CHAPTER III

THE CLASSIFICATION OF THE MAMMALIA

The terminology and nomenclature of science form a great barrier, which only too often shuts out the educated layman from following the course of investigation and keeping abreast of the discoveries in which he may be particularly interested. No more frequent and heartfelt complaint is uttered than that which decries the “scientific jargon,” and one might be tempted to think that this jargon was a superfluous nuisance, deliberately adopted to exclude the uninitiated and guard the secrets of the temple from the curious intruder. As a matter of fact, however, this terminology, though an unquestionable evil from one point of view, is an indispensable implement of investigation and description. Ordinary language has far too few words for the purpose and most of the words that might be used lack the all-important quality of precision. The vernacular names of animals and plants are notoriously inexact and, even when not inaccurately employed, are not sufficiently refined and distinctive for scientific use. This is pre-eminently true of the New World, where the European settlers gave the names of the creatures with which they had been familiar at home to the new animals which they found in the western hemisphere. Some of these names, such as deer, wolf, fox, bear, are accurate enough for ordinary purposes, while others are ludicrously wrong. The bird that we call the Robin is altogether different from his European namesake, and the great stag, or Wapiti, is commonly called “Elk,” a name which properly belongs to the Moose. In short, it is impossible to gain the necessary accuracy and abundance of vocabulary without devising an artificial terminology, drawn chiefly from Greek and Latin.

In dealing with fossils, the difficulty of nomenclature becomes formidable indeed. The larger and more conspicuous mammals of the modern world are more or less familiar to all educated people, and such names as rhinoceros, hippopotamus, elephant, kangaroo, will call up a definite and fairly accurate image of the animal in question. For the strange creatures that vanished from the earth ages before the appearance of Man _there are no vernacular names_ and it serves no good purpose to coin such terms. To the layman names like _Uintatherium_ or _Smilodon_ convey no idea whatever, and all that can be done is to attempt to give them a meaning by illustration and description, using the name merely as a peg upon which to hang the description.

The system of zoölogical classification which is still in use was largely the invention of the Swedish naturalist Linnæus, who published it shortly after the middle of the eighteenth century. As devised by Linnæus, the scheme was intended to express ideal relationships, whereas now it is employed to express real genetic affinities, so far as these can be ascertained. The Linnæan system is an organized hierarchy of groups, arranged in ascending order of comprehensiveness. In this scheme, what may be regarded as the unit is the _species_, a concept around which many battles have been waged and concerning which there is still much difference of opinion and usage. Originally a term in logic, it first received a definite meaning in Zoölogy and Botany from John Ray (1628-1705) who applied it to indicate a group of animals, or plants, with marked common characters and freely interbreeding. Linnæus, though not always consistent in his expressions on the subject, regarded species as objective realities, concrete and actual things, which it was the naturalist’s business to discover and name, and held that they were fixed entities which had been separately created. This belief in the fixity and objective reality of species was almost universally held, until the publication of Darwin’s “Origin of Species” (1859) converted the biological world to the evolutionary faith, which declares that the only objective reality among living things is the individual animal or plant.

According to this modern conception, a species may be defined as signifying a “grade or rank assigned by systematists to an assemblage of organic forms which they judge to be more closely interrelated by common descent than they are related to forms judged to be outside the species” (P. Chalmers Mitchell). The technical name of a species, which is either in Latin, or in latinized form, is in two words, one of which designates the genus (see below) and the other the particular species of that genus, as, for example, _Equus caballus_, the species Horse, _E. przewalskii_, the Asiatic Wild Horse, _E. asinus_, the species Ass, etc. In order to identify a species, the genus to which it belongs must be stated, hence the term, _binomial system_ of nomenclature, which Linnæus introduced, becoming _trinomial_ when the name of a subspecies is added, a modern refinement on the older method. A very large species (_i.e._ one which is represented by great numbers of individuals), extending over a very large area, is often divisible into groups of minor rank, as _varieties_, _geographical races_ or _subspecies_. Taking the species as the unit in the scheme of classification, the varieties and subspecies may be considered as fractions.

There is great difference of usage among writers on systematic zoölogy in the manner of applying the generally accepted concept of species, some making their groups very much more comprehensive than others, according as they are “lumpers” or “splitters,” to employ the slang phrase. The difficulty lies in the fact that there are no fixed and definite criteria, by which a given series of individuals can be surely distinguished as a variety, a species or a genus; it is a matter for the judgment and experience of the systematist himself. The individuals of a species may differ quite widely among themselves, provided that they are all connected by intergradations, and the more or less constant varieties or subspecies are to be distinguished from the individual variants, which are inconstant and fluctuating. No two specimens agree exactly in every particular, but if a very large suite of them be compared, it will be found that the great majority depart but little from the average or norm of the species, and the wider the departure from the norm, the fewer the individuals which are so aberrant. Taking so easily measured a character as size, for example, and measuring several hundred or a thousand representatives of some species, we see that a large majority are of average size, a little more or a little less, while very large or very small individuals are rare in proportion to the amount by which they exceed or fall short of the norm. Subspecies or varieties are marked by differences which are relatively constant, but not of sufficient importance to entitle them to rank as species.

A group of the second rank is called a _genus_, which may contain few or many species, or only a single one. In the latter case the species is so isolated in character that it cannot properly be included in the same genus with any other species. A large genus, one containing numerous species, is frequently divisible into several _subgenera_, each comprising a group of species which are more similar to one another than they are to the other species of the genus.

The third of the main groups in ascending order is the _family_, which ordinarily consists of a number of genera united by the possession of certain common characters, which, at the same time, distinguish them from other genera, though a single isolated genus may require a separate family for its reception. Just as it is often convenient to divide a genus into subgenera, so families containing many genera are usually divisible into _subfamilies_, as indicative of closer relationships within the family. The name of the family is formed from that of the genus first described or best known, with the termination _-idæ_, while that for the subfamily is _-inæ_. To take an example, all the genera of cats, living and extinct, are assembled in the family Felidæ (from the genus _Felis_) which falls naturally into two subfamilies. One of these, the Felinæ, includes the true cats, a very homogeneous group, both the existing and the extinct genera; the other subfamily, that of the highly interesting series of the “Sabre-tooth Tigers,” called the †Machairodontinæ, comprises only extinct forms.

The fourth principal rank or grade is the _order_, distinguished by some fundamental peculiarity of structure and usually including a large number of families. Some of the orders, however, contain but a single family, a single genus, or even, it may be, a single species, because that species is in important structural characters so unlike any other that it cannot properly be put into the same order with anything else. Such isolation invariably implies that the species or genus in question is the sole survivor of what was once an extensive series. As in the case of the family and the genus, it is often necessary to recognize the degrees of closer and more remote affinity by the use of _suborders_. Existing Artiodactyla, or even-toed hoofed animals, an enormous assemblage, may conveniently be divided into four suborders: (1) Suina, swine and the Hippopotamus; (2) Tylopoda, the Camel and Llama; (3) Tragulina, “mouse-deer,” or chevrotains; (4) Pecora, or true ruminants, deer, giraffes, antelopes, sheep, goats, oxen, etc. In nearly all of the orders such subordinal divisions are desirable and it is frequently useful to employ still further subdivisions, like _superfamilies_, which are groups of allied families within the suborder, _sections_ and the like.

In the Linnæan scheme, the next group in ascending rank is the _class_, which includes all mammals whatsoever, but the advance of knowledge has made it necessary to interpolate several intermediate grades between the class and the order, which, in the descending scale, are _subclass_, _infraclass_, _cohort_, _superorder_ and others, while above the class comes the _subkingdom_ of Vertebrata, or animals with internal skeletons, which includes mammals, birds, reptiles, amphibians and fishes.

A word should be said as to the conventions of printing technical names. The names of all species are, in American practice, printed in small letters, but many Europeans write specific terms which are proper nouns or adjectives with a capital. Generic, family and all groups of higher rank are always written with a capital, unless used in vernacular form, _e.g._ Artiodactyla and artiodactyls. It is also a very general custom to give capitals to vernacular names of species, as the Mammoth, the Coyote, the Black Bear. Genus and species are almost invariably in italics, groups of higher rank in roman.

Such a scheme of classification as is outlined above has a decidedly artificial air about it and yet it serves a highly useful purpose in enabling us to express in brief and condensed form what is known or surmised as to the mutual relationships of the great and diversified assemblage of mammals. The scheme has been compared to the organization of an army into company, battalion, regiment, brigade, division, army corps, etc., and there is a certain obvious likeness; but the differences go deeper, for an army is an assemblage of similar units, mechanically grouped into bodies of equal size. A much closer analogy is the genealogical or family tree, which graphically expresses the relationships and ramifications of an ancient and widespread family, though even this analogy may easily be pushed too far. Blood-relationship is, in short, the underlying principle of all schemes of classification which postulate the theory of evolution.

The system of Linnæus, as expanded and improved by modern zoölogists, has proved itself to be admirably adapted to the study of the living world; but it is much more difficult to apply it to the fossils, for they introduce a third dimension, so to speak, for which the system was not designed. This third dimension is the successive modification in time of a genetically connected series. The cumulative effect of such modifications is so great that only very elastic definitions will include the earlier and later members of an unbroken series. In attempting to apply the Linnæan system to the successive _faunas_ (_i.e._ assemblages of animals) which have inhabited the earth, palæontologists have employed various devices. One such method is to classify each fauna without reference to those which precede and follow it, but this has the great drawback of obscuring and ignoring the relationships, to express which is the very object of classification. Another and more logical method is to treat species and genera as though they belonged to the present order of things, for these groups, particularly species, were relatively short-lived, when regarded from the standpoint of geological time, and either became so modified as to require recognition as new species and genera, or died out without leaving descendants. Groups of higher rank, families, orders, etc., are treated as _genetic series_ and include the principal line or stock and such side-branches as did not ramify too widely or depart too far from the main stem. Under the first arrangement, the horses, a long history of which has been deciphered, would be divided into several families; under the second, they are all included in a single family.

One of the most interesting results of palæontological study is the discovery that in many families, such as the horses, rhinoceroses and camels, there are distinct series which independently passed through parallel courses of development, the series of each family keeping a remarkably even pace in the degree of progressive modification. Such a minor genetic series within a family is called a _phylum_, not a very happy selection, for the same term had been previously employed in a much wider sense, as equivalent to the subkingdom. In both uses of the term the underlying principle, that of genetic series, is the same; the difference is in the comprehensiveness of meaning.

It must be admitted that no method, yet devised, of applying the Linnæan scheme to the fossils is altogether satisfactory, and indeed it is only the breaks and gaps in the palæontological record which makes possible any use of the scheme. Could we obtain approximately complete series of all the animals that have ever lived upon the earth, it would be necessary to invent some entirely new scheme of classification in order to express their mutual relationships.

In the present state of knowledge, classification can be made only in a preliminary and tentative sort of way and no doubt differs widely from that which will eventually be reached. So far as the mammals are concerned, part of the problem would seem to be quite easy and part altogether uncertain. Some mammalian groups appear to be well defined and entirely natural assemblages of related forms, while others are plainly heterogeneous and artificial, yet there is no better way of dealing with them until their history has been ascertained. The mutual relations of the grand groups, or orders, are still very largely obscure.

The class Mammalia is first of all divided into two subclasses of very unequal size. Of these, the first, PROTOTHERIA, is represented in the modern world by few forms, the so-called Duck-billed Mole (_Ornithorhynchus paradoxus_) and Spiny Anteaters (_Echidna_) of Australia. They are the lowest and most primitive of the mammals and retain several structural characters of the lower vertebrates. Their most striking characteristic is that the young are not brought forth alive, but are hatched from eggs, as in the reptiles, birds and lower vertebrates generally.

The second subclass, EUTHERIA, which includes all other mammals, is again divided into two very unequal groups or infraclasses. One of these, DIDELPHIA, contains but a single order, the Marsupialia, or pouched mammals, now in existence, and is also very primitive in many respects, though far more advanced than the Prototheria. The young, though born alive, are brought forth in a very immature state and, with the exception of one genus (_Perameles_) the fœtus is not attached by a special structure, the placenta, to the womb of the mother. Like the Prototheria, the Marsupials, which were once spread all over the world, are at present almost entirely confined to Australia and the adjoining islands, the Opossums of North and South America, and one small genus (_Cænolestes_) in the latter continent being the exceptions to this rule of distribution. The second and vastly larger infraclass, the MONODELPHIA, is characterized by the _placenta_, a special growth, partly of fœtal and partly of maternal origin, by means of which the unborn young are attached to the mother and nourished during the fœtal period; they are born in a relatively mature state and are generally able to walk immediately after birth and resemble their parents in nearly all respects.

The vast assemblage of placental mammals, which range over all the continents, are divided into numerous orders, most of which appear to be natural groups of truly related forms, while some are but doubtfully so and others again are clearly unnatural and arbitrary. As has already been pointed out, the mutual relationships of these orders, as expressed in groups of higher than ordinal rank, offer a much more difficult problem, chiefly because our knowledge of the history of mammals is most deficient just where that history is most important and significant, namely, in its earlier portion. In many instances, the evolution of genera and families may be followed out within the limits of the order in a very convincing way, but very rarely can the origin of an order be demonstrated. When the history began to be full and detailed, the orders had nearly all been established, and, until the steps of their divergence and differentiation can be followed out, their mutual relationships can be discussed only from the standpoint of their likenesses and differences. In the valuation of these, there is much room for difference of opinion, and such difference is not lacking. On the other hand, concerning the number and limits of the orders themselves there is very general agreement.

In the following table only the major groups are included and those which are extinct are marked with a dagger (†). The scheme is almost identical with that given in Professor Osborn’s “Age of Mammals,” the few points in which I should prefer a somewhat different arrangement being waived in the interests of uniformity and avoidance of confusion. A few changes are, however, made in matters which I regard as too important to ignore.

I. SUBCLASS PROTOTHERIA. Egg-laying Mammals. 1. ORDER =†PROTODONTA=. 2. ORDER =MONOTREMATA=, _e.g._ the Duck-billed Mole and Spiny Anteaters. II. SUBCLASS EUTHERIA. Viviparous Mammals. _A._ INFRACLASS DIDELPHIA. Pouched Mammals. 1. ORDER =†TRICONODONTA=. 2. ORDER =MARSUPIALIA=. _a._ SUBORDER =Polyprotodonta=. Opossums, carnivorous and insectivorous Marsupials. _b._ SUBORDER =Diprotodonta=. Herbivorous Marsupials; Kangaroos, etc. _c._ SUBORDER =†Allotheria=. _B._ INFRACLASS MONODELPHIA. Placental Mammals. _AA._ COHORT UNGUICULATA. Clawed Mammals. 1. ORDER =†TRITUBERCULATA=. 2. ORDER =INSECTIVORA=. Insect-eating Mammals. _a._ SUBORDER =Lipotyphla=, _e.g._ Moles, Hedgehogs, Shrews, etc. _b._ SUBORDER =†Hyopsodonta=. _c._ SUBORDER =†Proglires=. _d._ =Suborder= =Menotyphla=, _e.g._ Tree and Jumping Shrews. 3. ORDER =†TILLODONTIA=. 4. ORDER =DERMOPTERA=. The Flying Lemur. 5. ORDER =CHIROPTERA=. Bats. 6. ORDER =CARNIVORA=. Beasts of Prey. _a._ SUBORDER =†Creodonta=. Primitive Flesh-eaters. _b._ SUBORDER =Fissipedia=. Wolves, Bears, Weasels, Cats, etc. _c._ SUBORDER =Pinnipedia=. Marine Carnivores—Seals and Walruses. 7. ORDER =RODENTIA=. Gnawing Mammals. _a._ SUBORDER =Duplicidentata=, _e.g._ Hares, Rabbits, Pikas. _b._ SUBORDER =Simplicidentata=, _e.g._ Squirrels, Marmots, Beavers, Rats, Mice, Porcupines, etc. 8. ORDER =†TÆNIODONTIA=. 9. ORDER =EDENTATA=. _a._ SUBORDER =Pilosa=. Hairy Edentates, _e.g._ Sloths, Anteaters, etc. _b._ SUBORDER =Loricata=. Armoured Edentates, _e.g._ Armadillos, †Glyptodonts. 10. ORDER =PHOLIDOTA=. Scaly Anteaters or Pangolins. 11. ORDER =TUBULIDENTATA=. The Aard Vark. _BB._ COHORT PRIMATES. Mammals with nails. 12. ORDER =PRIMATES=. _a._ SUBORDER =Lemuroidea=. Lemurs. _b._ SUBORDER =Anthropoidea=. Monkeys, Apes, Man. _CC._ COHORT UNGULATA. Hoofed Mammals. 13. ORDER =†CONDYLARTHRA=. 14. ORDER =†AMBLYPODA=. 15. ORDER =ARTIODACTYLA=. Even-toed Hoofed Mammals. _a._ SUBORDER =†Artiodactyla Primitiva=. _b._ SUBORDER =Suina=. Swine, Peccary, Hippopotamus. _c._ SUBORDER =Tylopoda=. Camels, Llama, Guanaco. _d._ SUBORDER =Tragulina=. Mouse-deer or Chevrotains. _e._ SUBORDER =Pecora=, _e.g._ Deer, Antelopes, Sheep, Oxen, etc. 16. ORDER =PERISSODACTYLA=. Odd-toed Hoofed Mammals. _a._ SUBORDER =Chelodactyla=, _e.g._ Horses, Tapirs, Rhinoceroses, etc. _b._ SUBORDER =†Ancylopoda.= †Chalicotheres. 17. ORDER =PROBOSCIDEA=. Elephants and †Mastodons. 18. ORDER =†BARYTHERIA=. 19. ORDER =†EMBRITHOPODA=. 20. ORDER =SIRENIA=. Sea-cows and Dugongs. 21. ORDER =HYRACOIDEA=. Conies. 22. ORDER =†TOXODONTIA=. _a._ SUBORDER =†Toxodonta=. _b._ SUBORDER =†Typotheria=. _c._ SUBORDER =†Entelonychia=. _d._ SUBORDER =†Pyrotheria=. 23. ORDER =†ASTRAPOTHERIA=. 24. ORDER =†LITOPTERNA=. _DD._ COHORT CETACEA. Whales, Dolphins, Porpoises. 25. ORDER =†ZEUGLODONTIA=. 26. ORDER =ODONTOCETI=. Toothed Whales, Dolphins, Porpoises. 27. ORDER =MYSTACOCETI=. Whalebone Whales.