schematic diagrams in which the essential elements of a generalized conclusion are summarized. Analytical and systematic essays should include diagrams of ideal forms, as well as pictures of corresponding actual forms. Grouped block diagrams showing successive stages of development are serviceable, because they so compactly present the normal succession of a series of complicated forms, and thereby so greatly aid the understanding of the text. It may, indeed, be fairly claimed for such diagrams that, by permitting the abbreviation of explanations, they save at least as much space as they occupy. It should be added, however, that there is good reason for thinking that the full value of graphic illustrations has not yet been reached; and that active invention as well as better training will surely lead to notable advances. Regional essays should be illustrated by maps and pictures, and especially by simplified and generalized diagrams, in which the distribution of the larger features may be so clearly shown. Outline map-diagrams and profiles are so much less serviceable than block diagrams, that the latter are to be preferred whenever it is possible to prepare them.
The guiding principle here, as in the preceding suggestions, is that everything possible should be done to make the author's meaning easily and clearly intelligible to the reader.