be said to have changed as much in substance as in form. The new conceptions introduced into the Biological Sciences have revolutionised their points of view, methods of procedure, and systems of classification. In the light of larger and more illuminating generalisations, sections of the subject, hitherto only partially explored, have acquired new prominence and value, and are cultivated with the keenest interest. It is enough to specify the researches into the ultimate structures, serial gradations, and progressive changes of organic forms, into the laws of their distribution in space and time, and into the causes by which these phenomena have been brought about. The results of persistent labour in these comparatively new fields of inquiry will largely determine the classifications of the future. Meanwhile the whole system of grouping, and many points of general doctrine, are in a transition state; and what is said and done in these directions must be regarded, to a certain extent at least, as tentative and provisional. In these circumstances, the really important thing is, that whatever may be said on such unsettled questions should be said with the authority of the fullest knowledge and insight, and every effort has been made to secure this advantage for the New Edition of the Encyclopædia.
The recent history of Physics is marked by changes both of conception and classification almost equally great. In advancing from the older dynamic to the newer potential and kinetic conceptions of power, this branch of science may be said to have entered on a. fresh stage, in which, instead of regarding natural phenomena as the result of forces acting between one body and another, the energy of a material system is looked upon as determined by its configuration and motion, and the ideas of configuration, motion, and force are generalised to the utmost extent warranted by their definitions. This altered point of view, combined with the far reaching doctrines of the correlation of forces and the conservation of energy, has produced extensive changes in the nomenclature and classification of the various sections of physics; while the fuller investigations into the ultimate constitution of matter, and into the phenomena and laws of light, heat, and electricity, nave created virtually new sections, which must now find a place in any adequate survey of scientific progress. The application of the newer principles to the mechanical arts and industries has rapidly advanced during the same period, and will require extended illustration in many fresh directions. Mechanical invention has, indeed, so kept pace with the progress of science, that in almost every department of physics improved machines and processes have to be described, as well as fresh discoveries and altered points of view. In recent as in earlier times, invention and discovery have acted and reacted on each other to a marked extent, the instruments of finer measurement and analysis having directly contributed to the finding out of physical properties and laws. The spectroscope is a signal instance of the extent to which in our day scientific discovery is indebted to appropriate instruments of observation and analysis.
These extensive changes in Physics and Biology involve corresponding changes in the method of their exposition. Much in what was written about each a generation ago is now of comparatively little value. Not only therefore does the system of grouping in these
