Balliet points out that science teaching has passed through three stages in the past generation. The first stage is characterized by the textbook method, occasionally supplemented by illustrative experiment, performed by the teacher. The second stage is characterized by individual laboratory experiment, a manual for a guide, and by a lack of application of the principles except for a few traditional cases. The third stage improves upon the second by leading the pupil, after formulating his generalizations, to apply them to the facts and phenomena of nature. "But", continues Balliet, "we must advance to a fourth stage. We must not only apply the generalizations, but make the _explanation_ of the facts and phenomena of nature--the interpretation of nature--the very goal of science teaching." All problems should be chosen then in the light of this last aim. The problems must be natural, not in any way artificial, and they should be those of the immediate environment of the pupil. To meet these obligations may be in some cases difficult, but it should not be impossible. In biological science there is a rich field permitting a considerable choice in method. There are observations, projects, experiments, excursions, individual reports, book readings, quizzes, and conferences. In a single well chosen problem or project nearly all of these will be employed. Biology lends itself ideally to the problem method of teaching. By using some every day problem of the pupil, his interest is assured. Even a seemingly simple problem if skilfully directed, will ramify into several fields of biology before its solution is completed. And the number of practicable problems is almost limitless, but not all are equally good for the purpose, so the teacher must often tactfully modify the pupils choice. Original choices are likely to be too complex for the pupil to solve at his stage of progress, so must be simplified, without his feeling that he has been interfered with, without causing a wane in his interest. It is clear that the real problem in the problem-method is the teacher's. Practically, it is quite impossible to handle _individual_ projects in large classes. In the writer's experience, he has had on the average 80 different pupils per day in four separate classes. It is clearly beyond the power of any teacher to direct simultaneously eighty different projects, and it would be a physical impossibility to furnish the necessary laboratory apparatus. So, for this reason the teacher may find it necessary to divide, as diplomatically as possible, the classes into congenial groups, each with its problem, so that the total number of problems will be so limited that each one may be given adequate attention. It seems that such must be the limitation of the problem-method under the conditions prevailing in the public schools today.