1. A just balance should be maintained between the difficulty of each lesson unit and the ability of the class. Matter that is too easy requires no effort in its mastery and hence is uninteresting. Matter that is too difficult discourages effort, and is, therefore, equally uninteresting. It should be sufficiently easy for every pupil to master, and sufficiently difficult to require real effort. 2. The amount of matter included should be carefully adjusted to the length of time taken for the lesson and to the attainments of the class. If too much is attempted, there will be insufficient time for adequate drill and review, and hence there will be lack of thoroughness. If too little is attempted, time will be wasted in needless repetition. 3. Each unit of instruction in any subject should, in general, grow out of the preceding unit taken in that subject, and be closely connected with it. It is in this way that a pupil's interest is aroused for the new problem and his knowledge becomes organized. Neglect in this regard results in the possession of disconnected and unsystematized facts. Each lesson should contain one or more central facts around which the other facts are grouped. This permits easy organization of the material of the lesson, and ensures its retention by the pupils. Further, the pupils are by this means trained to discriminate between the essential and the non-essential. CHAPTER XVII LESSON TYPES =The Developing Lesson.=--In the various lesson plans already considered, the aim has always appeared as an attempt to direct the learning process so that the pupil may both build up a new experience and also gain such control over it as will enable him to turn it to practical use. Because in all such lessons the teacher is supposed to direct the pupils through the four steps of the learning process in such a way that they discover for themselves some important new experience, or develop it out of their own present knowledge, the lessons are spoken of as developing lessons. Moreover, the two parts of the lesson in which the new experience is especially gained by the pupils, namely the selecting and relating processes, are often spoken of as a single step and called the step of _development,_ the lesson then being treated under four heads: Problem, preparation, development, and application.