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.