# The Encyclopedia Americana (1920)/Animals, Classification of

Edition of 1920. See also Animal on Wikipedia, and the disclaimer.
ANIMALS, Classification of. Classification is simply sorting out — arranging different things according to their likeness, putting them into groups. To these groups names are given for convenience in referring to them, and thus arises the terminology of this department of knowledge, which is the technical language of science broadly considered. In the present article, however, the writer purposes to confine himself to organic nature, and chiefly to zoology.
The “sorting out” alluded to above is very simple and easy at first. The child undertakes it in the kindergarten when he separates blue papers from red, square blocks from round ones, the long sticks from the short. But presently something combining more than one quality is encountered, and the question arises: Shall I put this on this pile or on that? An infant in a kindergarten who picked up a red ball might well hesitate whether to place it with the red things by virtue of its color, or on the spherical pile by virtue of its shape. This illustrates the cardinal difficulty that has embarrassed all classifiers, and has caused so great a diversity of schemes for the orderly arrangement of natural objects. Probably the only perfect classifier is nature herself, in the form of that mysterious and marvelous selector, chemical affinity. Human intelligence is less well informed as to the real constitution and affinities of the objects, animate and inanimate, that man's senses perceive, and, for the present, his arrangements of them must be imperfect and tentative. Hence classifications of the various groups, or even the limits of the groups themselves, may and do vary in at least two directions — first in the selection of a standard of comparisons, and second in knowledge or opinion of relationships.
At first, as was natural, superficial resemblances sufficed to group seemingly like objects. This brought worms, spiders and insects into one lot, classed bats with birds, whales with fishes and so on. But a closer examination soon revealed these and lesser incongruities. It was ascertained that every known bird was clothed in feathers and reproduced itself by eggs. The bat, although it flew in the air, was covered with hair, brought forth its young alive and suckled them; those were the most conspicuous and universal characteristics of
mammals. So the bat was a mammal that flew. Fishes were found to be always covered with a more or less scaly skin and breathed the air within the water by means of gills. The whales were found to breathe only atmospheric air, brought forth living young and suckled them. Plainly it was wrong to put them among the fishes, although they lived in the water — they were mammals. Thus arose the concept that fundamental structure was a better criterion by which to classify than external likeness.
Artificial Schemes.— In the early days the knowledge of animal structure was very limited. Comparative anatomy had hardly been thought of, and, indeed, arose into a definite science largely because of curiosity as to classification. Meanwhile naturalists wanted to group their facts, pigeon-hole and label their increasing information, which more and more was falling into coincidences and suggesting new comparisons and contrasts. Hence the early attempts at classification were frankly for convenience, hardly more scientific than the pigeon-holes in a clerk's desk. The crowning example of this, probably, is the “system” by which Linné (Linnæus) arranged the plants of the world.
The makers of all these “artificial” arrangements, as they were called, were ever seeking for the best, the most comprehensive features by which to work. Some were fantastic, as the “circle” theory of Macleay and Vigors; others sensible and useful. An arrangement of birds long in vogue was by the form of their feet, by which all birds were separated into several “orders” according as they were seizers, runners, climbers, etc. This was utilizing function as a criterion, and only hinted at structure.
But the untruth of this method was early perceived by some systematists who insisted that structure was the true foundation on which to erect what they styled a “natural” classification. Hence arose a classifying of classifications into two categories — the artificial, for convenience, as a scaffolding for study; and the natural as an expression of real truth. To some extent these lines of thought and work still exist, but the former has been nearly abandoned.
Seekers after truth of relationship by study of structure — forms of organs, morphology — increased in number from the early 17th century onward, and their accumulated information, published in various partial schemes, together with his own extensive investigations, gave material for the first grand generalization in zoology — that by Georges Cuvier (1769-1832), whose fame was popularized in this country by the most talented of all his pupils, Louis Agassiz, himself a great investigator and the author of a remarkable “Essay on Classification.”
Cuvier's “Plans of Structure.”— Cuvier thought himself able to separate animals into four groups, distinguished by four “plans of structure,” which had been assigned by the divine Creator, and which were of equal rank. These were: (1) Vertebrata, characterized by an internal skeleton, an essential part of which was a backbone; it comprised mammals, birds, reptiles (including batrachians) and fishes. (2) Mollusca, characterized by a massive type of body, without bones; the mollusks, brachiopods, tunicates,
etc. (3) Articulata, with bodies composed of ring-like segments; the insects, crustaceans, annelids and spiders. (4) Radiata, characterized by a radial arrangement of all the parts around a vital focus; star-fishes, polyps, worms, and animalcules.
Those four “plans of structure,” ordained from the beginning of things, Cuvier regarded as great facts; and Agassiz objected to all those like Leuckart, Vogt and others, who criticized or modified this arrangement, that they were considering too much complexity of structure and losing sight of “plan” or “type.” Agassiz pointed out that Cuvier's divisions combined their various subdivisions; and that this was a great step forward, even if he had not the correct measures for all his groups. “For we must remember,” said Agassiz, “that at the time he wrote naturalists were bent upon establishing one continuous, uniform series to embrace all animals, between the links of which it was supposed there were no unequal intervals.” The watchword of their school was “Nature makes no leaps”; they called their system the “Chain of Being.”
Nevertheless these views were not accepted by all investigators. One after another the leaders in biological science proposed modifications, especially as to the Radiata. Ehrenberg in 1836 departed altogether from the Cuvierian notion, and laid down the principle that the type of development is one and the same in all animals from nomad to man; that is, he set aside the idea of "plan," and erected a classification on purely anatomical grounds. That of Owen, a few years later, had a similar basis. Both assembled in their schemes groups that were heterogeneous, and gave varying rank to similar aggregations. The result was that neither was much accepted. Von Siebold, in 1845, added to Cuvier's four plans or types of structure two more — Zoophyta and Vermes — asserting that they had equal rank with the others, and Leuckart accepted them. Later Von Baer, the great embryologist, offered a classification based on mode of development from an egg to maturity, and other embryological systems were made by Van Beneden, Rolliker and Vogt.
All these men multiplied facts, cleared up subordinate relationships, and, by the different angles from which the subject was viewed, broadened knowledge immensely. The effort of all was to find a standard that could be applied uniformly, and would necessarily reveal the true place in nature of every living thing. If a perfect order in organic nature existed, then approaches from the point of view of the embryologist and from the point of view of the anatomist ought to arrive at the same result — the real condition — the truth. Thus far it had not done so except in favorable cases. One method would logically assign an animal, a plant or a well-defined group to a certain place in the system which would be quite negatived by other considerations.
Search for “Natural” Scheme.— Facts of biology accumulated, but a connected system for “natural” classification of them eluded all searchers. That this should be the case is not surprising when one recalls that it was believed by all the older zoologists, except a few advanced thinkers, whose views were usually scouted, that the individuals of each kind of animal were descended from their own peculiar
ancestor; that this original pair was totally unconnected with the ancestor of any other line, having, as Button asserted, “participated in the grace of a distinct creation”; that a species so created was forever unchangeable.
Classifiers sought to perfect their appreciation of resemblances, and constantly used the word relationship, but this term, when employed with reference to two kinds of animals having a wholly independent origin, was of course used in a purely metaphorical sense. It was not until the prevalent idea outlined above was discarded that relationship took on its real significance and became the key-word in classification; and this came about only when the doctrine and facts of organic evolution — that is, transmutation of forms of life through variation in descent — destroyed the earlier conception of the origin and history of living things on the globe. Then it was that all the avenues of inquiry (anatomy, embryology, geographical and geological distribution, adaptations to habits, etc.) led to the same point, indicated the true place in nature of each animal or group — true because it had become so by developmental heredity. Instead of four or six or some other number of parallel “plans” of immutable structure, it became plain that animal life and plant life represented only one progressive, enlarging and ever-varying scheme of adaptative and fruitful beings.
This compelled the discarding of another old assumption, one on which, in fact, all previous schemes of classification had rested, namely, the existence and fixity of “species.” This word came into use in zoology and botany at the beginning of the 18th century, when John Ray applied it to indicate a group of animals and plants with common characteristics that would interbreed freely. (This last test has been popularly considered definite, although not universally true.)
“Linnæus,” says Prof. William B. Scott in his ‘History of Land Mammals’ (1913), “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.”
Descent the Test of Relationship.— This conception upset completely all previous methods of determining relationship — the basis now, as always, of classification; and gave to that word a new and proper definition, namely, association by blood-connection derived through descent from the same stock. The term had been so used with reference to human affinities, and this meaning was now extended to all living things, as was right and natural. A man's relatives are those who belong to the same family-stock; the relationship between them is one of blood and inheritance. The same is true of horses, or sparrows, or fishes, or snails. Any recognizable relationships are those produced by common descent; and when these are unrecognizable the search for them must be along lines of ancestry.
Hence the vast service of palæontology has rendered to classification of living things — the same kind of service for the brute world that the study of genealogy has rendered to human history. Palaeontology reveals the genealogy of the animal world so far as its materials permit.
Genealogy, then, is the guide to the classification of the individual, whether human, quadruped, bird or lowly worm; and the old-fashioned “plans of structure” are merely helpful indications of probable community of descent. They exhibit the groups that have resulted from the more or less gradual variations, prehistoric divergencies, and frequent extinctions of intermediate lines, that have affected the descent of animal life from some original source. Biological classification is the expression of heredity.
The matter is commonly symbolized by a tree growing from seed and root to trunk, splitting into limbs that spread out in various directions, putting forth lesser divergent branches and finally innumerable twigs. Many branches flourish to the very tip; others remain short; others produce twigs; others die and disappear. Species are the latest twigs; the lesser branches from which they spring may represent genera, families, orders, and classes, until at last the root-trunk is reached. The figure is incomplete and inadequate, but is helpful
The newest, that is the most modern and simple manifestation of arrangement in the living world, is the species, which we may now define as an assemblage of individuals more closely related by common descent to one another than to anything outside their class. All the members of a species will interbreed, and, as a rule, will not interbreed with any other species, or at least will not produce fertile offspring. Some species are very distinct in their characteristics, others vague; some are exceedingly numerous, others contain few individuals; some are strictly local in their habitat, others exist over an immense area; some appear modern in their origin, others may be traced far back in the zoological record; some are remarkably uniform, others vary largely, so that systemists create “sub-species” or “geographical races” to demark their variations, which some regard as “nascent” species.
The species, then, is the unit of classification, and its characteristics are mainly external peculiarities of contour and color, which by their very nature are impermanent. Let us take as an example of a species our common cat, which is, in the main, simply a domesticated form of the Egyptian wildcat, known in zoology as Felis libyca.
The earliest naturalists gave long descriptive names to animals and plants. Ray, Linnæus, and students after them have reduced these to two, and have used Latin (or Latinized) terms in order to identify the subject in all languages, and thus avoid the confusion and inaccuracy of vernacular names, which often are applied ignorantly or carelessly to very different creatures. Our Canadian moose is virtually the same animal as the elk of northern Europe; but the “elk” of this country is not that at all but a deer closely related to the red deer of the Old World. A Japanese has no trouble in distinguishing them, however, when he reads of Alces malchis (the European elk)
and Cervus canadensis (the American “elk”). This is called the binomial system of nomenclature; and sometimes geographical varieties are designated by a third (trinomial) name, as Sturnella Magna argutola, our Western meadow-lark.
After this digression on the form of specific names let us resume our account of the development of the scheme of arrangement of animal life.
Relative Rank in Classification.— As one surveys the world he finds two or more species with a certain close resemblance that separates them from others, and these he unites into a secondary group called a genus.
For instance, kinds of cats, different in habitat, size, coloring, etc., have the same general features of a comparatively elongated body, long tail, and small, plain ears; they are united in the genus Felis, with different specific names, as Felis libyca, Felis catus, Felis leo, etc.— the binomial being the generic and the specific name used together like Smith, John; Smith, George, and the like. But there are other kinds of cats that agree in having heavy bodies, short tails and tufted ears. These are recognized as forming another genus Lynx, and we have of these several species, as Lynx canadensis, Lynx pardalina, etc.
These several genera may now be united by common characteristics into a higher group called a family, the name of which in zoology always ends in the suffix idæ, and is likely to be taken from the name of the most prominent genus as Felidæ, the cat family. (Large famines are often subdivided into sections or sub-families designated by the termination inæ). This family group is based on a combination of the features that ally all its constituents, and at the same time separate it from other groups of equal rank, and these are usually features of structure, rather than of form or appearance, such as the form and number of the teeth, adaptations of limbs to a special mode of life, etc. Sometimes these are very marked. For example, the family of the cats differs from that of the dogs or bears or weasels in having claws that may be withdrawn into a sheath — a distinction of family rank. The cattle family (Bovidæ) includes a wide variety of forms — neat cattle, sheep, goats, and antelopes — but is separated from the deer family (Cervidæ) by the fact that all have hollow but permanent horns, while the deer have solid, deciduous horns (antlers). It is such broad characteristics that make family rank; and now and then families represent divergencies from, their allies so great that only a single genus, perhaps containing but one species, is given family rank. This is the case with our American pronghorn.
The fourth rank is the order, containing a collection of families believed to have consanguinity or descent from a common stock. To determine this by the detection of a common characteristic, usually of intimate structure, is sometimes easy, as in the case of the order Carnivora, or beasts of prey, whose sign is the character of the teeth, and particularly the presence of the prominent canine tooth, so strongly developed in the cats, which are highly representative of that order. Orders are usually large and comprehensive groups, and often may be divided into well-marked sub-orders, as,
in this instance, the Pinnipedia, or fin-footed carnivora (seals and walruses) and the Fisipedia, or toed carnivora (land beasts of prey); and most orders include several extinct families. It is probable, also, that the Carnivora and certain others ought to form a super-order Creodonta, ancestral to them. When one is dealing with groups as large and widely separated as orders, is it possible to discover any features possessed by all that will enable us to collect them into a still higher group? It seems doubtful, when one recalls the diversity in the one group alone chosen for our illustration. Here are 11 orders (not counting extinct ones) represented by animals so diverse as the duckbill, kangaroo, sloth, ox, manatee, whale, cat mouse, mole, bat, and man. Have they anything in common? Much; and particularly two prominent features that separate them from all other animals — their covering of hair, and their nourishing their newly-born young on mother's milk. Hence they go together into a still larger group called a Class — the class of mammals (Mammalia). In this class are two sub-classes, Prototheria (duckbill and the echidnas), and Eutheria (all other mammals).
The mammals are equal in rank with the classes Birds, Reptiles, Amphibia, Fishes, Round-mouths, and several others formerly regarded as worms or mollusks, all of which possess one feature of prime importance — a notochord, the prototype of the spinal cord and backbone of the more highly developed forms. This unites these otherwise so varied classes into a still broader aggregation or “phylum,” the Chordata. Beyond this are only two words — Animal and Plant, indicating the two grand kingdoms of life in the world. Even these, however, are so essentially alike in their simplest form, the monad, that they are doubtfully distinguishable at that point of contact, and may be combined under the term organism.
Here, then, ends our inquiry into the place in nature of our example. The animal organism called "cat" is a chordate eutherian, carnivorous mammal of the family Felidæ, genus Felis, and species libyca.
Such is the schematic history of a single animal, and it represents not only its place in nature, but its phyllogeny, that is the line of its evolution from a primitive monad to the complexity and finish of an animal highly adapted to a certain manner of life.
Biological classification is now, therefore, on a sure, philosophical, scientific basis. It has but one principle — that of descent — blood affinity. The problems are those of perception: to detect the evidence of genetic relationship. To this every fact of investigation, from the embryologist's microscope and the anatomist's scalpel to the geologist's hammer and the field-naturalist's notebook, contributes; and no longer is any artificial grouping possible, except on account of an ignorance that study will correct, or as a theoretical suggestion that must be proved before it is acceptable.
In the present article only a single kind of animal has been considered. The process is the same for all others, but different criteria must be employed in the various groupings, and even different terms used, or the same term with different limits. Thus “family” is a far more comprehensive and important group in
botany than in zoology. Moreover the diverse temperaments of classifiers lead them to vary in closeness of distinction. Some men see specific or generic differences that others do not admit, wishing to ignore the minute differences that the former deem important. The tendency is toward the broader view.