Page:Popular Science Monthly Volume 31.djvu/531

Rh ; while the increasingly important study of meteors and comets has brought us close to the very threshold of the great ultimate mystery of star-genesis and world-forming. The extreme tenuity of the mass of comets, the inconceivable rarity of the matter composing their gaseous tails, the curious phenomena of the instantaneous reversal on passing their perihelion, the proof that their light is partly reflected and partly direct, the spectroscopic determination of their composition, the discovery of the essentially planetary nature of meteor-streams, and the recognition of their vast numbers swarming through space, are among the most striking novelties of the last half-century in this direction.

In sidereal astronomy, besides the mere mechanical increase of mapping, the chief advances have been made in observations upon double stars, spectroscopic analysis of fixed stars and of nebulæ, and consequent proof of the fact that truly irresolvable nebulæ do really exist, the gaseous raw material of future stars and solar systems. It must be added that within the half-century the hypothetical ether has amply vindicated its novel claim to take its place as a mysterious entity side by side with matter and energy among the ultimate components of the objective universe.

In geology, the chief theoretical advances have been made by the recognition of the cosmical aspects of the earth's history; its relations to nebula, sun, and meteor; the importance of eccentricity and precession of the equinoxes, and the possible results of ancient changes in its rates of motion, tides, and so forth. Dynamical geology has made vast strides, especially in the investigation of volcanic phenomena, mountain-building, and the birth and growth of islands and continents. The science of earth-sculpture has been developed from the very beginning. Stratigraphical geology has been largely improved. And in paleontology an immense number of the most striking and interesting of fossil forms have been brought to light. Among them may be specially mentioned those which have proved of critical importance as evidences of the truth of organic evolution—the toothed birds of the Western American cretaceous deposits, the lizard-like bird or bird-like feathered lizard of the Solenhofen slates, Marsh's remarkable series of ancestral horses, Cope's beautiful reconstruction of the fossil progenitors of existing camels. Monkeys certainly, anthropoid apes clearly, man doubtfully, have been detected in the fossil state. India, Australia, Canada, the United States have been explored and surveyed, geologically and paleontologically; and the exploitation of the far West in particular has not only added immensely to our knowledge of life in past times, but has also revolutionized our conceptions as to the gradual growth and development of continental areas, and the occasional vast scale of volcanic phenomena. The permanence of all great continents and oceans is now a proved truth of geology. It has been re-enforced and extended from a totally different point of view by