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Show GENERAL CONSIDERATIONS UPON CLASSIFICATION. 83 an idea of its differences and community with other objects of the same category, so far as those differences and community are understood. The differences and affinities (that is to say, community of attributes and relations) between the members of a category are ordinarily not few, much less single, but numerous and complex; and the value and utility of a system of classification is about proportional to the number of differences and affinities which it truthfully expresses. Systems of classification are spoken of as "artificial" and "natural." My understanding is that an artificial system is- one which takes account of the agreements and disagreements of the classified objects with respect to only one characteristic or one very limited set of characteristics. The meaning of the expression "natural system of classification" is much more difficult to assign. Most probably different authors would entertain widely differing conceptions as to its meaning, none of which would be very definite or precise. They might, however, agree that a natural system as contradistinguished from an artificial one takes cognizance of all the characteristics and relations of the members to each other; the difference and affinity in any case being rated and valued, therefore, in accordance with the totality of characters and not dependent upon merely one of them. But it is far easier to say this much about a system of classification than it is to comprehend it! The truth is, that a natural system in any such length and breadth is impossible for any category, unless we know all the members of it and the totality of their relations; and there is no reason to believe that human knowledge has ever reached to that perfection. But as knowledge is ever increasing, we may at least hope for the time when it shall be sufficient to enable us to find and designate the greater and more important relations with absolute verity; and if the systema naturce is fitted and keyed together in order and harmony, as we are fain to believe, the outstanding facts will fall readily into their places; just as the final parts of a puzzle are quickly placed when the true arrangement of the other parts is discovered. A purely artificial system marks the initial stage of generalization of knowledge; a perfect natural system is for the time being unattainable. The growth of knowledge and philosophy, however, is marked by a transition, long, laborious and very gradual, from one to the other; a transition, which is marked by an indefi- \ |