A history of land mammals in the western hemisphere

by William Berryman Scott

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.