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 fer tile offspring. Some species are very distinct in their characteristics, others vague; some are exceedingly numerous, others contain few in dividuals; some are strictly local in their hab itat, 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 classifica tion, 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 zo ology as Felis libyca.
The earliest naturalists gave long descrip tive names to animals and plants. Ray, Lin naeus, and students after them have reduced these to two, and have used Latin (or Latin ized) 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 dif ferent creatures. Our Canadian moose is vir tually the same animal as the elk of north ern Europe; hut 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 This is called the binomial system of nomen clature; and sometimes geographical varieties are designated by a third (trinomial) name, as Sturnella magna argutula, our Western mead ow-lark.
After this digression on the form of specific names let us resume our account of the develop ment 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 separtes 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 elon gated body, long tail, and small, plain ears; they are united in the genus Fells, with differ ent specific names, as Fells libyca, Fells canes, Fells 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 ids, and is likely to be taken from the name of the most prominent genus as Felider, the cat family. (Large fami
lies are often subdivided into sections or sub families designated by the termination Ma). 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 (Boulder) includes a wide variety of forms— neat cattle, sheep, goats, and antelopes — but is separated from the deer family (Gerold(?) by the fact that all have hollow but permanent horns, while the deer have solid, deciduous horns (antlers). It is such broad characteris tics that make family rank; and now and then families represent divergencies from, their al lies 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 con sanguinity or descent from a common stock. To determine this by the detection of a com mon characteristic, usually of intimate struct ure, 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 Fisi pedia, or toed carnivora (land beasts of prey); and most orders include several extinct fam ilies. It is probable, also, that the Camivora and certain others ought to form a super-order ancestral to them. When one is dealing with groups as large and widely sep arated 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 illustra tion. 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 par ticularly 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 to gether into a still larger group called a Class the class of mammals (Mamma/M). In this class are two sub-clas'ses, Prototheria (duckbill and the echidnas), and Eutheria (all other mammals).