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ordination of characters.' In this arrangement, the higher the place which any class holds in the classification, the more important are the characters which constitute it. This arrangement will prevent any widely dissimilar groups from being brought together in the lower divisions. The ox and the frog will be held apart in the classification, as in Nature. Thus, if we are considering flowering plants, we notice that plants in which the ovules are enclosed in a protective structure resemble one another (and differ from those whose ovules are unprotected) not only in this particular, but in a large number of other points as well, such as the structure of their vascular tissue, the form of the stamens, the germination of the pollen-grain, and the development of the endosperm. In classifying flowering plants, we therefore divide them first of all (according as they have protected or unprotected ovules) into Angiospermæ or Gymnospermæ. In subdividing the Angiospermæ, we choose the character of the presence of two primary leaves, or of only one, and thus form the two alternative sub-classes, Dicotyledons and Monocotyledons. After this we go on to other characters in descending order of importance, and so form our Orders, Sub-orders, Genera, and Species.

The characterization of Angiosperms, according as they are dicotyledonous or monocotyledonous, admits of being stated in a relatively untechnical way. Thus:

(i.) Dicotyledons have the following characters :

(1) The embryo has two seed-leaves or cotyledons.

(2) The first or primary root of the embryo branches after

it leaves the seed.
(3) The stem branches repeatedly.

(4) The stem, when perennial, has a distinct pith, con-
tinuous rings of wood, and separable bark. The
stem increases in thickness by the formation of fresh
rings of wood outside those already formed and
inside the bark. The hardest wood is inside.

(5) The outer parts of the flower are most commonly in fives-i.e., have five members in each whorl.

(6) The leaves are net-veined.

(ii.) Monocotyledons have the following characters : (1) The embryo has only one seed-leaf.

(2) The primary root branches before it leaves the

seed.

(3) The stem, as a rule, shows little branching, and in the monocotyledonous trees (such as Palms) it may be quite unbranched, growing only from a bud at its apex, the buds produced in the axils of the leaves remaining undeveloped.

(4) The stem is without any distinct pith, continuous rings of wood, or separable bark. The wood consists of bundles of fibres and vessels, which are separately embedded in cellular tissue. The hardest bundles are outside.

(5) The outer parts of the flower are in threes.

(6) With few exceptions, the leaves are straight-veined.

It is to be noticed that the most important characters are by no means (usually) the most obvious. Our natural groups seem, at first sight, to include extremely heterogeneous kinds. To an unbotanical mind, the yellow cowslip, the scarlet pimpernel, and the purple cyclamen would seem as unlike as flowers could be; yet these three species are closely related, and we class them all in the Natural Order Primulaceæ. So also the daisy, the goldenrod, and the thistle belong to one Natural Order, the Compositæ ; and two flowers so unlike as the blue cornflower and the purple knapweed belong, not only to the same family, but even to the same genus (Centaurea). We do not, in Classification, give the preference to the most obvious, but to the most significant and the least variable characters. Thus, in both Zoological and Botanical Classification, Analogy (resemblance arising from adaptation to similar functions) is of far less importance than Homology or morphological identity. Hence the paramount necessity, for purposes of Classification, of the study of Development. In classing any organism, we must consider not only its characters at any one moment of observation, but also those exhibited by its past history; for thus alone is it possible to ascertain the homologies of structure upon which Comparative Morphology is founded.

The importance to Classification of a close study of Development has been tenfold increased by the discovery of the connexion between ontogeny and phylogeny, the establishment of the theory that each individual organism (at least among animal forms) 'recapitulates' in its development the whole history of its race. If we were to meet, for the first time, a full-grown hen, we might be uncertain of her exact place in the Animal Kingdom; but when we have watched day by day the development of the chick, in the egg, from the single cell which represents some protozoan ancestor, through the fish-like stage which exhibits a swimming tail and conspicuous gill-slits, and again through the reptilian form with its four limbs and hands, each with its five digits distinctly shown by the microscope, on to the first emergence of the characteristic birdlike form, then we have no difficulty in relegating our adult fowl to her proper position in our zoölogical classification.

Thus, an ideal natural classification of animals or plants would represent, not only the present affinities, but the whole ancestral history of the organisms dealt with. It would indicate no mechanical arrangement of isolated types, but an organized continuum in which some of the missing links would be supplied by palæontological research, and others would be ideally reconstructed with more or less of probable exactness. Our scheme of classification would thus become a genealogical tree, showing the vital relation of each kind to all the others, and thus making evident the organized unity of the whole.*

Definition in Connexion with Natural Classification. - The Problem of Classification involves the necessity of defining the names which constitute the Nomenclature. In the case of Natural Classification, Definition takes the form of Characterization-i.e., of giving an inventory of the known characteristics common to all the typical members of the class indicated by the term to be defined a result to be obtained only through those thorough-going analyses and syntheses which are called for when we study Nature, with reconstructive intention, as a complex and developing system of which all the parts and aspects are intimately interrelated. In a natural classification, as we have seen, every group, from the primary divisions downwards, will possess a number of common characteristics. Thus, the definition of the term 'Dicotyledon' might be stated somewhat as follows :

The term 'Dicotyledon' stands for a plant possessing the distinguishing characters of the Angiospermæ (genus), and further characterized by the following marks :

Embryo with two cotyledons.

Stem, when perennial, having a distinct pith, continuous rings of wood, and separable bark, and branching repeatedly.

Leaves net-veined.

Parts of the flower usually in fives.

So, again, the definition of the term 'Vertebrate' in Zoölogy would be somewhat as follows :

The term 'Vertebrate' stands for a multicellular animal (genus) characterized by the following marks :

1. The possession (at some stage of the animal's development) of a smooth, elastic, dorsally placed rod (the Notochord) lying ventral

* As a particularly important and impressive instance of Natural Classification in the realm of inorganic Nature, we may mention the classification of the chemical elements according to Mendeléeff's Periodic Law. This same instance is also an excellent example of Classification by Series (vide Professor Duncan's 'The New Knowledge,' ch. iii.; Hodder and Stoughton, 1906). As another, perhaps still more important, instance of Natural Classification-this time in the realm of spiritual values-we would refer to an article on 'The Classification of the Virtues,' by H. W. Wright (The Journal of Philosophy, Psychology, and Scientific Methods, vol. iv., No. 6, March 14, 1907). However, Mr. Wright does not so much furnish the classification itself as the principles for making it. 'As the species are classified according to the part they play in the process of organic evolution, so the virtues are classified according to the office they discharge in the organization of conduct. Thus our ideal of a principle of classification organic to the field of its application is realized' (ibid., p. 160).

to the nerve-cord. This may be replaced by a cartilaginous (i.e., gristly) rod, or by a column of distinct 'vertebræ.' These, again, may either remain cartilaginous or be replaced later by vertebræ of bone. (These vertebræ grow round and protect the nerve-cord.) 2. The possession, at some stage of development, of gill-slits in the anterior part of the alimentary canal.

3. An unpaired dorsal nerve-cord, which is tubular, having a central canal, and is protected by the notochord or the vertebræ. In the more advanced forms the brain and sense organs are highly developed, the latter being paired.

4. A highly organized circulation. The heart is always ventral to the alimentary canal.

5. Symmetrical segmentation.

Definition by characterization tends, in the case of the developmental sciences, to take the form of Definition by Type, a type being defined as 'an example of any class for instance, the species of a genus-which is considered as eminently possessing the character of the class' (Whewell). Thus, Dr. P. Chalmers Mitchell says that Morphologists 'are slowly coming to some such conception as that a species is the abstract central point around which a group of variations oscillate, and that the peripheral oscillations of one species may even overlap those of an allied species' ('Enc. Brit.,' 10th edn., vol. xxviii., article on 'Evolution,' p. 343). Definition by Type is no doubt to this extent logically defective-that it does not provide ideally against ambiguity; and, in its insistence on a central as distinguished from a peripheral definiteness of characterization, it resembles Description rather than Definition proper; but it is none the less the Definition natural to classificatory Science.

There is good reason why, in Botanical and Zoölogical classification, the reference to organized reality should call for definition by type. Typical structures possessing a complete fitness for existence survive, and the intermediate forms tend to disappear, though there may be many deviations from type that are not important enough to interfere with that fitness to survive upon which the persistence of the type depends. Hence, in the developmental sciences, Real Definition-the definition of a class or concept that is framed to bear the searchlight which Science throws upon Nature is essentially central in character. The central qualities and tendencies determine the definition; but in its application the definition takes in all variations which show a more marked approximation to the central requirements in question than to those of any other definition. This Definition by Type, we may add, forms a transitional link between a rigid peripheral definition, or definition by boundaries, and the more inward and vital definition by ends or ideals. In this latter kind of Definition, the defining marks, far from being possessed in common by all the members of the class defined, may be possessed, in strictness, by none. If 'Man' is defined by the ideal his nature is capable of reaching, it is not necessary that any single individual man should possess the marks in question.*

Real Definition, finally, is essentially provisional and progressive. The widening of knowledge implies the remodelling of scientific principles and scientific classifications, and this implies that the definitions of essential concepts and of natural kinds undergo a sympathetic renewal. Where the aim of Definition is to characterize according to Nature, and the knowledge of Nature is continually deepening, the definition must adapt itself if it is not to stultify the very reason for its existence.

Diagnostic Classification and Definition.

A natural classification, in order to be really useful, should be accompanied by an analytical key. Such a key is a diagnostic classification, its function being to serve as an index or searcher for the corresponding natural classification. The essential distinction between these two types of Classification is that, whilst the marks which serve to locate a given species within the system are, in the case of Natural Classification, fundamental, in the case of Diagnostic Classification they may be merely superficial. The marks here are external and salient, and easily recognized. A diagnostic classification meets an important practical requirement -that of easy diagnosis-diagnosis being the method of determining the place of a natural kind in a classification, finding the correct name or label for the object by means of its characteristics.

The botanical system of Linnæus is essentially of the diagnostic type. It has been called artificial, because its classification-connexions do not stand for natural affinities. This is true, but it was not the author's intention that his classification should be natural. He intended that it should serve as a complete practical index or catalogue. 'It is an index to a department of the book of nature, and as such is useful to the student. It does not aspire to any higher character, and although it cannot be looked upon as a scientific and natural arrangement, still, it has a certain facility of application which commends it to the tyro. In using it, however, let it ever be remembered that it will not of itself give the student any view of the true relations of plants as regards structure and properties, and that, by leading to the discovery of the name of a plant, it is only a stepping-stone to the natural system. Linnæus

* As a suggestive illustration of what is involved in a philosophic definitiona definition, that is, which is framed under the control of such categories as those of development and personality-see Edward Caird's 'Evolution of Religion,' Lecture II. For the so-called 'pragmatic' definition, see C. S. Peirce, 'Illustrations of the Logic of Science,' in The Popular Science Monthly, vol. xii., under the two headings, The Fixation of Belief' (November, 1877), and 'How to Make our Ideas Clear (January, 1878).

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