=150. The law of extremes.= When a stimulus approaches either the maximum or minimum of the factor for the species concerned, response becomes abnormal. The resulting modifications approach each other and in some respects at least become similar. Such effects are found chiefly in growth, but they occur to some degree in structure also. It is imperative that they be recognized in nature as well as in field and control experiment, since they directly affect the ratio between response and stimulus. The data which bear upon the similarity of response to extremes of different factors are too meager to permit the formulation of a rule. It is permissible, however, to suggest the general principle that extreme stimuli produce similar growth responses, and to emphasize the need of testing its application to adaptation proper. =151. The method of working hypotheses.= In the study of stimulus and response, where the unimpeachable facts are relatively few, and their present correlation slight, the working hypothesis is an indispensable aid. “The true course of inductive procedure ... consists in _anticipating nature_, in the sense of forming hypotheses as to the laws which are probably in operation, and then observing whether the combinations of phenomena are such as would follow from the laws supposed. The investigator begins with facts and ends with them. He uses such facts as are in the first place known to him in suggesting probable hypotheses; deducing other facts which would happen if a particular hypothesis is true, he proceeds to test the truth of his notion by fresh observations or experiments. If any result prove different from what he expects, it leads him either to abandon or to modify his hypothesis; but every new fact may give some new suggestion as to the laws in action. Even if the result in any case agrees with his anticipations, he does not regard it as finally confirmatory of his theory, but proceeds to test the truth of the theory by new deductions and new trials.”[10] In the treatment of adjustment and adaptation which follows, the method of multiple working hypotheses is uniformly employed. No apology is felt to be necessary for this, since the whole endeavor is to indicate the proper points of attack, and not to distinguish between that which is conjectural and that which is known. If an hypothesis occasionally seem to be stated too strongly, it is merely that it appears, after a survey of the problem from all sides, to explain the facts most satisfactorily. The final proof of any hypothesis, however, rests not only upon its ability to explain all the facts, but also upon the inability of other hypotheses to meet the same test. The discovery and examination of all possible hypotheses, and the elimination of those that prove inadequate are the essential steps in the method of working hypotheses. HYDROHARMOSE _ADJUSTMENT_