When the vibratory theory was accepted, it became necessary to investigate the nature of the ether and to determine its characteristic properties. Neumann, MacCullagh, Green, and Stokes then developed an elastic solid theory of the ether. The experiments of Lord Rayleigh, Lorentz, Drude, Larmor, and others suggested that light is identical with electromagnetic disturbances and, consequently, is an electrical phenomenon. Some of the finest developments in physics during the nineteenth century were in the realm of electricity. They resulted in an enormous extension of the use of electricity in industry and commerce and led to the investigation of radioactivities of various kinds and these in turn are developing investigations of a most brilliant character. CHAPTER XI THE NATURAL SCIENCES Manifestations of animal life are everywhere visible. They may be seen on mountain peaks, in desert plains, and by the seashores. Even the bleak arctic ice fields have their faunas. This extraordinary distribution of life has attracted attention since the dawn of history. Primitive man, by his often beautiful cave drawings, indicated that he studied intimately the wild life surrounding him. The basic facts of natural history were studied by the early peoples of the Near East. The Greeks prepared many books on natural history and anticipated modern evolutionary theories. The natural sciences, however, made slow progress until toward the end of the eighteenth century when Linnæus and Buffon began their great works. When the nineteenth century opened, the broader fields of nature were segregated, classified, and described. Linnæus took broad views regarding the principles of classification based upon general structure, and his work was enlarged and improved by Cuvier. Buffon contributed suggestions regarding the probable mutability of species with respect to changes in environment, and improved on the old Greek evolutionary ideas by formulating a definite theory of the causes of mutability. He was an important agent in promoting the modern theories of evolution in zoölogy and botany, which have done more than anything else to augment our knowledge of terrestrial life. The numerous scientific exploring expeditions in the eighteenth and nineteenth centuries collected an enormous amount of data regarding animals and animal life. Early in the nineteenth century this data was worked up and classified. It soon became apparent that the range of any given species of animal is strictly limited. A new science, that of the geographical distribution of life, was developed. This has been very fruitful in defining the true home areas of all species of animals, insects, birds, and fish, and locating their principal paths of migration.