tive affinities of organic elements, the equilibrization of organs—such are the designations of the leading doctrines which are unfolded in the preliminary discourse of his" (St.-Hilaire's) "'Anatomical Philosophy.' Elective affinities of organic elements are the forces by which the vital structures and varied forms of living things are produced; and the principles of connection and equilibrium of the forces of the various parts of the organization prescribe limits and conditions to the variety and development of such forms."[1] Now for the first time we hear such phrases as "unity of plan," and (more significant still) "unity of composition." Then came Von Baer's law of progression of structural development from the general to the special, afterward extended to functional development, and giving rise to the conception of the specialization of functions. Out of this, too, arose the term "evolution," and, though confined to organic development, implied an advance in generalization. The mere mention of such further advances as are implied in the establishment of the functional identity between the contractile tissues of plants and those of the higher animals; in the use of the phrase "psychical powers" to designate the sensorial and mental endowments of animals; in the proof of the absence of specialized sensibility among the lower tribes of animals, and of the hereditary transmission of certain characters acquired under the influence of external circumstances; in the parallel traced between the progressive complication of the psychical manifestations during the early life of a human being, and the gradual increase in mental endowment to be observed in ascending the animal scale[2]—may serve to indicate the conceptions forming the matrix in which a philosophically constructed Psychology was to be moulded. How great a revolution had taken place in biology, and how far we have now got from the natural history method, may appear from Prof. Huxley's definition of "zoological physiology," which, though made some years after the first publication of the "Principles of Psychology," at least points out the direction in which thought had been moving. He says:
"It regards animal bodies as machines impelled by certain forces, and performing an amount of work, which can be expressed in terms of the ordinary forces of Nature. The final object of physiology is to deduce the facts of morphology, on the one hand, and those of distribution on the other, from the laws of the molecular forces of matter."[3]
With a prescient insight into the future of science which has probably few parallels, Mr. Spencer founded his Psychology on the hypothesis of development. To all but a few deep-thinking observers there can have seemed few signs in 1855 that that hotly-disputed theory was ever likely to be in the ascendant. The exposition of none of the
