§ 4. O. Kuelpe, Introduction to Philosophy, 1897, 55 ff.; Wundt, as above; G. T. Ladd and R. S. Woodworth, Elements of Physiological Psychology, 1911; E. W. Fiske, An Elementary Study of the Brain, 1913; K. Dunlap, An Outline of Psychobiology, 1914. § 5. W. S. Jevons, The Principles of Science, 1900, bk. iv., chs. xviii., xix.; E. B. Titchener, Prolegomena to a Study of Introspection, in American Journal of Psychology, xxiii., 1912, 427 ff.; O. Kuelpe, Outlines of Psychology, 1909, § 2; W. A. Hammond, Aristotle’s Psychology, 1902; C. A. F. Rhys Davids, A Buddhist Manual of Psychological Ethics, 1900. § 6. M. Howe and F. H. Hall, Laura Bridgman, 1903, 49 f.; G. Stoerring, Mental Pathology in its Relation to Normal Psychology, 1907 (the quotations from this work are sometimes condensed in the text); S. I. Franz, Handbook of Mental Examination Methods, 1912, 68, 80. § 7. Add, as typical, to works already cited: W. Preyer, The Mind of the Child, 1888-9 (human special); J. M. Baldwin, Mental Development in the Child and the Race, 1906 (human genetic); _id._, Social and Ethical Interpretations in Mental Development, 1906 (social); G. Le Bon, The Psychology of Peoples, 1898 (ethnic); A. Moll, Hypnotism, 1891 (derangement); G. Le Bon, The Crowd, 1910; J. Jastrow, Fact and Fable in Psychology, 1900 (collective derangement); E. L. Thorndike, The Principles of Teaching Based on Psychology, 1906; H. Münsterberg, Psychology, General and Applied, 1914. For the history of psychology, see O. Klemm, A History of Psychology, 1914; M. Dessoir, Outlines of the History of Psychology, 1912. CHAPTER II SENSATION Now that these points have been determined, let us proceed to a general discussion of the whole subject of Sensation.—ARISTOTLE § 9. =Sensations from the Skin.=—The skin is part of our organic birthright. One of the great differences between the living and the not-living lies in the possession of a skin; stone and iron weather and rust, but even the naked amœba has its ectosarc, and flowers of tan their plasmoderm. The skin is also the oldest of the sense-organs, and the mother of all the rest; how old, we dare hardly guess; but we know that the chemical elements which make up living tissue took form early in the history of our planet, earlier than the heavy metals. So it is natural to begin our survey of sensations by questioning the skin.