Page:Popular Science Monthly Volume 78.djvu/242

232 method were adopted: and it is desirable that this point should be clearly brought forth in a systematic presentation.

The general principle of classification, alluded to above, is that in first subdividing a group of phenomena, advantage should be taken of the different values of some element common to all of them. For example, all land forms are the surface expression of some kind of structure; hence structure may be well taken as the basis of a first subdivision; and its values may run from simple to complex along some appropriate order. All forms, thus classed according to structure, have been more or less affected by the action of some external process; hence each of the former structural divisions may now be again divided according to the kind of process that has acted upon it. But inasmuch as any process working upon any structure requires time for the accomplishment of effect, a third subdivision may be made according to the stage of advance reached by the external process in its work upon the structural mass; and so on, with relief and texture, or any other elements that are to be considered. It may often happen that, after one or more subdivisions have been made in this way, no single element is found which runs with different values through all the last formed groups; then each of these groups may be subdivided according to the different values of an element that it alone possesses.

Each final kind of land forms is usually represented by a typical example, which may be either an actual occurrence or an idealized instance. The more important types should be illustrated by diagrams, and all the type diagrams should be drawn according to a common plan, uniform in style and scale, so as to subordinate irrelevant dissimilarities and emphasize essential likenesses. The aid of deduction must be frequently called upon, in order to fill out a series of forms, for which only a few members are provided by observation.

Technical terms are necessarily employed rather frequently in a systematic presentation. If they are presumably new to the hearers, it is desirable first to give some account of the thing that the term names, with graphic illustration by simple diagram when possible; then the thing being clearly conceived, the technical term may be introduced as a name for it. Thus the hearers will acquire both the thing and the term in their proper relation. If the term is introduced first, the hearers are placed in the dangerous position of trying to attach a concept to a name, instead of being led to the much safer position of attaching a name to a concept.

It was pointed out in the account of narrative presentation, that a student may to advantage exercise himself in that simple method when making his first appearance before an audience. Let it now be added that he ought surely to have had practise in analytic presentation before he undertake systematic, and in systematic before he undertakes regional, for regional presentation, next to be described, is the most