may be divided according to their place of birth, as English, Welsh, Scotch, Irish, Colonial-born and aliens. The ethnographer would divide them into Anglo-Saxons, Cymri, Gaels, Picts, &c. The statist arranges them according to age; to condition, as married, unmarried, widowed, etc.; to state of body, as able, incapacitated, blind, imbecile. In the natural world, again, we may make various classifications. Plants may be arranged according to the country from which they are derived; the kind of place or habitat in which they flourish; the time they live, as annual, biennial, perennial; their size, as herbs, shrubs, trees; their properties, as esculents, drugs, or poisons all these are distinct from the classifications which the botanist devises to represent the natural affinities or relationships of plants. It is thus evident that in making a classification we have no one fixed method which can be ascertained by rule, but that an indefinite number of choices or alternatives are usually open to us. Logic cannot in such cases do much; and it is really the work of the special sciences to investigate the character of the classification required. All that logic can do is to point out certain general requirements and principles. The first requisite of a good classification is, that it should be appropriate to the purpose in hand; that is to say, the points of resemblance selected to form the leading classes shall be those of importance to the practical use of the classification. All those things must be arranged to gether which require to be treated alike, and those things must be separated which require to be treated separately. Another and, in a scientific point of view, the most important requisite of a good classification, is that it shall enable the greatest possible number of general assertions to be made. This is the criterion, as stated by Dr. Whewell, which distinguishes a natural from an artificial system of classification, and we must carefully dwell upon its meaning. It will be apparent that a good classification is more than a mere orderly arrangement; it involves a process of induction which will bring to light all the more general relations which exist between the things classified. An arrangement of books will generally be artificial; the octavo volumes will not have any common character except being of an octavo size. An alphabetical arrangement of names again is exceedingly appropriate and convenient to many purposes, but is artificial because it allows of few or no general assertions. We cannot make any general assertions whatever about persons because their names happen to begin with an A or a B, a P or a W. In a classification of plants again we meet with most deep and natural distinctions between the great classes called Exogens, Endogens, and Acrogens. These are the very widest classes in what is called the natural system of botanical arrangement; but similar principles are observed in all its minor classes. The continual efforts of botanists are directed to bringing the great multitudes of plants together in species, genera, orders, classes, and in various intermediate groups, so that the members of each group shall have the greatest number of points of mutual resemblance and the fewest points of resemblance to members of other groups. Thus is best fulfilled the great purpose of classification, which reduces multiplicity to unity, and enables us to infer of all the other members of a class what we know of any one member, provided we distinguish properly between those qualities which are likely or are known to belong to the class, and those which are peculiar to the individual. It is a necessary condition of correct classification, as remarked by Prof. Huxley, that the definition of a group shall hold exactly true of all the members of a group, and not of the members of any other group. Natural classifications give us the deepest resemblances and relations, and may lead us ultimately to a knowledge of the way in which the varieties of things are produced. They are, therefore, essential to a true science, and may almost be said to constitute the framework of the science. Closely connected with the process of ClassifiIcation is that of abstraction. To abstract is to separate the qualities common to all individuals of a group from the peculiarities of each individual. The notion" triangle" is the result of abstraction in so far as we can reason concerning triangles, without any regard to the particular size or shape of any one triangle. All classification implies abstraction, for in framing and defining the class I must separate the common qualities from the peculiarities. When I abstract, too, I form a general conception, or one, which, generally speaking, embraces many objects. If, indeed, the quality abstracted is a peculiar property of the class, or one which belongs to the whole and not to any other objects, I may not increase the extent of the notion, so that Mr. Herbert Spencer is, perhaps, right in holding that we can abstract without generalizing. We often use this word generalization, and the process may be defined as inferring of a whole class what we know only of a part. Whenever we regard the qualities of a thing as not confined to that thing only but as extended to other objects; when, in fact, we consider a thing only as a member of a class, we are said to generalize. Dr. Whewell added to the superabundance of terms to express the same processes when he introduced the expression Colligation of facts. Whenever two things are found to have similar properties so as to be placed in the same class they may be said to be connected together. We connect together the places of a planet as it moves round the sun, when we conceive them as points upon a common ellipse. Whenever we thus join together previously disconnected facts, by a suitable general notion or hypothesis, we are said to colligate them. Dr. Whewell |