CLASSIFICATION OF ANIMALS. Classi fication is the act of sorting out and putting into groups kindred ideas, observations, or ob jects. As many classifications, then. are pos sible as are the categories which different per sons may erect; for no two persons see things in the same light, nor have the same thoughts. Besides, there are many aspects that the one ob server may take in viewing the same thing. Thus, we may classify plants and animals ac cording to their anatomical structure, the meth od of their embryological development, their food or habitats. All classifications, then, are arbitrary: are the attempts of man to arrange, in an orderly fashion. his conceptions and ob servations. Classification in biology is, conse quently. to be regarded as a subjective process. Nevertheless, the constant attempt of all mod ern naturalists is to conform, in the formation of their groups, as closely as possible to the facts of nature, and thereby express the natural kinship or blood-relationship of animals, as learned through the investigation of their struc ture and phylogeny. A natural and true •lassi fieation, then, is a statement of near or remote relationships. according to time degree of differ entiation the forms sought to be classified have undergone in their descent from a more or less remote common ancestor. If a complete classi fication of animals is ever made. it will be a com plete genealogy of the animal kingdom. In this sense, classification is neither arbitrary nor artificial, but only tentative: and it will be per manent in so far as it conforms to the facts of kinship in descent. Ilence, a nearer and nearer approach to a natural and real classification has been made with each forward step of knowl edge in embryology, morphology, and paleon tology.
"There is no question in natural history." said Louis Agassiz, in his classic Essay on Classi fication, "on which more diversified opinions are entertained than on that of elassification—not that naturalists disagree as to the necessity of some sort of arrangement in describing animals or plants. for since nature has become the object of special studies, it has been the universal aim of all naturalists to arrange the objects of their investigations in the most natural order pos sible. Even Buffon, who began the publication
of his great Natural History by denying the ex istence in nature of anything like a system, closed his work by grouping the birds according to certain general features exhibited in common by ninny of them. It is true, authors have dif fered in their estimation of the characters on which their different arrangements are founded: and it is equally true that they have not viewed their arrangements in the same light, some hav ing plainly acknowledged the artificial character of their systems, while others have urged theirs as the true expression of the natural relation; which exist between the objects themselves." By collaboration and by common consent, cer tain categories have been agreed upon ; and these we generally have in mind when we speak of the classification of the animal or vegetable kingdom. These categories are founded mainly on anatomical and embryological relationships and differences: and in zoi5logical classification. for example, animals that resemble one another in structure, development, or both. are grouped closely together, or are more distantly associat ed, according to the nearness or remoteness of the kinship as evidenced by the facts of struc ture or development. whether in existing or in extinct forms. These grade- or of kinship we speak of as species, genera. families, orders. classes. etc. Species are more nearly re lated than genera. genera than families. etc.
The total number of the kinds of animals, or specie-. that are recognized is not far from 400. 000. In order to think of them all, it is neces sary that they should he grouped into cate gories of larger size than species, and these, in turn. into still larger ones. This is rendered possible 1w the fact that animals show nearer and remoter a tlinit ies. The ordinary lowest category is species; but this may be subdivided into or varieties: several similar species arc grouped into a genus: related genera con stitute a family; several families may make up an order. Orders constitute a class; classes a phylum. For example see table preceding.