These views, however absurd they seem to most people now, were almost universally held so recently as during the seventeenth and eighteenth centuries, and were thus coincident with one of the most brilliant epochs of our literature and our dawning science. It was only towards the beginning of the nineteenth century, when geology became widely studied and its results were fully appreciated, that the more rational conception of a very slow development of the earth’s surface during countless ages began to be generally accepted. [Sidenote: Changing Conditions of the Earth] The grand nebular hypothesis of Laplace came to reinforce the views of the geologists, by showing how the earth itself may have originated as a gaseous or molten globe; and its slow process of cooling, with the reaction of the interior and exterior on each other, served to elucidate the facts of the heated interior, as shown by hot springs and volcanoes, as well as many of the phenomena presented by the distorted and metamorphosed strata which formed its crust. Hence it gradually came to be perceived that the condition of the earth, with all its endless variations of surface, of continents and oceans, of seas and islands, of vast plateaux and lofty mountain ranges and extensive low plains, with their ravines and cataracts, their great lakes and stately rivers, was subject to perpetual change from that remote epoch when it seems to have been actually the case that “the earth was without form and void,” and that owing to the greater density of the vapour-laden atmosphere, “darkness was upon the face of the deep.” [Sidenote: Changing Forms of Life] Another field of geological research forced us to the conclusion that the same continued process of change had affected the forms of life upon the earth. When carefully investigated, the crust was found to abound in the fossilised remains of animals and plants. Careful study of these showed that the oldest of all were of comparatively simple structure, and that the higher forms only appeared in more recent epochs; while the highest of all were probably very little older than man himself. It is only during the last half century that the theory of Evolution has been elaborated and has become generally accepted as applicable to the whole of the vast cosmic process--from the development of the nebulæ into stars and suns and systems, with a corresponding development of planets from an early condition of intense heat, through a more or less lengthy period of cooling and contraction, to an ultimate state of refrigeration, the earlier and later stages being alike unsuited to the existence of life. [Sidenote: Theory of Natural Selection]