CHAPTER III CLASSIFICATION OBJECTS of experience make their appearance in an order which seems to be almost chaotic; and in memory they are often reproduced very much in the order in which they originally occurred. But even in memory, and still more in reflection, there is a tendency to arrange things according to their likenesses and differences. This is the beginning of classification. Classification "is not identical with collection. It denotes the systematic association of kindred facts, the collection, not of all, but of relevant and crucial facts." 1 on. A classification is necessarily based on a similarity of some sort of quality or structure or origin, and so Any given collection of things may be classified in many different ways. Books, for example, may be grouped according to subject, size, style of binding, publisher, and so on; minerals, according to composition, value or chemical properties; the people of a city, according to race, income, occupation or religion. Any quality or relation whatever may serve as a basis of classification. In the abstract, one may be as good as another, and the one to be employed in a given instance will be that which best serves the purpose we have in hand. There are several different types of classification, each serving a special purpose. Types of Classification.-1. INDEX CLASSIFICATION.— 1 Karl Pearson, The Grammar of Science, chap. III., n. 1. CLASSIFICATION 33 We may notice briefly the "Index Classification." 2 The purpose of this mode of grouping is to enable us to get hold of a given fact quickly and easily. Catalogues are usually constructed with this end in view, and they illustrate the principles involved. Certain obvious characteristics are selected, and very often a given item may appear under several different heads, as in cross-references. Alphabetical catalogues are the most familiar examples of the index classification. 2. DIAGNOSTIC CLASSIFICATION.-A second type is the "Diagnostic Classification"; its purpose is the identification of an object or the discovery of the group to which it belongs. "Nature Study" books abound in classifications of this sort. Here, too, certain obvious characteristics are made the basis of classification. Flowers, for example, may be classified according to color or time of appearance or habitat; or the main divisions may be made upon one basis, as color, the first subdivision on another, as time of appearance, etc. The identification of ailments by the physician depends upon a classification of symptoms made upon this plan. 3. NATURAL" AND ARTIFICIAL CLASSIFICATIONS. -Both index and diagnostic classifications are useful, but they do not, by themselves, lead directly to any greater knowledge of the facts or of their essential relations. They are based, for the greater part, upon superficial and easily noticed characteristics, and have little relation to the essential properties of the things classified. It is often possible, however, so to group 66 66 2 See Jevons, Principles of Science, chap. xxx. 3 3 A diagnostic classification which is to be a sure means for the identification of any and all cases should be based on essential qualities. Those based on superficial and striking qualities serve phenomena as to display at once their most significant characteristics. Compare, for example, the popular classification of the whale as a fish with the scientific classification of the same animal as a mammal. To call a whale a fish is to imply that it lives in the water, but tells little more; to call it a mammal tells us that it has warm blood, lungs instead of gills, a four-chambered heart, certain peculiarities of the skeleton, and so on. Grouping data in such a way as to make manifest at once their essential characteristics is the aim of classification in science. Since science aims at complete and systematic knowledge, it will obviously select as the basis of classification in any given case that quality which does correlate the greatest amount of knowledge about the facts under consideration. Scientists usually make a distinction between "artificial" and "natural" classifications. "It would be possible to classify all living things according to color, as white, yellow, green organisms, etc. Such a classification would, however, be artificial and destitute of scientific value because based upon a purely artificial and highly inconstant character. An interesting example of an artificial classification formerly employed is the system of Linnæus, who classified flowering plants into Monandria, Diandria, Triandria, Tetrandria, etc., according to the number of stamens. This was sufficiently convenient for a first rough arrangement, but was soon found to lead to the most incongruous association of plants agreeing in the number of stamens but differing in almost all characters. From such cases it is plain that plants and animals cannot be naturally classified for ready identification of many cases, but not for all. See Bosanquet's Logic for a discussion of Diagnostic Classification as one based upon deeper and more essential qualities. NATURAL CLASSIFICATIONS 35 by likenesses or differences in a single character artificially selected. The entire organisms must be taken into account, and the natural classification differs from the artificial one in representing real relationship and not merely superficial likeness. Modern biology teaches that this relationship is of precisely the same nature as human relationship, i.e., that it is due to community of descent from ancestral plants or animals. The labor of determining the natural classification is much lightened by the fact that certain structures are often found as a matter of experience to be constantly associated or correlated, so that the presence of one indicates the presence of the others. In such cases a single character may be taken as the basis of a classification which is natural, because agreement in one character has been previously proved empirically to indicate agreement in the others. For example, it has been proved that the differences or resemblances of animals are correlated with corresponding differences or resemblances in their teeth. Hence mammals, to a great extent, can be classified according to the structure and disposition of the teeth. And so in many groups it is usually possible to discover empirically some one or few characters on which, by reason of their constant association with other characters, a natural classification can be based." 4 Biology furnishes one of the best illustrations of a field in which a natural classification can be made, although even here in many cases there is no universal agreement as to which is the natural classification. For each of two characters or sets of characters might be correlated with a number of others, and it might be 4 Sedgwick and Wilson, Biology, p. 175. difficult to decide which of the two correlated the greater number or the more important ones. Even if there were such agreement, it would not necessarily be permanent; new information might result in the selection of a new basis of relationship. The difference between natural and artificial classifications is, as Jevons points out, one of degree only: "It will be found almost impossible to arrange objects according to any circumstance without finding that some correlation of other circumstances is thus made apparent." The principle employed in classification for scientific purposes is well stated in Huxley's definition, which was modified somewhat by Jevons and stated in the following form: "By the classification of any series of objects is meant the actual or ideal arrangement together of those things which are like and the separation of those which are unlike, the purpose of the arrangement being, primarily, to disclose the correlations or laws of union of properties and circumstances, and, secondarily, to facilitate the operations of the mind in clearly conceiving and retaining in memory the characters of the objects in question." A scientific classification is ordinarily designed to serve the purposes here enumerated, but there may be cases, especially in everyday life, where our primary interest is not in getting a complete knowledge of things, but in getting together things which have a relation to some common purpose or problem; and in such cases the grouping together of things which are, in most respects, very dissimilar, may be justifiable. Custom-house regulations, for example, proverbially group together things which, apart from certain economic considerations, may be totally unlike. The so |