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62 could be reached best not by deductions from an attempted synthesis of observations on all the animals that creep on the land or swim in the sea (in the fashion of collectors of postage stamps), but by a complete study of a few organisms. Cuvier by a kind of guess-work used to reconstruct an entire animal from a single bone: full knowledge would enable us to do this in a complete, definite, qualitative and quantitative fashion. When such a knowledge has been attained, each single character will at once define and limit for us the possibilities of the other characters. Such a true and logical extension of the prin- ciple of correlation in morphology is really an application of the theory of functions to the living world. It would not exclude the study of causation, but limit it to its proper sphere. No doubt the "causes" of the correlations of organisms must be sought for in the idioplasm.

The possibility of applying the principle of correlated variation to psychology depends on differential psychology, the study of psychological variation. I believe, moreover, that a combination of study of the anatomical "habit," and the mental characteristics will lead to a statical psychophysics, a true science of physiognomy. The rule of investigation in all the three sciences will have to be that the question is posed as follows; given that two organisms are known to differ in one respect, in what other respects are they different? This will be the golden rule of discovery, and, following it, we shall no longer lose ourselves hopelessly in the dark maze that surrounds the answer to the question "Why?" As soon as we are informed as to one difference, we must diligently seek out the others, and the mere putting of the question in this form will directly bring about many discoveries.

The conscious pursuit of this rule of investigation will be particularly valuable in dealing with problems of the mind. Mental actions are not co-existent in the sense of physical characters, and it has been only by accidental and fortunate chances, when the phenomena have presented themselves in rapid succession in an individual, that discoveries of