232 TEE POPULAR SCIENCE MONTHLY 



pirical 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 sub- 

 division; 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 accom- 

 plishment 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 fre- 

 quently 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 hear- 

 ers 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 be- 

 fore he undertake systematic, and in systematic before he undertakes 

 regional, for regional presentation, next to be described, is the most 



