Professor Huxley has defined the process of classification in the following terms b. By the classification of any series of objects, is meant the actual or ideal arrangement together of those which are like and the separation of those which are unlike; the purpose of this arrangement being to facilitate the operations of the mind in clearly conceiving and retaining in the memory the characters of the objects in question.'

This statement is doubtless correct, so far as it goes, but it does not include all that Professor Huxley himself implicitly treats under classification. He is fully aware that deep correlations, or in other terms deep uniformities or laws of nature, will be disclosed by any well chosen and profound system of classification. I should therefore propose to modify the above statement, as follows:- By the classification of any series of objects, is meant the actual or ideal arrangement together of those which are like and the separation of those which are unlike, the purpose of this arrangement being, primarily, to disclose the correlations or laws of union of properties or circumstances, and, secondarily, to facilitate the operations of the mind in clearly conceiving and retaining in the memory the characters of the objects in question.'

Multiplicity of Modes of Classification.

In approaching the question how any given group of objects may best be classified, let it be remarked that there must generally be an unlimited number of modes of classifying any group of objects. Misled, as we shall see, by the problem of classification in the natural sciences, philosophers often seem to think that in each subject there must be one essentially natural classification which

bLectures on the Elements of Comparative Anatomy,' 1864, p. 1.

is to be selected, to the exclusion of all others. This erroneous notion probably proceeds also in part from the limited powers of thought and the inconvenient mechanical conditions under which we labour. If we arrange the books in a library catalogue, we must arrange them in some one order; if we compose a treatise on mineralogy, the minerals must be successively described in some one arrangement; if we describe even such simple things as geometrical figures, they must be taken in some fixed order. We shall naturally therefore select that classification which appears to be most convenient and instructive for our principal purpose. But it does not follow that this system of classification possesses any exclusive excellence, and there will be usually many other possible arrangements, each valuable in its own way. A perfect intellect would not confine itself to one order of thought, but would simultaneously regard a group of objects as classified in all the ways of which they are capable. Thus the elements may be classified according to their atomicity into the groups of Monads, Dyads, Triads, Tetrads, Pentads, and Hexads, and this is probably the most instructive classification; but it does not prevent us from also classifying them according as they are metallic or non-metallic, solid, liquid or gaseous at ordinary temperatures, useful or useless, abundant or scarce, ferro-magnetic or diamagnetic, and so on.

Mineralogists have spent a great deal of labour in trying to discover a so-called natural system of classification for minerals. They have constantly encountered the difficulty that the chemical composition did not run together with the crystallographic form, and the various physical properties of the mineral. Substances identical in the form of their crystals, especially those belonging to the first or cubical system of crystals, were often found to have no resemblance in chemical compo

sition. The identically same substance, again, is occasionally found crystallized in two essentially different crystallographic forms; calcium carbonate, for instance, appearing as calc-spar and arragonite. Now the simple truth is that if we are unable to discover any correspondence, or, as we shall call it, any correlation between the several properties of a mineral, we cannot make any one arrangement which will enable us to treat at any one time all these properties. We must really classify minerals in as many different methods as there are different unrelated properties of sufficient importance. Even if, for the purpose of describing minerals successively in some one order in a treatise, we select one system, that, for instance, having regard to chemical composition, we ought mentally at least to regard the same minerals as classified in all other possible modes.

Exactly the same may be said of the classification of plants. An immense number of different modes of classifying plants have been proposed at one time or other, an exhaustive account of which will be found in Rees' 'Cyclopædia,' article Classification,' or in the Introduction to Lindley's 'Vegetable Kingdom.' There have been the Fructistæ, such as Casalpinus, Morison, Hermann, Boerhaave or Gaertner, who arranged plants according to the form of the fruit. The Corollista, Rivinus, Ludwig, and Tournefort, paid attention chiefly to the number or arrangement of the parts of the corolla. Magnol selected the calyx as the critical part, while Sauvage arranged plants according to their leaves; nor are these instances more than a small selection from the actual variety of modes of classification which have been tried. Of such attempts it may be said that every proposed system will probably yield some information concerning the relations of plants, and it is only after trying many modes that it is possible to approximate to the best.

Natural and Artificial Systems of Classification.

It has been usual to distinguish systems of classification as natural and artificial, those being called natural which seemed to express the order of existing things as determined by nature. Artificial methods of classification, on the other hand, included those formed for the mere convenience of men in remembering or treating natural objects.

The difference, as it is commonly regarded, has been well described by Ampère, as follows: 'We can distinguish two kinds of classifications, the natural and the artificial. In the latter kind, some characters, arbitrarily chosen, serve to determine the place of each object; we abstract all other characters, and the objects are thus found to be brought near to or to be separated from each other, often in the most bizarre manner. In natural systems of classification, on the contrary, we employ concurrently all the characters essential to the objects with which we are occupied, discussing the importance of each of them; and the results of this labour are not adopted unless the objects which present the closest analogy are brought most near together, and the groups of the several orders which are formed from them are also approximated in proportion as they offer more similar characters. In this way it arises that there is always a kind of connexion, more or less marked, between each group and the group which follows it.'

There is much, however, that is vague and logically false in this and many other definitions which have been proposed by naturalists to express their notion of a natural system. We are not informed how the import

e 'Essai sur la Philosophie des Sciences', p. 9.

ance of a resemblance is to be determined, nor what is the measure of the closeness of analogy. Until all the words employed in a definition are made clear in meaning, the definition itself is worse than useless. Now if the views concerning classification here upheld are true, there can be no sharp and precise distinction between natural and artificial systems. All arrangements which serve any purpose at all must be more or less natural, because, if closely enough scrutinized, they will involve more resemblances than those whereby the class was defined.

It is true that in the biological sciences there would be one arrangement of plants or animals which would be conspicuously instructive, and in a certain sense natural, if it could be attained, and it is that after which naturalists have been in reality striving for nearly two centuries, namely, that arrangement which would display the genealogical descent of every form from the original life germ. Those morphological resemblances upon which the classification of living beings is almost always based are inherited resemblances, and it is evident that descendants will usually resemble their parents and each other in a great many points.

I have said that a natural is distinguished from an arbitrary or artificial system only in degree. It will be found almost impossible to arrange objects according to any one circumstance without finding that some correlation of other circumstances is thus made apparent. No arrangement could seem more arbitrary than the common alphabetical arrangement according to the initial letters of the name. But we cannot scrutinize a list of names of persons without noticing a predominance of Evans's and Jones's, under the letters E and J, and of names beginning with Mac under the letter M. The predominance is so great that we could not attribute it to chance, and inquiry would of course show that it arose from im