately after birth, and remain there until able to take care of themselves.
The second or Metachirine group includes three species found all over the tropical parts of the New World. They are of medium size, with short close fur, very long, scaly, and naked tails, and less developed ridges on their skulls than in the type species. As a rule there is no pouch adapted to carry the young, which commonly ride on their mother’s back, holding on by winding their prehensile tails round hers. The Philanderine group is closely allied to the preceding, but is readily distinguished by the woolly hair, and the brown streak down the middle of the face. The Woolly Opossum (D. lanigera), which is represented in the accompanying woodcut (Fig. 38) carrying its young in the fashion mentioned above, is one of the two species of this group. In the fourth or Micoureine group the numerous species are all smaller than in the preceding groups, and have short and close hair, and no dark streak down the face. The best known species is the Murine Opossum (D. murina), little larger than a House-Mouse, and of a bright red colour, which is found as far north as central Mexico, and extends thence right down to the south of Brazil. The last or Peramyne group contains several extremely shrew-like species, of very small size, with short, hairy, and usually non-prehensile tails, not half the length of the trunk, and with wholly unridged skulls. The most striking member of the group is the Three-striped Opossum (D. americana), from Brazil, which is of a reddish-gray colour, with three clearly-defined deep-black bands down its back, very much as in some of the striped mice of Africa.
The numerous fossil species of Opossum found in the Upper Eocene and Lower Miocene of Europe are of especial interest from a distributional point of view, since they indicate how the Opossums of America may have been connected with the Australian Marsupials. These forms were originally referred to Didelphys, but have been subsequently described as Peratherium and Amphiperatherium. The characters of the molar teeth on which these genera are based do not appear to be sufficiently important to justify their separation from Didelphys. Allied forms occur in the Tertiaries of North America, which were originally described under the name of Herpetotherium, but have been subsequently referred to Peratherium. Remains of many of the existing species of Opossum are found in a fossil condition in the Pleistocene cave-deposits of Brazil.
Family DASYURIDÆ
Dentition: i ⁴⁄₃, c ¹⁄₁, p and m numerous, variable. Incisors small; canines well developed; molars with pointed cusps. Limbs equal. Fore feet with five subequal toes terminating in claws. Hind feet with the four outer toes well developed, and distinct from each other and bearing claws; the first (or hallux) clawless, generally rudimentary, sometimes entirely wanting. Stomach simple. No cæcum. Predatory carnivorous or insectivorous animals, inhabitants of Australia, Tasmania, and the southern parts of New Guinea and some of the adjacent islands. The aberrant genus Myrmecobius, though clearly a member of this family, is so sharply distinguished from all the others as to render a division into two subfamilies necessary.
Subfamily =Dasyurinæ=.—This comprises the more typical Dasyuridæ, in which the premolars and molars never exceed the normal number of seven on either side of each jaw, and in which the tongue is not specially extensile.
Thylacinus.[42]—Dentition: i ⁴⁄₃, c ¹⁄₁, p ³⁄₃, m ⁴⁄₄ = 46. Incisors small, vertical, the outer one in the upper jaw larger than the others. Summits of the lower incisors, before they are worn, with a deep transverse groove dividing them into an anterior and a posterior cusp. Canines long, strong, and conical. Premolars separated from one another by intervals, with compressed crowns, increasing in size from before backwards. True molars in general characters resembling those of Dasyurus, but of more simple form, the cusps being not so distinct nor sharply pointed. Milk-molar very small, and shed before the animal leaves the mother’s pouch. Humerus with an entepicondylar foramen. General form very Dog-like. Head elongated. Muzzle pointed. Ears moderate, erect, triangular. Fur short and closely applied to the skin. Tail of moderate length, thick at the base and tapering towards the apex, clothed with short hair. Hallux (including the metacarpal bone) wanting. Vertebræ: C 7, D 13, L 6, S 2, C 23. Marsupial bones represented only by small unossified fibro-cartilages.
The only known existing species of this genus, T. cynocephalus (Fig. 39), though smaller than a common Wolf, is the largest predaceous Marsupial at present living. It is now entirely confined to the island of Tasmania, although fragments of bones and teeth found in caves afford evidence that a closely allied species once inhabited the Australian mainland. The general colour of the Thylacine is grayish brown, but it has a series of transverse black bands on the hinder part of the back and loins, whence the name of “Tiger” frequently applied to it by the colonists. It is also called “Wolf,” and sometimes, though less appropriately, “Hyæna.” Owing to the havoc it commits among the sheepfolds, it has been nearly exterminated in all the more settled parts of Tasmania, but still finds shelter in the almost impenetrable rocky glens of the more mountainous regions of the island. The female produces four young at a time. The pouch opens backwardly, and there are four mammæ. The figure of the skull exhibits the peculiar Dog-like form so characteristic of the genus.
Sarcophilus.[43]—Dentition: i ⁴⁄₃, c ¹⁄₁, p ²⁄₂, m ⁴⁄₄. Upper incisors nearly equal, and placed vertically, the first not differentiated from the rest. Premolars rounded and closely crowded between the canine and molars, with broad crowns; molars broad and heavy, the last one without a distinct hind talon. Form thick and powerful; head disproportionately large for the body; muzzle short and broad; ears broad and rounded; tail of moderate length, and evenly hairy. Hallux wanting; soles of feet naked, without defined pads. Humerus with entepicondylar foramen.
This genus is now represented only by a single species (S. ursinus) found in Tasmania, where, from its ferocious and destructive habits, it is commonly known under the name of the “Devil.” A front view of the skull is shown in Fig. 35.
The prevailing colour of this animal is black, and the size about equal to that of an English Badger; its habits are fossorial, and it is very destructive to sheep. On account of the similarity in the number of its teeth this genus has been generally included in the next one, but in the structure of the teeth it is much nearer to Thylacinus. An extinct species is found in the Pleistocene deposits of the mainland of Australia.
It may be observed that the two premolars missing from the typical series of four in this and the next genus are the second and the fourth; the fourth milk-molar being likewise absent. In Thylacinus and other Polyprotodonts with three premolars it is the second that is missing.
Dasyurus.[44]—Dentition: i ⁴⁄₃, c ¹⁄₁, p ²⁄₂, m ⁴⁄₄; total 42. Upper incisors nearly equal, and placed vertically; first slightly longer, narrower, and separated from the rest. Lower incisors sloping forwards and upwards. Canines large and sharply pointed. Premolars with compressed and sharp-pointed crowns, and slightly developed anterior and posterior accessory basal cusps. True molars with numerous sharp-pointed cusps. In the upper jaw the first three with crowns having a triangular oral surface, the fourth small, simple, narrow, and placed transversely. In the lower jaw the molars more compressed, with longer cusps; the fourth not notably smaller than the others. Form viverrine. Ears long and narrow, prominent, and obtusely pointed. Hallux rudimentary, or absent; its metatarsal bone always present. Tail long and well clothed with hair. Humerus without an entepicondylar foramen. Vertebræ: C 7, D 13, L 6, S 2, C 18-20.
The Dasyures are small Civet-like animals with a gray or brown pellage profusely spotted with white; they are mostly inhabitants of the Australian continent and Tasmania, where in the economy of nature they take the place of the smaller predaceous Carnivora, the Cats, Civets, and Weasels of other parts of the world. They hide themselves in the daytime in holes among rocks or in hollow trees, but prowl about at night in search of the small living mammals and birds which constitute their prey. The species are not numerous, and include D. maculatus, about the size of a common Cat, inhabiting Tasmania and the southern part of Australia; D. viverrinus, Tasmania and Victoria; D. geoffroyi, nearly all Australia; D. hallucatus, North Australia; D. albopunctatus, New Guinea.
Remains referred to D. viverrinus occur in the Australian Pleistocene deposits.
Phascologale.[45]—This genus comprises a considerable number of small Marsupials, none of them exceeding a common Rat in size, differing from the Dasyures in possessing an additional premolar—the dentition being i ⁴⁄₃, c ¹⁄₁, p ³⁄₃, m ⁴⁄₄; total 46,—and having the teeth generally developed upon an insectivorous rather than a carnivorous pattern, the upper middle incisors being larger and inclined forwards, the canines relatively smaller, and the molars with broad crowns, armed with prickly tubercles. The muzzle is pointed. Ears moderately rounded and nearly naked. Feet broad and short. Fore feet with five subequal toes, having compressed, slightly curved, pointed claws. Hind feet with the four outer toes subequal, having claws similar to those in the fore feet; the hallux always distinct and partially opposable, though small and nailless. Tail long, very variable in its covering, being either bushy, crested, or nearly naked. Pouch represented merely by a few folds of skin. Mammæ varying from four to ten in number. The food of these animals is almost entirely insects; some species pursuing their prey among the branches of trees, while others are purely terrestrial. They are found throughout Australia, and also in New Guinea and the Aru and some of the adjacent islands.
P. cristicaudata, a species with a thick compressed tail ornamented upon its apical half with a crest of black hair, differs from the others by the very reduced size of the fourth premolar in the upper, and its complete absence in the lower jaw, thus forming an interesting transition in dentition towards Dasyurus. It constitutes the genus Chætocercus of Krefft, but is included by Mr. O. Thomas in Phascologale, the frequent absence of the fourth lower premolar in P. thorbeckiana indicating that the total absence of this tooth in the known specimens of this species cannot be regarded as of generic importance. All the members of this and the two following genera can be at once distinguished from Dasyurus by the absence of white spots on the fur.
Sminthopsis.[46]—The genus Sminthopsis includes several small species allied to Phascologale but characterised by the narrowness of the hind foot, and by the soles of the feet being either granulated or hairy, instead of naked.
Antechinomys.[47]—The last genus of the Dasyurinæ is Antechinomys, represented only by A. laniger of Queensland and New South Wales. This elegant little mouse-like creature, which has large oval ears and a long tail with the terminal part bushy, is distinguished from Sminthopsis by the absence of the hallux and the great elongation of the limbs. The tympanic bullæ of the skull are also unusually large, with the mastoid portion much swollen. A full account of the habits and anatomy of this animal, which appears to be of very rare occurrence, is given in the Proc. Zool. Soc. 1880, p. 454.
Subfamily =Myrmecobiinæ=.—Molars and premolars exceeding the normal number of seven on each side. Tongue, long cylindrical, and extensile.
Myrmecobius.[48]—Dentition: i ⁴⁄₃, c ¹⁄₁, p ³⁄₃, m ⁵⁄₅ or ⁶⁄₆; total 52 or 56, being the largest number of teeth in any existing Marsupial. The distinction between the molars and premolars is founded not on a knowledge of the succession of the teeth, but on their form. The teeth are all small and (except the four posterior inferior molars) separated from each other by an interval. Head elongated, but broad behind. Muzzle long and pointed. Ears of moderate size, ovate, and rather pointed. Fore feet with five toes, all having strong, pointed, compressed claws, the second, third, and fourth nearly equal, the fifth somewhat, and the first considerably, shorter. Hind feet with no trace of hallux externally, but the metatarsal bone present. Tail long, clothed with long hairs. Fur rather harsh and bristly. Female without any pouch, the young when attached to the nipples being concealed only by the long hair of the abdomen. Vertebræ: C 7, D 13, L 6, S 3, C 23. A gland on the under surface of the body just in advance of the sternum.
Of this singular genus but one species is known, M. fasciatus (Fig. 41), found in western and southern Australia. It is about the size of an English squirrel, to which animal its long bushy tail gives it some resemblance; but it lives entirely on the ground, especially in sterile, sandy districts, feeding on ants. Its prevailing colour is chestnut red, but the hinder part of the back is elegantly marked with broad, white, transverse bands on a dark ground.
The special interest of this form lies in its apparent relationship to those Mesozoic mammals which possess a large number of true molars (see p. 114); and it is suggested by Thomas that it may eventually be found advisable to include some of the latter in the present subfamily.
Family PERAMELIDÆ.
Dentition: i ⁴⁻⁵⁄₃, c ¹⁄₁, p ³⁄₃, m ⁴⁄₄; total 46 or 48. Upper incisors small, with short broad crowns. Lower incisors moderate, narrow, proclivous. Canines well developed. Premolars compressed, pointed. Molars with quadrate tuberculated crowns. Fourth premolar preceded by a small molariform tooth, which remains in place until the animal is nearly full grown. Fore feet with two or three of the middle toes of nearly equal size, and provided with strong, sharp, slightly curved claws; the other toes rudimentary. Hind feet long and narrow; the hallux rudimentary or absent; the second and third toes very slender, and united in a common integument; the fourth very large, with a stout elongated conical claw; the fifth smaller than the fourth (see Fig. 43). The ungual phalanges of the large toes of both feet cleft at their extremities (as in Manis among the Edentata, but in no other Marsupials). Head elongated. Muzzle long, narrow, and pointed. Stomach simple. Cæcum of moderate size. Pouch complete, opening backwards. Alone among Marsupials they have no clavicles.
The Peramelidæ form a very distinct family, in some respects intermediate between the sarcophagous Dasyuridæ and the phytophagous Macropodidæ. In dentition they resemble the former, but they agree with the latter in the peculiar structure of the hind feet. In the construction of the fore feet they differ from all other Marsupials.
The Bandicoots, as these Marsupials are popularly termed, are of fossorial habits, and subsist either on an insectivorous or omnivorous diet. It has been generally considered that their syndactylous feet indicate direct affinity with the Diprotodonts, but owing to the essentially Polyprotodont character of the organisation—which extends even to their carpal and tarsal bones—Thomas dissents from this view, and concludes that their syndactylism is an independently acquired character, and that they are really a direct offshoot from the Dasyuridæ. Some individuals are remarkable for the presence of a longitudinal groove in the root of the canines, by which feature they approximate to some of the Mesozoic Polyprotodont forms. They may be divided into three genera.
Perameles.[49]—Anterior and posterior extremities not differing greatly in development. Fore feet with the three middle toes well developed, the third slightly larger than the second, the fourth somewhat shorter, provided with long, strong, slightly curved, pointed claws. First and fifth toes very short and without claws. Hind feet with hallux of one or two phalanges, forming a distinct tubercle visible externally; the second and third toes very slender, of equal length, joined as far as the ungual phalanges, but with distinct claws; the fifth intermediate in length between these and the largely developed fourth toe. Ears of moderate or small size, ovate, pointed. Tail rather short, clothed with short adpressed hairs. Fur short and harsh. Vertebræ; C 7, D 13, L 6, S 1, C 17. Skull long and narrow, with the bulla single, and its mastoid portion not inflated.
The animals of this genus are all small, and live entirely on the ground, making nests composed of dried leaves, grass, and sticks in hollow places. They are rather mixed feeders; but insects, worms, roots, and bulbs constitute their ordinary diet. The various species are widely distributed over Australia, Tasmania, New Guinea, and several of the adjacent islands, as Aru, Kei, and New Ireland. The best known are—P. gunni (Fig. 42), bougainvillei, nasuta, obesula, and macrura from Australia, and P. doreyana, raffrayana, and longicaudata from New Guinea.
Remains apparently referable to existing species are found in the cave-deposits of New South Wales.
Peragale.[50]—Molar teeth curved, typically with longer crowns and shorter roots than in the last. Hinder extremities proportionally longer, and hallux without claw. Muzzle much elongated and narrow. Fur soft and silky. Ears very large, long, and pointed. Tail long, its apical half clothed on the dorsal surface with long hairs which form a crest. Vertebræ: C 7, D 13, L 6, S 2, C 23. Skull distinguished from that of Perameles by the large size and double structure of the auditory bulla, of which the mastoid portion is inflated. There is also an abrupt contraction of the muzzle at the third premolar.
The type species of Rabbit-Bandicoot (P. lagotis), as these animals are called, is found in Western Australia, and also occurs fossil in the cave-deposits of New South Wales. It is the largest member of the family, being about the size of the common Rabbit, to which animal it bears sufficient superficial resemblance to have acquired the name of “Native Rabbit” from the colonists. It burrows in the ground, but in other respects resembles the true Bandicoots in its habits.
The smaller P. leucura has short-crowned molars, with distinct cusps, which are almost obsolete in the type species.
Chœropus.[51]—Dentition generally resembling that of Perameles, but the canines are less developed, and in the upper jaw two-rooted. Limbs very slender; posterior nearly twice the length of the anterior. Fore feet with the functional toes reduced to two, the second and third, of equal length, with closely united metacarpals and short, sharp, slightly curved, compressed claws. First toe represented by a minute rudiment of a metacarpal bone; the fourth by a metacarpal and two small phalanges without a claw, and not reaching the middle of the metacarpal of the third; fifth entirely absent. Hind foot (Fig. 43) long and narrow, mainly composed of the strongly developed fourth toe, terminating in a conical pointed nail, with a strong pad behind it; the hallux absent or represented by a rudimentary metatarsal; the remaining toes completely developed, and with claws, but exceedingly slender; the united second and third reaching a little way beyond the metatarso-phalangeal articulation of the fourth; the fifth somewhat shorter. Tail not quite so long as the body, and covered with short hairs forming a slight crest. Ears large and pointed, and folded down when the animal is at rest. Fur soft and loose. Vertebræ: C 7, D 13, L 6, S 1, C 20. Skull short and wide, with a small and single bulla, and a contraction of the muzzle at the third premolar.
The only known species of this genus (Fig. 44), chiefly remarkable for the singular construction of its limbs, is an animal about the size of a small Rat, found in the interior of the Australian continent. Its general habits and food appear to resemble those of the other Peramelidæ. It was first described as C. ecaudatus by Ogilby from a mutilated specimen, but the specific name was afterwards changed, as being inappropriate, by Gray to castanotis.
Suborder DIPROTODONTIA.
For the leading characters of this group, see page 132.
Family PHASCOLOMYIDÆ.
Dentition: c ¹⁄₁, i ⁰⁄₀, p ¹⁄₁, m ⁴⁄₄ = 24. All the teeth with persistent pulps. The incisors large, scalpriform, with enamel only on the front surface, as in the Rodentia. The molars strongly curved, forming from the base to the summit about a quarter of a circle, the concavity being directed outwards in the upper and inwards in the lower teeth. The first of the series, or premolar, appears to have no milk-predecessor, and is single-lobed; the other four composed of two lobes, each subtriangular in section. Limbs equal, stout, and short. Fore feet with five distinct toes, each furnished with a long, strong, and slightly curved nail, the first and fifth considerably shorter than the other three. Hind feet with a very short nailless hallux, the second, third, and fourth toes partially united by integument, of nearly equal length, the fifth distinct and rather shorter; all four provided with long and curved nails. In the skeleton of the foot, the second and third toes are distinctly more slender than the fourth, showing a slight tendency towards the peculiar character so marked in the next two families. Tail rudimentary. Stomach simple, provided with a special gland situated near the cardiac orifice. Cæcum very short, wide, and with a peculiar vermiform appendage. Pouch present. The auditory bullæ of the skull are imperfect, open behind, with their anterior wall formed by a descending process of the squamosal, instead of the alisphenoid. Masseteric fossa of mandible with a perforation and a deep pit.
Phascolomys.[52]—The existing Wombats (Fig. 45) comprise three species, all of which are included in the one genus Phascolomys, and all of which date from the Pleistocene.
In the typical group we find the following characters. Fur rough and coarse. Ears short and rounded. Muffle naked. Postorbital process of the frontal bone obsolete. Ribs fifteen pairs. Vertebræ: C 7, D 15, L 4, S 4, C 10-12. The Wombat of Tasmania and the islands of Bass’s Straits (P. ursinus) and the closely similar but larger animal of the southern portion of the mainland of Australia (P. mitchelli) belong to this group.
In the second group the characters are as follows. Fur smooth and silky. Ears large and more pointed. Muffle hairy. Frontal region of skull broader than in the other group, with well-marked postorbital processes. Ribs thirteen. Vertebræ: C 7, D 13, L 6, S 4, C 15-16. One species, P. latifrons, the Hairy-nosed Wombat of Southern Australia.
In their general form and actions the Wombats resemble small bears, having a somewhat similar shuffling manner of walking, but they are still shorter in the legs, and have broader, flatter backs than bears. They live entirely on the ground, or in burrows or holes among rocks, never climbing trees, and feed entirely on grass, roots, and other vegetable substances. They sleep during the day, and wander forth at night in search of food, and are shy and gentle in their habits generally, though they can bite strongly when provoked. The only noise the common wombat makes is a low kind of hissing, but the Hairy-nosed Wombat is said to emit a short quick grunt when annoyed. The prevailing colour of the last-named species, as well as of P. ursinus of Tasmania, is a brownish gray. The large wombat of the mainland is very variable in colour, some individuals being found of a pale yellowish brown, others dark gray, and some quite black. The length of head and body is about three feet.
It is noteworthy that P. mitchelli was first described from the evidence of fossil remains, the living form subsequently described as P. platyrhinus being found to be indistinguishable. Other extinct species occur in the Pleistocene of Australia.
Phascolonus.[53]—Remains of a large extinct Wombat, which must have nearly equalled the dimensions of a Tapir, occur in the Pleistocene of Queensland, and have been described as Phascolonus. It is probable that the expanded and flattened upper incisors from the same deposits upon the evidence of which the presumed genus Sceparnodon was founded, are likewise referable to the same form. The characters of both the upper and lower incisors distinguish Phascolonus from Phascolomys.
Family PHALANGERIDÆ.
Dentition extremely variable, owing to the presence of minute rudimental teeth not constant in the same species, or even in the two sides of the jaws of the same individual; exclusive, however, of Tarsipes, the formula i ³⁄₁, c ¹⁄₀, p ²⁻³⁄₀₋₂, m ³⁻⁴⁄₃₋₄ represents fairly the general condition of the functional teeth. First incisors long and stout; the lower pair very large and pointed, but without the scissor-like action found in the existing Macropodidæ; second and third lower incisors minute and probably functionless. Fourth premolar generally secant; milk-molar generally minute and deciduous at an early period. Molars either with sharp cutting-crests or bluntly tuberculate; fourth sometimes absent. Mandible without pit, and at most a very minute perforation in the masseteric fossa. Limbs subequal. Fore feet with five distinct, subequal toes, furnished with claws. Hind feet short and broad, with five well developed toes; the hallux large, nailless and opposable; the second and third slender, and united by a common integument as far as the claws. Tail generally long, and frequently more or less prehensile. Stomach simple. Cæcum present (except in Tarsipes), and usually large. Pouch complete. Animals of small or moderate size and arboreal habits, usually feeding on a vegetable or mixed diet, inhabiting Australia and the Papuan Islands.
The homologies of the lower functionless teeth between the first incisor and fourth premolar are very difficult to determine, but it is probable that one represents a canine only when the largest known number is present; this tooth, according to Mr. Thomas, being the first to disappear.
Phalangers are small woolly-coated animals, with long, powerful, and often prehensile tails, large claws, and, as in the American opossums, with opposable nailless great toes. Their expression seems in the day to be dull and sleepy, but by night they appear to decidedly greater advantage. They live mostly upon fruit, leaves, and blossoms, although some few feed habitually upon insects, and all relish, when in confinement, an occasional bird or other small animal. Several of the Phalangers possess flying membranes stretched between their fore and hind limbs (Fig. 48), by the help of which they can make long and sustained leaps through the air, like the Flying Squirrels, but it is interesting to notice that the possession of these flying membranes does not seem to be any indication of special affinity, the characters of the skull and teeth sharply dividing the flying forms, and uniting them with other species of the non-flying groups. Their skulls (Fig. 47) are as a rule broad and flattened, with the posterior part swollen out laterally, owing to the numerous air-cells situated in the substance of the squamosal.
The Phalangers are interesting from an historical point of view, since the Gray Cuscus (Phalanger orientalis) was the first of the Marsupials of the eastern hemisphere brought to the notice of Europeans, having been described in a work published at Leyden in 1611, from an account of a specimen seen at Amboyna during the third expedition of Admiral Van der Hagen.
The present family corresponds to the Dasyuridæ among the Polyprotodonts as presenting, on the whole, the most generalised types of the suborder. The existing forms may be divided into three subfamilies.
Subfamily =Tarsipedinæ=.—Cheek-teeth almost rudimentary and variable in number. Tongue long, slender, pointed, and very extensile. Tail long. Cæcum absent.
Tarsipes.[54]—So named from some supposed resemblance of its foot to that of the Lemurine genus Tarsius; but it must be remarked that it has none of the peculiar elongation of the calcaneum and navicular so characteristic of that genus. Head with elongated and slender muzzle. Mouth-opening small. The two lower incisors are long, very slender, sharp-pointed, and horizontally placed. All the other teeth are simple, conical, minute, and placed at considerable and irregular intervals apart in the jaws, the number appearing to vary in different individuals and even on different sides of the same individual. The formula, in a specimen in the Museum of the Royal College of Surgeons, is i ²⁄₁, c ¹⁄₀, p and m ³⁄₂ on one side, and ⁴⁄₃ on the other; total 20. Rami of the mandible extremely slender, nearly straight, and without coronoid process or inflected angle. Fore feet with five well-developed toes, furnished with small, flat, scale-like nails, not reaching to the extremity of the digits. Hind feet rather long and slender compared with those of the Phalangerinæ, having a well-developed opposable and nailless hallux; second and third digits syndactylous, with sharp compressed curved claws; the fourth and fifth free, and with small flat nails. Ears of moderate size and rounded. Tail longer than the body and head, scantily clothed with short hairs, prehensile. Vertebræ: C 7, D 13, L 5, S 3, C 24.
Of this singular genus but one species, T. rostratus (Fig. 46), is known, about the size of a common Mouse. It inhabits Western Australia, lives in trees and bushes, uses its tail in climbing, and feeds on honey, which it procures by inserting its long tongue into the blossoms of Melaleucæ, etc. One kept in confinement by Mr. Gould was also observed to eat flies.
Subfamily =Phalangerinæ=.—Teeth normal. One or more rudimentary teeth between the upper canine and fourth premolar, and between the first lower incisor and fourth premolar. Tongue of ordinary structure. No cheek-pouches. Stomach and ascending colon simple. Cæcum long, simple. Tail well-developed, generally prehensile.
A numerous group of animals, varying from the size of a mouse to that of a large cat, arboreal in their habits, and abundantly distributed throughout the Australian region. The members of this group are the typical representatives of the family, and are commonly known to the colonists as Opossums.
Phalanger.[55]—The typical genus Phalanger (Cuscus) presents the following characters. No flying membrane; size large or medium, and build stout and clumsy; fur thick and woolly. Ears short or medium, hairy externally, and in some cases also internally. Toes of fore feet subequal, their relative lengths in the order 4, 3, 5, 2, 1. Claws long, stout, and curved. Soles of feet naked and striated, with large ill-defined pads. Tail stout and markedly prehensile, with the proximal half furred like the body, and the terminal portion entirely naked. Four mammæ. Skull (Fig. 47) stout and strong, with large vacuities in the hinder half of the palate, and the auditory bullæ thick and inflated. Dentition usually i ³⁄₂, c ¹⁄₀, p ³⁄₃, m ⁴⁄₄. First upper incisor with nearly circular section, or only slightly flattened in front; canine more or less closely approximated to third incisor (which is very small), and situated partly in front of the suture between the premaxilla and maxilla. Fourth premolar large, secant, and placed obliquely to line of molars. Molars four-cusped, with the inner cusps of the upper ones crescentoid, and imperfect transverse ridges connecting each pair of cusps.
The Cuscuses are curious sleepy-looking animals, inhabiting the various islands of the East Indian Archipelago as far west as Celebes, and being the only Marsupials found west of New Guinea. As already noted, it was a member of this genus, the Gray Cuscus (P. orientalis), a native of Amboyna, Timor, and the neighbouring islands, which was the first Australasian Marsupial known to European naturalists. There are altogether five species known, all of about the size of a large cat; their habits resemble those of other Phalangers, except that they are said to be somewhat more carnivorous.
Trichosurus.[56]—The members of the genus Trichosurus are of relatively large size, and are distinguished from Phalanger by the following characters. Ears more or less hairy behind. Relative lengths of toes of fore feet in the order 4, 3, 2, 5, 1. Hair on the soles of the hind feet beneath the heel, but not elsewhere. Tail thick, not tapering, covered with bushy hair up to the extreme tip, which is naked, but with a naked strip on the inferior surface in the distal third or half. A gland on the chest. Dentition usually i ³⁄₂, c ¹⁄₀, p ²⁄₂, m ⁴⁄₄. Upper incisors of nearly uniform length, the first much flattened in front. Canine situated some distance behind the third upper incisor, which it scarcely exceeds in size. Last premolar and molars very similar to those of Phalanger.
The true Phalangers comprise two species, of which the best known is the Vulpine Phalanger (T. vulpecula), so common in zoological gardens, where, however, it is seldom seen, owing to its nocturnal habits. It is of about the size and general build of a small fox, whence its name. In the typical variety the colour is gray, with a yellowish white belly, white ears, and a black tail. This variety is a native of the greater part of the continent of Australia, but is replaced in Tasmania by the closely allied Brown Phalanger (var. fuliginosa). Its habits are very similar to those of the Yellow-bellied Flying-Phalanger (Petaurus australis) described below, except that it is unable to take the flying leaps of that animal. Like all the other phalangers, its flesh is freely eaten both by the natives and the lower class of settlers.
Pseudochirus.[57]—The genus Pseudochirus agrees with the preceding in the absence of a flying membrane, and presents the following leading characters. Size large or medium. Fur comparatively short and woolly. Ears medium or short, hairy behind, although seldom closely furred over all this aspect. Claws medium. Fore toes subequal, the first two distinctly opposable to the other three. Soles of feet naked, with large, striated, round pads, and hair beneath the heels. Tail tapering, markedly prehensile, with its distal third and the whole of the under surface short-haired; tip naked underneath for a short distance. Four mammæ. No gland on chest. Skull with larger nasals than in the preceding genera; the posterior part of the palate in most cases fully ossified, and the auditory bullæ generally somewhat inflated. Dentition (at most) i ²⁻³⁄₂, c ⁰⁻¹⁄₀, p ³⁄₃, m ⁴⁄₄. Upper teeth nearly uniform in length, but the first incisor distinctly longer than second. Upper premolars variable. Molars with both inner and outer cusps distinctly crescentoid, and recalling those of the Selenodont Artiodactyle Ungulates.
Range.—Tasmania, Australia, and New Guinea.
There are about ten species of this genus known, of which the commonest is Cook’s Ring-tailed Phalanger (Pseudochirus peregrinus), an animal discovered by Captain Cook during his first voyage, at Endeavour river, North Queensland.
The complex and sub-selenodont character of the molars of this and the following genus readily distinguish them from the more typical Phalangers, and show an approximation to the type of dentition prevailing in Phascolarctus; according, however, to Mr. O. Thomas, a tendency towards the same structure is observable in unworn molars of young Cuscuses. The genus may be divided into three groups, of which the first, as typified by the common P. peregrinus, is restricted to Australia and Tasmania, while the third, as represented by P. canescens, is only found in New Guinea. P. albertisi may be taken as the type of the second group, which is represented by that species in New Guinea, and by P. archeri in Queensland. With the exception of P. peregrinus, the species have a more or less restricted range. Remains of Pseudochirus, probably referable to existing species, are found in the cave-deposits of New South Wales.
Petauroides.[58]—With the genus Petauroides, containing only the single species P. volans, we come to the first of the Flying-Phalangers, characterised by the possession of a living membrane along the flanks. The characters of this genus are as follows. Size large. Fur very long and silky. Ears large and oval, thickly furred on the back, but naked internally. Flying-membrane reaching from wrist to ankle, but very narrow along the sides of the forearm and lower leg. Fore toes subequal, their relative lengths in the order 4, 3, 5, 2, 1. Claws long, curved, and sharp. Tail long, cylindrical, and bushy, except near its tip, where it is naked and prehensile. Skull short and broad, with the nasals short, and not extending nearly as far forwards as the premaxillæ. Large vacuities in hinder part of palate. Auditory bullæ inflated and smooth. Dentition usually i ³⁄₂, c ¹⁄₀, p ²⁄₂, m ⁴⁄₄. General characters of teeth very similar to those of Pseudochirus, but the first upper incisor scarcely longer than the second.
The single species is found in Australia, from Queensland to Victoria, and is commonly known as the Taguan Flying-Phalanger. The structure of the skull and teeth indicates close affinity with Pseudochirus, although the external form is widely different in the two genera. This Phalanger seems, indeed, to be, so to speak, a very specialised Pseudochirus, in which the teeth have become somewhat further diminished and the flying membrane has been developed.
Dactylopsila.[59]—The genus Dactylopsila is one of the forms without any trace of a flying membrane, its characters being as follows. Size medium. Body striped black and white. Ears oval, nearly naked at the ends. Fore toes of very unequal length, the fourth being enormously elongated; fourth and fifth toes of pes also markedly elongated. Claws long, moderately curved. Tail long, cylindrical, and evenly bushy, with the extremity more or less naked below. Skull narrow, but with the zygomatic arches greatly expanded; palate fully ossified. Dentition: i ³⁄₃, c ¹⁄₀, p ³⁄₂, m ⁴⁄₄. Upper incisors very large, the third being directed horizontally forwards; canine small and approximated to the third incisor, which it resembles. The fourth premolar of moderate size, with its longer axis placed obliquely. First lower incisor longer than in any other genus. Molars oblong, with four cusps.
The typical D. trivirgata, or Striped Phalanger, inhabits the Papuan and North Australian sub-region; a second species (D. palpator), characterised by the still greater elongation of the fourth finger, occurring in South New Guinea. These animals are said to be of insectivorous habits, the elongated fourth finger, as in the analogous instance of the Lemuroid genus Chiromys, being apparently specially adapted for extracting insects and larvæ from their hiding places.
Petaurus.[60]—Size medium or small. Fur very soft and silky. A broad flying membrane extending from the outer side of the fifth digit of the manus to the ankle. Fore toes usually increasing regularly in length from the first to the fifth, but in some of the smaller species the fourth is the longest. Claws strong, sharp, and much curved. Tail long, evenly bushy to the extremity. Glands on the chest and between the ears. Skull short and wide, with the nasals expanded posteriorly, and usually two small palatal vacuities near the second molars. Auditory bullæ inflated, and variable in size. Dentition: i ³⁄₂, c ¹⁄₀, p ³⁄₃, m ⁴⁄₄. First upper incisors very large, and taller than canine. Molars with square crowns rounded at the angles, and four cusps, except in the last, which is triangular.
This genus, which ranges from New Ireland to South Australia, but is not found in Tasmania, contains three species, the largest of which is the Yellow-bellied Flying-Phalanger (P. australis), whose habits are recorded by Mr. Gould as follows. “This animal is common in all the brushes of New South Wales, particularly those which stretch along the coast from Port Philip to Moreton Bay. In these vast forests trees of one kind or another are perpetually flowering, and thus offer a never-failing supply of the blossoms upon which it feeds; the flowers of the various kinds of gums, some of which are of great magnitude, are the principal favourites. Like the rest of the genus, it is nocturnal in its habits, dwelling in holes and in the spouts of the larger branches during the day, and displaying the greatest activity at night while running over the small leafy branches, frequently even to their very extremities, in search of insects and the honey of the newly-opened blossoms. Its structure being ill adapted for terrestrial habits, it seldom descends to the ground except for the purpose of passing to a tree too distant to be reached by flight. When chased, or forced to flight it ascends to the highest branch and performs the most enormous leaps, sweeping from tree to tree with wonderful address; a slight elevation gives its body an impetus which, with the expansion of its membrane, enables it to pass to a considerable distance, always ascending a little at the extremity of the leap; by this ascent the animal is prevented from receiving the shock which it would otherwise sustain.”
A second species, P. sciureus, in some ways one of the most beautiful of all mammals, has been chosen for the accompanying woodcut.
Gymnobelideus.[61]—Like Petaurus in every respect, but without any trace of a flying membrane, and with the fifth digit of the manus slightly shorter than the third. This genus is represented only by G. leadbeateri of Victoria, and according to Mr. Thomas, may be regarded as the primitive form from which the specialised Petaurus has been developed.
Dromicia.[62]—Size small, and general appearance dormouse-like. Ears large and thin, almost naked, and without internal or basal tufts. No flying membrane. Digits of normal proportions, the relative lengths of those of the manus in the order 3, 4, 2, 5, 1; fore claws rudimentary, hind ones long and sharp. Tail mouse-like, cylindrical, furry at base, the remainder scaly, with fine hairs, except at the tip, which is naked and prehensile. Skull short and broad, with the hinder part of the palate incomplete, and the auditory bullæ large, much inflated, and transparent. Dentition: i ³⁄₂, c ¹⁄₀, p ³⁄₃, m ³⁻⁴⁄₃₋₄. First upper incisor spatulate, and much longer than either of the others. Canine large, placed at some distance behind the third incisor. Molars (except the last) with evenly rounded crowns, carrying four small smooth cusps.
This genus, which occurs in New Guinea, Western Australia, and Tasmania, is represented by four species. It seems to be intermediate between Petaurus and Acrobates, and it has apparently had to yield place to those more highly organised types in regions where they have come in contact with one another.
Distœchurus.[63]—Size small. Ears rather short, thinly covered with hair, but with small tufts at the base. No flying membrane. Digits of normal proportions, without expanded terminal pads. Claws curved and sharp. Tail, skull, and dentition as in Acrobates, with the exception that the fourth premolar is small in the upper, and absent in the lower jaw.
The one species of Feather-tailed Phalanger (D. pennatus) is found in New Guinea.
Acrobates.[64]—Size very small. Ears moderate, thinly covered with hair, but with small tufts round the base and on the internal prominences. A narrow flying membrane, fringed with long hairs, running from the elbow to the flank, and from the latter to the knee. Four mammæ. Digits furnished with expanded and striated terminal pads, the relative length of those of the manus being in the order 4, 3, 5, 2, 1. Claws sharp, although somewhat concealed by the terminal pads. Tail short-haired above and below, with a broad fringe on either side. Skull short, wide, and depressed. Posterior portion of palate very imperfectly ossified; anterior palatal vacuities almost confined to the maxillæ. Auditory bullæ low, rounded, and but slightly prominent. Dentition: i ³⁄₂, c ¹⁄₀, p ³⁄₃, m ³⁄₃. Teeth sharp, and of an insectivorous type. Upper canine long, and approximated to third incisor. The three upper premolars large, functional, and taller than the molars. Molars small and rounded, with smooth unridged cusps.
There is only one species in this genus, the beautiful little Pigmy Flying-Phalanger (A. pygmæus), not so big as a Mouse, which is found in Queensland, New South Wales, and Victoria, and feeds on the honey it abstracts from flowers, and on insects. Its agility and powers of leaping are exceedingly great, and it is said by Mr. Gould to make a most charming little pet.
Subfamily =Phascolarctinæ=.—Teeth large, normal; no rudimentary premolars before the last upper premolar, or any teeth between the first lower incisor and fourth premolar. Tongue of ordinary structure. Distinct cheek-pouches. Stomach with a special gland near the cardiac orifice. Cæcum very long, and (with the upper portion of the colon) dilated and provided with numerous longitudinal folds of mucous membrane. In many anatomical characters, especially the possession of a special gastric gland, this group resembles the Phascolomyidæ.[65]
Phascolarctus.[66]—Dentition: i ³⁄₁, c ¹⁄₀, p ¹⁄₁, m ⁴⁄₄; total 30. Upper incisors crowded together, cylindroidal, the first much larger than the others, with a bevelled cutting edge (Fig. 36). Canine very small; a considerable interval between it and the premolar, which is as long from before backwards but not so broad as the true molars, and has a cutting edge, with a smaller parallel inner ridge. The molars slightly diminishing in size from the first to the fourth, with square crowns, each bearing four pyramidal cusps, with curved ridges radiating from them, and having a structure very similar to these of Pseudochirus. The lower incisors are semiproclivous, compressed and tapering, bevelled at the ends. Premolars and molars in continuous series, as in the upper jaw. Milk-tooth very minute, and almost functionless. Fore feet with the two inner toes slightly separated from and opposable to the remaining three, all with strong, curved, and much compressed claws. Hind foot (Fig. 49) with the hallux placed very far back, large and broad, the second and third (united) toes considerably smaller than the other two; the fourth the largest. No external tail. Fur dense and woolly. Ears of moderate size, thickly clothed with long hairs. Vertebræ: C 7, D 11, L 8, S 2, C 8. Ribs eleven pairs, a rare exception to the usual number (13) in the Marsupialia.
There is but one species, the Koala or Native Bear of the Australian colonists (P. cinereus), an animal of comparatively large size and heavy build (Fig. 50), found in the south-eastern parts of the Australian continent. It is about two feet in length, and of an ash-gray colour, an excellent climber, and residing generally in lofty Eucalyptus trees, on the buds and tender shoots of which it feeds, though occasionally descending to the ground in the night.
EXTINCT PHALANGEROIDS.
Numerous imperfect remains recently described by De Vis are regarded as indicating large extinct types of Phalangeridæ, but further evidence is required before all these determinations can be definitely accepted. Thus part of an upper jaw is provisionally referred to a large species of Pseudochirus, while part of a scapula is made the type of a genus Archizonurus which appears to be allied to the former. Another fragmentary scapula is considered to indicate a large Phalanger. Finally, part of a fibula, described under the name of Koalemus is regarded as affording evidence of the former existence of a large ancestral form allied to the Koala, and it is suggested that an upper jaw with teeth may belong to the same or an allied type.
Thylacoleo.[67]—Dentition of adult: i ³⁄₁, c ¹⁄₀, p ³⁄₃, m ¹⁄₂; total 28. First upper incisor much larger than the others; canine and first two premolars rudimentary. In the lower jaw the two small anterior premolars are functionless, and often deciduous; posterior premolars of both jaws formed on the same type as those of Potorous, but relatively much larger; true molars rudimentary, tubercular. One species, T. carnifex. This animal presents a most anomalous condition of dentition, the functional teeth being reduced to one pair of large cutting incisors situated close to the median line, and one great, trenchant, compressed premolar, on each side above and below. It was first described as a carnivorous Marsupial, and named, in accordance with its presumed habits, “as one of the fellest and most destructive of predatory beasts”; but, as its affinities are certainly with the Phalangeridæ and Macropodidæ, and its dentition completely unlike that of any known predaceous animal, this view has been called in question.
The dentition is nearer to that of the existing Phalangeridæ than to that of the Macropodidæ, and the genus may be provisionally regarded as the type of a distinct subfamily of the former.
Family MACROPODIDÆ.
Dentition i ³⁄₁, c ⁰⁻¹⁄₀, p ²⁄₂, m ⁴⁄₄. Incisors sharp and cutting, those of the lower jaw frequently having a scissor-like action against one another; upper canine, if present, small. Penultimate premolar shed with the fourth milk-molar, which is molariform and long persistent. Molars wide, and either transversely ridged or bluntly tuberculate. Premolars and molars moving forwards in the skull as the age of the animal increases, this being most marked in the larger species. Masseteric fossa of mandible hollowed out below into a deep cavity walled in externally by a plate of bone, and communicating with the inferior dental canal by a large foramen. Hind limbs usually larger than the anterior ones, and progression generally saltatorial. Fore feet with five digits; hind feet syndactylous, the fourth digit being very large and strongly clawed; hallux usually absent. Tail generally long and hairy, occasionally prehensile; stomach sacculated. Pouch large and opening forwards.
The Macropodidæ or Kangaroos, taken as a whole, form a very well-marked family, easily distinguished from the other members of the suborder by their general conformation, and by peculiarities in the structure of their limbs, teeth, and other organs. They vary in size from that of a sheep down to a small rabbit. The head, especially in the larger species, is small, compared with the rest of the body, and tapers forward to the muzzle. The shoulders and fore limbs are feebly developed, and the hind limbs usually of disproportionate strength and magnitude, which gives them a peculiarly awkward appearance when moving about on all fours, as they occasionally do when feeding. Rapid progression is, however, performed only by the powerful hind limbs, the animal covering the ground by a series of immense bounds, during which the fore part of the body is inclined forwards, and balanced by the long, strong, and tapering tail, which is carried horizontally backwards. When not moving they often assume a perfectly upright position, the tail aiding the two hind legs to form a sort of supporting tripod, and the front limbs dangling by the side of the chest. This position gives full scope for the senses of sight, hearing, and smell to warn of the approach of enemies, from which these animals save themselves by their bounding flight. The fore paws have live distinct digits, each armed with a strong curved claw.
The hind foot (Fig. 52), as being a typical example of the syndactylous modification, may be noticed in some detail. It is extremely long and narrow, and (with only one exception) without any hallux or great toe. It consists mainly of one very large and strong toe, corresponding to the fourth of the human or other typically developed foot, ending in a strong, curved, and pointed claw. Close to the outer side of this lies a smaller fifth digit, and to the inner side two excessively slender toes (the second and third), bound together almost to the extremity in a common integument. The two little claws of these toes, projecting together from the skin, may be of use in scratching and cleaning the fur of the animal, but the toes themselves must have quite lost all connexion with the functions of support or progression.
The dentition of the Kangaroos, functionally considered, consists of sharp-edged incisors, most fully developed near the median line of the mouth, for the purpose of cropping the various kinds of herbage on which they feed, and ridged and tuberculated molars for crushing it, there being no tusks or canines for offensive or defensive purposes.
The number of vertebræ is—in the cervical region 7, dorsal 13, lumbar 6, sacral 2, caudal varying according to the length of the tail, but generally from 21 to 25. In the fore limb the clavicle and the radius and ulna are well developed, allowing of considerable freedom of motion of the hand. The pelvis has large epipubic or “marsupial” bones. The femur is short, and the tibia and fibula are of great length, as is the foot, the whole of which is applied to the ground when the animal is at rest in the upright position.
The stomach is of large size, and very complex, its walls being puckered up by longitudinal muscular bands into a great number of sacculi, like those of the human colon. The alimentary canal is long, and the cæcum well developed. All the species have a marsupium or pouch formed by a fold of the skin of the abdomen, covering the mammary glands with their four nipples. In this pouch the young are placed as soon as they are born; there their growth and development proceeds; and to it they resort temporarily for the purpose of shelter, concealment, or transport, for some time after they are able to run and jump about the ground and feed upon the same herbage which forms the nourishment of the parent. During the early period of their sojourn in the pouch, the blind, naked, helpless young creatures (which in the Great Kangaroo [Fig. 53] scarcely exceed an inch in length) are attached by their mouths to the nipples of the mother, and are fed by milk injected into their stomach by the contraction of the muscle covering the mammary gland.
The Kangaroos are all vegetable feeders, browsing on grass and various kinds of herbage, the smaller species also eating roots. They are naturally timid, inoffensive creatures; but the larger ones when hard pressed will turn and defend themselves, sometimes killing a dog by grasping it in their fore paws, and inflicting terrible wounds with the sharp claws of their powerful hind legs, sustaining themselves meanwhile upon the tail. A few aberrant forms are arboreal. The great majority are inhabitants of Australia and Tasmania, forming one of the most prominent and characteristic features of the fauna of these lands, and in the scenery of the country, as well as the economy of nature, performing the part of the deer and antelopes of other parts of the world, which are entirely wanting in Australia. Kangaroos were very important sources of food-supply to the natives, and are hunted by the colonists, both for sport and with a view to their destruction, on account of the damage they naturally do in consuming the grass, now required for feeding cattle and sheep. Notwithstanding this, they have in some districts increased in numbers, owing to the suppression of their former enemies, the aborigines and the Dingo or native dog. A few species are found in New Guinea and the adjacent islands, which belong, in the zoological sense, to the Australian region.
Before noticing the various generic types of the Macropodidæ, a few words are necessary in respect of the tooth-change, and we may here quote the observations of Mr. O. Thomas on this subject. “The full dentition of the members of this family consists, in the upper jaw, first of three incisors, then of a small canine (often, however, suppressed, as in Fig. 55), and then of six cheek-teeth, of which the second in the series is the only one which has a milk or deciduous predecessor, and is therefore the one to be regarded as the last premolar of the typical mammalian dentition. The special characteristics that render the development and succession of the teeth in the Macropodidæ, and especially in the genus Macropus, so puzzling to systematic zoologists, are: firstly, a general progression forwards in the jaw of the whole tooth-row, comparable to that found elsewhere only in the Elephants and some Sirenians; and, secondly, the fact that before the tooth-change the first tooth of the series (p 3) and the single milk-tooth (dm 4) placed next to it, both of which fall out at the change, are respectively so very similar in shape and size to the first and second teeth of the permanent series, viz. the permanent premolar (p 4) and the first molar (m 1), as to be most naturally mistaken for, or compared with, them in specific descriptions.... The necessary knowledge as to the stage of dentition in which any skull may be, can often be gained only by cutting open the bone either above and behind the first tooth of the series to see if the true permanent p 4 be still buried there (in which case, of course, that first tooth is only p 3), or behind the last visible molar to see if there be yet another tooth behind it, showing it to be m 3 and not m 4. The first plan is, as a rule, the better, since p 4 is generally by far the most important tooth for diagnostic purposes, and its characters have, therefore, in any case to be taken into account.”
The Macropodidæ are divided into three well-marked sections: (1) the true Kangaroos (Macropodinæ); (2) a group consisting of smaller animals, commonly called Rat Kangaroos, or (improperly) “Kangaroo Rats,” or sometimes Potoroos; and (3) the Hypsiprymnodontinæ, now represented only by a single species.
Subfamily =Hypsiprymnodontinæ=.—Size very small. Claws small, feeble, and subequal. Hind feet with an opposable hallux. Tail naked and scaly. The fourth premolar twisted obliquely outwards, as in Phalanger. Other teeth as in the Potoroinæ.
This subfamily is now represented only by the genus Hypsiprymnodon,[68] which is a form of great interest, as showing a structure of foot connecting that of the Kangaroos with that of the Phalangers. The single known species, H. moschatus, was described by Ramsay from specimens discovered in north-east Australia. It was described almost simultaneously by Owen under the name of Pleopus nudicaudatus. From the resemblance in the structure of the foot and the obliquity of the premolars to the Phalangers Mr. Thomas has some hesitation as to which family should receive this genus, but the macropine characters of the mandible preponderate in favour of the Macropodidæ.
Triclis.[69]—A lower jaw of a much larger form from the Pleistocene deposits of Australia apparently indicates another member of this subfamily, having the outwardly directed and grooved premolar characteristic of Hypsiprymnodon. It differs, however, from that genus, and also from all other known Macropodidæ, in having a small tooth between the incisor and fourth premolar, which apparently represents a canine, or perhaps an anterior premolar. This form indicates, therefore, a closer connexion between the Phalangeridæ and Macropodidæ than any other.
Subfamily =Potoroinæ=.—The second section or subfamily, the Potoroinæ, have the first upper incisor narrow, curved, and much exceeding the others in length (Fig. 54). Upper canines always persistent, flattened, blunt, and slightly curved. Premolars of both jaws always having large, simple, compressed crowns, with a nearly straight or slightly concave free cutting edge, both outer and inner surfaces usually marked by a series of parallel, vertical grooves and ridges, these teeth being either set in the same line with the molars, or slightly bent outwards. Molars with quadrate crowns, having a blunt, conical cusp at each corner, the fourth notably smaller than the third, sometimes rudimentary, and appearing early. Fore feet narrow; three middle toes considerably exceeding the first and fifth in length; their claws long, compressed, and but slightly curved. Hind feet as in Macropus. Tail long and hairy, sometimes partially prehensile, being used for carrying bundles of grass with which these animals build their nests.
The Potoroos or Rat Kangaroos are all small animals, none of them exceeding a common rabbit in size. They inhabit Australia and Tasmania, are nocturnal, and feed on the leaves of various kinds of grasses and other plants, as well as roots and bulbs, which they dig up with their fore paws. Nine species are known, presenting a considerable range of diversity in minor characters, and admitting of being grouped in four principal sections, which may be allowed the rank of genera. These are:
Potorous.[70]—Head long and slender. Auditory bullæ somewhat inflated. Ridges on premolars few and perpendicular. Large palatine foramina. Tarsus short. Muffle naked. Three species, viz. P. tridactylus, P. gilberti, and P. platyops; the last two being confined to West Australia.
Bettongia.[71]—Head comparatively short and broad. Ears short and rounded. Auditory bullæ generally much inflated. Large palatine foramina. Tarsus long. Ridges on premolars numerous and oblique. Tail more or less prehensile, thickly haired, and the hairs on the upper surface longer than those on the lower, and forming a crest. Muffle naked. Four species, viz. B. penicillata, B. cuniculus, B. gaimardi, B. lesueuiri.
Caloprymnus.[72]—Muffle naked, as in Bettongia, but the edge of the hairy part less emarginate backwards in the middle line. Ears short, rounded, and hairy. Auditory bullæ much inflated, and of large size. Nasals larger and wider behind than in the other genera. Very long anterior palatine foramina. Limbs as in Bettongia. Tail thin, cylindrical, evenly coated with short hair, without trace of a crest. Skull broad and flat, with a remarkably short and conical muzzle. The sole representative of this genus is C. campestris of South Australia, originally referred to Bettongia.
Æpyprymnus.[73]—Head short and broad. Auditory bullæ not inflated. No palatine foramina. Tarsus long. Muffle partially hairy. Tail evenly hairy, not crested above. Molars oblong, less distinctly quadritubercular, and not decreasing so much in size posteriorly as in the other genera. Represented only by Æ. rufescens.
Remains of Æ. rufescens occur in the Pleistocene cave-deposits of New South Wales.
Subfamily =Macropodinæ=.—This subfamily includes the largest forms. The cutting edges of the upper incisors are nearly level, or the first pair but slightly longer than the others (Fig. 55). The canines are rudimentary and often wanting. The premolars are usually not longer (from before backwards) than the true molars and less compressed than in the last subfamily; they are placed in precisely the same line with the molars. The crowns of the molars always have two prominent transverse ridges; and these teeth increase in size from before backwards, the fourth molar appearing very late. The fore limbs are small, with subequal toes armed with strong, moderately long, curved claws. Hind limbs very long and strongly made. Head small, with more or less elongated muzzle. Ears generally rather long and ovate.
Upwards of forty-four existing species of this group have been described, and many attempts have been made to subdivide them into smaller groups or genera for the convenience of arrangement and description, but these have generally been based upon such trivial characters that it is preferable to speak of many of them as sections of the genus Macropus, reserving generic rank only to forms somewhat aberrant in structure. According to this arrangement the genera will be as follows:
Lagostrophus.[74]—Represented only by the Banded Wallaby (L. fasciatus) of Western Australia, which presents the following distinctive features. Size small. Muffle naked. Hind feet covered with long bristly hairs, concealing the claws. Lower part of back marked by dark cross-bands. Skull with a narrow pointed muzzle and inflated auditory bullæ; symphysis of mandible firmly united. No canine. Upper incisive series meeting at a sharp angle, and diverging but slightly behind. First incisor smaller in section than either of the others and scarcely longer, bluntly pointed; second with a flattened oral surface; third smaller, similarly flattened, but with a groove on oral surface forming a notch at its postero-external angle. Fourth premolar short, with a distinct inner ledge. Molars as in Macropus.
Dendrolagus.[75]—General proportions of limbs and body normal and unlike those of other members of the family. Muffle broad and only partly naked. Fur on nape, and sometimes on back, directed forwards. Fore limbs nearly as large as the hind; hind feet with the syndactylous second and third digits relatively large; claws of fourth and fifth hind digits curved like those of the manus. Tail very long, and thickly furred. Skull stout, with a short and wide muzzle; the posterior part of the palate fully ossified, and the auditory bullæ not inflated. A small canine. Fourth premolar large, but much shorter antero-posteriorly than in the next genus; molars as in the latter.
This genus includes four species of Tree-Kangaroos, three of which occur in New Guinea, while D. lumholtzi is found in North Queensland. They differ greatly from all the other forms in being chiefly arboreal in their habits, climbing with facility among the branches of large trees, and feeding on the bark, leaves, and fruit. They are confined to the tropical forests of the regions mentioned; and it would appear that we must regard their resemblance in the proportions of the limbs and habits to the Phalangers as having been independently acquired.
Dorcopsis.[76]—Hind limbs relatively less large than in Macropus. Muffle large, broad, and naked. Ears small. Fur on nape directed wholly or partially forwards. Hind claws not concealed by hair. Tail with a nearly naked tip. Skull long and narrow, with the auditory bullæ not inflated. A well-developed canine. First upper incisor somewhat short; second and third nearly equal, notched externally. Fourth premolar greatly elongated antero-posteriorly, its length generally exceeding the united lengths of the first and second molars; a distinct inner ledge, and vertical grooves on both sides. Molars low and rounded, with the median longitudinal bridge between the ridges almost or quite aborted, and the talon in front of the first transverse ridge very narrow, and not extending to the inner side. The two series of cheek-teeth parallel, or nearly so, instead of converging at the extremities.
Three species of this genus are known, all of which are from New Guinea; the type being D. muelleri. In the characters of the dentition, the forward inclination of the fur on the nape, and other points, this genus is allied to Dendrolagus; but Dorcopsis macleayi connects the other species with Macropus.
Lagorchestes.[77]—Muffle entirely or partially covered with hair. Fourth hind digit with a long claw, not concealed by hair. Tail rather short, evenly furred, without a spur. Skull with short muzzle and diastema, and inflated auditory bulla. Canine present, sometimes very small. Fourth premolar large, not constricted in the middle, with a continuous inner ledge.
This genus includes the Hare-Kangaroos, a group of small hare-like animals, great leapers and swift runners, which mostly affect the open grassy ridges, particularly those of a stony character, sleeping in forms or seats like the common hare. Their limbs are comparatively small, their claws sharp and slender, and their muffle is clothed with velvet-like hairs. Three species—M. leporoides, M. hirsutus, M. conspicillatus.
The range extends over the whole of Australia, but does not embrace Tasmania.
Onychogale.[78]—Muffle hairy. Fourth hind claw long, narrow, compressed, and sharp. Tail long and tapering, covered with short hair, and furnished at the tip with a horny spur. Skull nearly as in Macropus, with the auditory bullæ more or less inflated. Canine small or wanting. Upper incisors small, decreasing in size from first to third. Fourth premolar small, hour-glass shaped, and without inner ledge. Molars as in Macropus.
This genus contains three species, having the same distribution as Lagorchestes. Mr. O. Thomas observes: “The spur-tailed Wallabies form a natural little group, distinguished both by the shape of the incisors and the peculiar horny excrescence at the tip of the tail. The latter character is altogether unique among Marsupials, and is only found among other mammals in the Lion, which occasionally has a somewhat similar horny spur at the end of its tail. In the case of the Wallabies it is difficult to conceive what can be the use of this spur; and observations on the living animal are much needed with regard to this interesting point.”
Petrogale.[79]—Muffle naked. Fur of nape directed backwards. Claw of fourth hind digit very short. Tail long, cylindrical, thinner than in Macropus, and thickly haired and pencilled at the extremity. Skull as in the smaller species of Macropus, with large posterior palatal vacuities, and the bullæ sometimes inflated. No canine. Upper incisors small, the third resembling that of Macropus. Fourth premolar large and stout, as in some of the Wallabies, with a continuous inner ledge, and two or three indistinct vertical ridges externally. Molars as in the Wallabies.
This genus is represented by six species, of which P. penicillata is a well-known example, ranging over the whole of the mainland of Australia. The Rock-Wallabies, as its members may be called, are very closely allied to some of the true Wallabies; and some hesitation may be expressed as to the advisability of accepting their generic separation from Macropus. They inhabit rocky regions, making their retreats in caverns and crevices, leaping with surprising agility from one narrow ledge to another, and browsing upon the scanty herbage that the neighbourhood of such situations affords. The species are P. xanthopus, P. penicillata, P. lateralis, P. concinna, P. brachyotis, P. inornata.
Remains of P. penicillata are found in a fossil state in the Pleistocene cave-deposits of New South Wales.
Macropus.[80]—Muffle generally completely naked. Ears large. Fur on nape (with an occasional exception in two species) directed backwards. Claw of fourth hind digit very long. Tail thick, tapering, and evenly furred. Four mammæ. Skull (Fig. 55) long, smooth, and rounded; the nasals expanded behind; generally large palatal vacuities; and the auditory bullæ not inflated. Canine minute, and shed at an early period. Incisor series forming an open curve; the first the tallest, and the third nearly always the longest antero-posteriorly, and generally with an infolding of enamel near its postero-external angle. Fourth upper premolar with a secant edge, and an inner basal ledge or tubercle; corresponding lower tooth secant; both maybe longer or shorter than first molar. Molars (except very occasionally) with a distinct longitudinal bridge connecting transverse ridges. Lower incisors long and scalpriform, with inner secant edges opposable, owing to the loose articulation of the mandibular symphysis.
This genus includes the true Kangaroos and Wallabies, the size of the individual existing species varying from that of a Rabbit to that of a Man. There are no less than twenty-three existing species, which may be divided into three groups, as well as many extinct ones. The genus is found in Australia and New Guinea, as well as in the eastern half of the Austro-Malayan transitional region.
The first group, or true Kangaroos, comprises the largest existing forms, which are generally of a uniform and sombre colour.
The skull is of a large and massive type, with the palate more or less well ossified posteriorly, while the molars frequently have a median longitudinal bridge connecting the first transverse ridge with the anterior talon, and no antero-external bridge between the same ridge and talon. The history of the discovery of the typical representative of this group, as being of considerable interest, may be given at some length. When Captain Cook, during his first memorable voyage of discovery, was detained for the purpose of refitting his ship at Endeavour river on the north-east coast of Australia, a strange-looking animal, entirely unknown to them, was frequently seen by the ship’s company; and it is recorded in the annals of the voyage that, on the 14th of July 1770, “Mr. Gore, who went out this day with his gun, had the good fortune to kill one of the animals which had been so much the subject of our speculation, ... and which is called by the natives kanguroo,” a name which, though it does not appear to be now known to any of the aboriginal tribes of the country, has been adopted for this animal in all European languages, with only slight modifications of spelling. With the exception of a passing glimpse in the beginning of the same century by the Dutch traveller Bruyn of some living examples of an allied species, this was the first introduction to the civilised world of any member of a group of animals now so familiar. The affinities of the species, skins of which were brought home by Captain Cook and subsequent voyagers, were recognised by Schreber as nearer to the American opossums (then the only known Marsupials) than to any other mammals with which zoologists were acquainted, and consequently it was placed by him, in his great work on the Mammalia, then in the course of publication, in the genus Didelphys, with gigantea for a specific designation,—the latter having been bestowed upon it by Zimmermann under the impression that it was a huge species of jerboa. Soon afterwards (1791) Dr. Shaw very properly formed a new genus for its reception, which he named Macropus, in allusion to the peculiar length of its hind foot. By the name thus formed, Macropus giganteus, this kind of Kangaroo has ever since been known in zoological literature. It is the common Gray Kangaroo, called “boomer,” “forrester,” or “old man” by the colonists, and frequents the open grassy plains of the greater part of eastern Australia and Tasmania; a figure being given in the woodcut on p. 160. The muffle is partly covered with hair, and the fourth premolar very short. Several varieties are known.
A sub-group, distinguished from the above by the naked muffle, includes some very large and handsome species, which principally dwell in rocky mountain ranges, as M. rufus, the great Red Kangaroo, M. antilopinus, and M. robustus. The fourth premolar is of large or medium size in these forms. Remains of M. giganteus occur fossil in the Pleistocene of Australia, where we also find the allied extinct M. titan, which attains somewhat larger dimensions. M. robustus also dates from the same geological epoch, where it was accompanied by two allied types known as M. altus and M. cooperi.
The second group includes the larger Wallabies, which are smaller than the true Kangaroos, with a brighter and more variegated coloration. The palate is generally more incomplete than in the typical group; and in the molars the anterior talon is connected with the first transverse ridge by an external instead of a median longitudinal bridge. The members of this group are frequenters of forests and dense impenetrable brushes and scrubs, and hence are often called Brush Kangaroos, though a native name, “Wallaby,” is now generally applied to them. There are several species, of which M. ruficollis, M. ualabatus, M. parryi, and M. agilis are the best known.
M. ualabatus and M. parryi are found fossil in the Pleistocene deposits of Australia. In those beds we also meet with remains of several very large extinct species, which appear to be allied to those Wallabies in which the fourth premolar is large and elongated, all of them agreeing with the Wallabies in the absence of the median bridge between the first ridge and talon of the molars. These fossil forms comprise M. brehus, in which the skull was probably about one foot in length, and M. rœchus, and M. anak, which were of somewhat inferior dimensions. In the last-named species the length of the fourth upper premolar is equal to that of the first and half of the second molar.[81]
The third and last group of the genus includes the small Wallabies, which are small and lightly-built animals, in some instances not larger than a Rabbit. Their muffles are always naked, and in the skull the anterior palatine foramina are small and the posterior vacuities very large, while the posterior expansion of the nasals is very marked. The third upper incisor is smaller than in the last group. This group extends farther into the tropics than either of the others, being found in the New Britain and Aru islands, as well as in New Guinea. M. brachyurus is remarkable for its comparatively short and slender tail and small ears. The earliest known species of Kangaroo, referred to before, M. bruni, belongs to this section. Several examples were seen by Bruyn in 1711 living in captivity in the garden of the Dutch governor of Batavia, and described and figured in the account of his travels (Reizen over Moskovie, etc.) under the name of “Filander.” It was quite lost sight of, and its name even transferred by S. Müller to another species (Dorcopsis muelleri), until rediscovered in 1865 by Rosenberg, who sent a series of specimens to the Leyden Museum from the islands of Aru and Great Key, thus determining its true habitat. M. thetidis is a well-known Australian representative of this group.
Extinct genera.—In addition to the fossil forms already mentioned which can be referred to existing genera, there are others from the Australian Pleistocene indicating extinct generic types of Macropodidæ, to which brief reference may now be made. The first of these is Sthenurus,[82] represented by a single large species (S. atlas), and characterised by the presence of a complete inner lobe to the fourth upper premolar, and of an outer one in the opposing lower tooth, so that these teeth present a flat and oval grinding surface when worn. The median longitudinal bridge connecting the transverse ridges of the molars is very imperfect; and in the upper molars there is no bridge between the first ridge and talon. In Procoptodon[83] the premolars resemble those of Sthenurus, but the molars are elongated, and usually have their enamel thrown into numerous vertical foldings. The most distinctive feature is, however, the complete ankylosis of the mandibular symphysis; the mandibular rami being deep, and the diastema in the dental series short. The lower incisors are nearly cylindrical, and the palate has large vacuities. Three species are known. The largest representation of the whole family is the type of the genus Palorchestes[84] (P. azael), in which the length of the skull is estimated at sixteen inches. It is distinguished from Procoptodon by the longer mandibular symphysis and diastema, and the spatulate lower incisors. The true molars have no distinct anterior talon, and are not grooved, while the palate was fully ossified.
EXTINCT FAMILIES.
Here may be noticed two genera of extinct Marsupials, the remains of which have been found in the Pleistocene deposits of Australia, which agree with the Macropodidæ and the Phalangeridæ in having ³⁄₁ incisors, those of the lower jaw being very large and proclivous. As the whole of their structure, especially that of the hind feet, is not yet known, their precise affinities cannot be determined.
Diprotodon.[85]—Dentition: i ³⁄₁, c ⁰⁄₀, p ¹⁄₁, m ⁴⁄₄; total 28. The first upper incisor very large and scalpriform (Fig. 56). True molars with prominent transverse ridges, as in Macropus, but wanting the longitudinal connecting bridge. Anterior and posterior limbs less disproportionate than in the Kangaroos. Humerus elongated, and differing from that of nearly all Marsupials in the absence of an entepicondylar foramen. The palate is fully ossified, and there is no pit or perforation in the masseteric fossa of the mandible. D. australis is the largest known Marsupial, being fully equal in bulk to a Rhinoceros. It may be regarded as the type of a family—Diprotodontidæ—having affinity on the one hand with the Phalangers and on the other with the Kangaroos.
Nototherium.[86]—Represented by a species of somewhat smaller size than the type of Diprotodon, with a shorter skull, in which the zygomatic arches are very wide and the nasals curiously expanded at their extremities. The mandibular symphysis is ankylosed; and, as in Diprotodon, there appears to have been no tooth-change. The humerus probably referable to Nototherium is of a short and widely expanded type, with a large entepicondylar foramen, and coming nearer to that of the Wombat than to that of any other existing form. The Nototheriidæ may apparently be regarded as a distinct family connecting the Diprotodontidæ with the Phascolomyidæ and Phalangeridæ.
Bibliography of Marsupialia.—G. R. Waterhouse, Nat. Hist. of the Mammalia, vol. i. “Marsupiata,” 1846; J. Gould, Mammals of Australia, 1863; R. Owen, article “Marsupialia,” in Cyclop. of Anatomy and Physiology, and various memoirs “On Extinct Mammals of Australia” in Philosophical Transactions; W. H. Flower, “On the Development and Succession of the Teeth in the Marsupialia,” Phil. Trans. 1867; O. Thomas, “On the Homologies and Succession of the Teeth in the Dasyuridæ,” Phil. Trans. 1887; and “Catalogue of Marsupialia and Monotremata in the British Museum,” 1888.