Variety, in natural history, is the designation of a group subordinate to species, distinguished from a race chiefly by the circumstance that a race is artificial. Varieties are regarded as the first step in the formation of species. They are not to be confused with variations, since, like species, they are groups of similar individuals. The question of when to call two differing lots of animals species and when varieties is not (-nimble of satisfacttry solution, except by erect ing an artificial anti arbitrary bar separating them. In the absence of any such arbitrary rule, we find some naturalists 'splitting' one species into many, by elevating the varieties to specific rank; while another naturalist will 'lump' several species, reducing them to the rank of varieties of one species. _Many authors would define varieties to be founded on differences due to environment—differences that will disappear with changing environment. Those who are radical in recognizing by and tran sitory differences of this kind confess that they cannot recognize with certainty their own varieties, unless they know the habitat of the specimen. Other authors regard the difference between variety and species to be solely one of degree of divergence in characters. As varieties became recognized, the custom was formed of adding a varietal name to the specific name, as has already been explained.
Genits.—A category having a rank between species and family, and consisting of a group of species deemed to he more closely related to each other than to anything else. There is abso lutely no criterion by which we can distinguish between a genus and a species, on the one hand, and a family on the other; but it is often a fact that, when a collection of related species is ar ranged in an orderly fashion, more or less sharp breaks occur, which enable naturalists to draw lines. Consequently, the boundaries of genera are being constantly widened or restricted; and generic names change and disappear as a result of fusion of genera or of splitting up of a genus. Since the genus is that under which species are usually indexed, this instability of generic names is highly regrettable. What is needed is a rec ognition of the subjectiveness of generic names and of the prime necessity of the stability of nomenclature: no changes in generic names should be made without the best of reasons. The name of the genus (always written with a capital initial) is combined with that of the species. The name of the cat genus, for ex ample, is Felis; and Felis leo. Fens tigris, Felis •atus are the names of the lion, tiger, and wildcat species. These eats are inure alike to one another than any one of them is to the dogs. The latter belong to the genus Canis. The size of a genus varies with the number of species it con tains; some genera containing, a hundred spe cies or more, while others contain only one. Dar win believed that the species of a large genus are more variable than those of a small one. In any ease, the species within a genus are not regarded as having diverged very widely from their common stock, and the criteria for generic distinetions are largely superficial •hara•teris t ies.
Pamily—.1 group of genera having a certain resemldance to one another, whieh indicates common descent, or 'family connection.' Fea tures of structure more or less extetnal, and be longing to parts, as limbs, wings, teeth, horns, characteristic of methods of life, arc of the kind of characters used in judging of family limits. Thus, the prime family distinction of the cats (Felidw) is the arrangement for re tracting the claws, which sharply defines them as a group from the dogs, on one hand, and the civets on the other: but the cheeta is able to retract its claws only partly, and this fact, in connection with other distinc'tive features, causes the animal to he placed in a genus separate from Feb, Order.—Several related families may fall to gether. by the possession in common of char acters denied to others, into a larger category known as an order. A familiar example is the order Carnivora, embracing the families of cats, dogs, civets. weasels, bears, etc., because all these exhibit an organization developed along Ines of differentiation from other mammals, tending to fit them to prey upon other animals and digest a flesh diet. The various lines and degrees of specialization are recognized by sub ordinate groupings in families and genera.
class.—Groups of orders are found to agree in certain very general and fundamental charac teristics of organization, such as having six legs or eight legs; the production, by the skin, of hair, feathers, or scales: the integument being leathery or calcareous—and so on; and such groups form the next larger category, called a class. Even here, however. there is large room for difference of opinion as to limits: and, in sonic classes, an intermediate category called subclass seems necessary, as in the case of the prime division of the class "Alammalia into mono delphic and didelphie, or of the class Cephalo poda into dibranehiate and tetrabranchiate. The same difficulty has led to naming groups of in termediate rank elsewhere—such as superfaini lies, subfamilies, subgenera, etc.
Phyla.—Classes combine into several grand divisions of the animal kingdom, called sub kingdoms, or phyla, by having in common a single or a few characteristics so broad and an cestral that they are spoken of as 'plan of struc ture.' Thus, all the mammals. birds, reptiles, amphibians, and fishes, together with several lowly representatives. such as the ascidians, arc united by a single characteristic of structure possessed by no other animals—the presence of a backbone, or its essential equivalent, the noto chord. This character is so wide-reaching and fundamental that it springs from the very root of the phylogenetie tree, and is of prime impor tance. By similar broad and fundamental 'plans of structure' are the other phyla of the twelve now recognized by most zoidogists characterized. Their origin is lost to view in the mists of primeval time, but even here two divisions may be recognized—the Protozoa and the tletazoa the former embracing the single phylum Pro tozoa, or one-celled animals; the latter, all the rest which agree in consisting of many cells, hay ing a two-layered embryonic development.