234 W. L. DAVIDSON I

like with like and keeping separate things that are dissimilar ; but it is in the second only that the idea of gradation comes in, and so the conception of higher and lower in generality. Thus, the letters of the alphabet, as they stand in the order familiar to us all, are unclassified. There is no reason why A should precede B, or B should be followed by C : we might equally well begin with B as with A, with M or with P, as with either ; and but for the matter of habit, a " beta-alph " would be as appropriate as an " alpha-bet ". We proceed to classify only when we group distinct letters together, on the score of their possessing some striking peculiarity in common ; as when we pick out the vowels from the consonants, or when we form classes of labials, dentals, liquids and so forth. Not yet, however, have we reached the full sense of classification. This would be attained only if we could arrange the groups of letters on some distinct plan, so that each group should be seen to occupy its own proper place, and to have definite relations to all others around it. Speaking strictly, we form a class when we bring together a collection of individuals held in union by the bond of one or more points of community, and when we take care that nothing that is destitute of the point or points of community is admitted into the class : we classify when we arrange classes thus constructed on the principle of higher and lower, wider and narrower. Hence, Classifica- tion naturally assumes the form of a series of grades. We ascend from the lower to the higher, or descend from the higher to the lower, in a continuous order ; and the relations that obtain between groups are those of subordination, superordination and co-ordination. One group is subor- dinate to another when it is contained under that other as a part of a compound whole, whose mark it possesses but which has in addition distinguishing characteristics of its own. One group is superordinate to another when it is regarded as the higher under which the other takes its place as lower. Two or more groups are co-ordinate when they stand upon the same level or occupy positions of equal authority such as Orders of different Classes, in botany, or Genera of different Orders. And if we ask what is the full signification of this classifying process, we find it is simply this that the different groups have different degrees of generality, and that the greater the generality the less the meaning conveyed, while the less the generality the richer the meaning. Thus, we take the grade ' Class ' in the botanical grouping. This is a division very high in the scale, and includes an enormous number of sub-divisions