LITERARY NOTICES. 



4*7 



diversity of animal life is the result of pro- 

 cesses of evolution or continuity of develop- 

 ment. This involves adaptation, which has 

 to be conformed to a changing environment. 

 When the change is in the direction of com- 

 plexity, we have elaboration ; when it is in 

 the direction of simplicity, we have degen- 

 eration. 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 fac- 

 tors of evolution are those of origin and 

 guidance. The origin of variations lies in 

 mechanical stresses and chemical or physi- 

 cal 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 affec- 

 tions of those organs, which are styled ex- 

 plosive disturbances in the brain or other 

 aggregated mass of nerve-cells. In the men- 

 tal 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 con- 

 ceptual analysis, by which wc isolate partic- 

 ular qualities of it, forming concepts of the 

 isolates. The formation of a conceptual in- 

 ference 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 ra- 

 tional 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 fac- 

 tor becomes one of the very highest impor- 

 tance. He becomes a conscious participator 

 in the evolution of man, in the progress of 

 humanity." But he can never be wholly in- 

 dependent of natural selection, for biological 

 laws still hold true, though moral consid- 

 vol. xxxix. 29 



erations and the law of duty may modify 

 them ; but, however profound the modifica- 

 tion 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 dis- 

 cussed in the last chapter. 



The Theory of Light. By Thomas Pres- 

 ton, 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 trea- 

 tise. In his preface the author refers to 

 the difficulty experienced by students of 

 science in obtaining the scattered publica- 

 tions 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 element- 

 ary form, as well as the experiments on 

 which it is founded, will be found in suffi- 

 cient 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 re- 

 flection, 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 char- 

 acter, and is adapted to students well ad- 

 vanced in the science. 



