diversity of animal life is the result of processes of evolution or continuity of development. This involves adaptation, which has to be conformed to a changing environment. When the change is in the direction of complexity, we have elaboration; when it is in the direction of simplicity, we have degeneration. Continued elaboration, involving a tendency to differentiation that gives rise to individuality, and a tendency to integration giving rise to association, is progress; and this is opposed to degeneration. The factors of evolution are those of origin and guidance. The origin of variations lies in mechanical stresses and chemical or physical influences. Whether these act on the body, and are transmitted by inheritance, or only on the germ, is not decided. It is also debatable whether use and disuse are factors of origin. The almost universally admitted factor in guidance is natural selection. The physiology of the senses and sense-organs of animals is studied as preliminary to the psychical or mental accompaniments of affections of those organs, which are styled explosive disturbances in the brain or other aggregated mass of nerve-cells. In the mental processes of man a distinction is made between perceptual construction, by which we construct an image of an object from the complex of our perceptions of it, and conceptual analysis, by which wc isolate particular qualities of it, forming concepts of the isolates. The formation of a conceptual inference or a judgment is regarded as the first stage of reason, and any mental process involving conceptual inference is rational. In contradistinction to this, an intelligent act is an act performed as the outcome of merely perceptual inference. The quality in animals is intelligence; their faculties are only perceptual. In man alone, and in no other animal, it is contended, is the rational faculty thus defined, developed; and that, "among human folk, that process of natural selection which is so potent a factor in the lower reaches of organic life, sinks into insignificance. For him the moral factor becomes one of the very highest importance. He becomes a conscious participator in the evolution of man, in the progress of humanity." But he can never be wholly independent of natural selection, for biological laws still hold true, though moral considerations and the law of duty may modify them; but, however profound the modification by the introduction of newer and higher factors, the older and lower factors are still at work beneath the surface. The relations of mind and the material organism are discussed in the last chapter.
The Theory of Light. By Thomas Preston, M. A. London and New York: Macmillan & Co. Pp. 465. Price, $3.26.
The history and present condition of the science of optics form the field of this treatise. In his preface the author refers to the difficulty experienced by students of science in obtaining the scattered publications which contain the latest advances in their respective specialties, and states that in no branch of experimental physics is the English student placed at such a disadvantage as in the theory of light. "Influenced by these considerations," he continues, "I have been induced to undertake the present work, with the hope of furnishing the student with an accurate and connected account of the most important optical researches from the earliest times up to the most recent date. I have, however, avoided entering into the more complicated mathematical theories, yet the mathematical theory, in its most elementary form, as well as the experiments on which it is founded, will be found in sufficient detail to enable the student, furnished with the necessary knowledge of higher mathematics, to attack at once with profit the original memoirs and theories recently elaborated by various English and foreign writers." The book gives a few pages to the views of the ancient philosophers, and comes down so far as to include the recent experiments of Prof. Hertz. The divisions of the book, however, are topical rather than historical; thus, the second chapter describes the propagation of light-waves and the composition of vibrations; the rectilinear propagation of light is the subject of the third; and succeeding chapters deal with reflection, refraction, interference, polarization, etc., the topic which concludes the volume being electro-magnetic radiation. The book is supplied with over two hundred diagrams; it is descriptive, not controversial in character, and is adapted to students well advanced in the science.