CHAPTER 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.