120. The case is different when later on larger sections of a course of successful instruction are to be repeated. During all the earlier stage particular facts were moved far apart (68) for the sake of clearness; by means of conversation, or of incidental mention in other recitations, or through experience itself (110), provision was made also for association of various kinds. Now it becomes the business of repetition in the first place to gather together into a smaller compass what has been expanded; next it subserves the purpose of systematic arrangement, and lastly, is often of use for making the instruction more complete and for adding the difficult to the comparatively easy. Here the mode of presentation itself changes to meet the requirements of a more advanced grade of work. But repetition immediately after the presentation, or, perhaps, during the next hour, will, as a rule, remain necessary even at this higher stage. 121. Here, where compression and insertions are to modify the material of instruction, we need to inquire into the forms of connection peculiar to the objects, together with those essential for use, and to determine accordingly the series and web of ideas to be formed in the mind of the pupil. For such organization of ideas, repetition is, at all events, far better adapted than presentation, which can traverse only one of several series at a time, and which passes into repetition the moment an effort is made to bring the other series forward also. In natural history, for example, various classifications occur, in history the ethnographic divisions are crossed by the synchronistic, while the history of culture demands yet another basis of association; in geography each noted city is to be a landmark, enabling the pupil to take his bearings in every direction, but cities on rivers suggest river basins and mountain ranges; in mathematics each theorem is to be kept ready for separate application, but it has also its special place in the chain of demonstrations; grammatical rules, too, should be available when called for, but it is very necessary at the same time that the pupil become perfectly at home in his grammar and know where to look for information. The teacher who, by skilful repetition, does justice to these multiform associations, is not always the one who shows most skill in systematic presentation, and who knows best how to make prominent the main thoughts, and to link to them those that are subordinate.