Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/226

212 section i., and part vii.). He perceives that the present area of land on the earth’s surface is the result of the balance of two antagonistic processes—the destruction caused by superﬁcial agents on every portion of land ex- posed to their inﬂuence, and the periodic elevation, by subterranean action, of the land so wasted, or of new land from beneath the sea.

5. Distribution of Animal and Vegetable Life.—It is usual to include in treatises on physical geography an outline of the distribution of plants and animals, with an account of the great regions or provinces into which zoologists and botanists have divided the continents. The question naturally arises why the distribution should be as it is. Two answers obviously suggest themselves—1st, climate, and 2d, the power possessed by plants and animals of diffusing themselves. Yet climate only explains a part of this problem, and it is evident that migration cannot possibly account for the diffusion of innumerable organisms. There is a large residuum of unexplained phenomena on which much light is thrown by geological inquiry. Thus, for example, the presence of living Arctic forms of vegetation on the mountains of central Europe can be eonnected with the occurrence of the remains of Arctic animals in the superﬁcial deposits of that region, and with other facts which make it clear that at no very distant date an Arctic climate prevailed over most of Europe, that at that time a northern vegetation spread southwards and covered the plains and heights of Europe even as far south as the Alps and Pyrenees, and that as the climate gradually ameli- orated the northern vegetation was cxtirpated from the low grounds by the advance of plants better suited to the milder temperature, but continued to maintain its ground amid the congenial frosts and snows of the mountains, where to this day it still ﬂourishes (see ).    

   EOLOGY is the science which investigates the history of the earth. Its object is to trace the progress of our planet from the earliest beginnings of its separate existence, through its various stages of growth, down to the present condition of things. It seeks to determine the manner in which the evolution of the earth’s great surface features has been effected. It unravels the complicated processes by which each continent has been built up. It follows, even into detail, the varied sculpture of mountain and valley, crag and ravine. Nor does it conﬁne itself merely to changes in the inorganic world. Geology shows that the present races of plants and animals are the descend- ants of other and very different races which once peopled the earth. It teaches that there has been a progress of the inhabitants, as well as one of the globe on which they dwelt; that each successive period in the earth‘s history, since the introduction of living things, has been marked by characteristic types of the animal and vegetable kingdoms; and that, however imperfectly they have been preserved. or may be decipliered, materials exist for a history of life upon the planet. The geographical distribution of existing faunas and ﬂoras is often made clear and intelligible by geological evidence; and in the same way light is thrown upon some of the remoter phases in the history of man himself. A subject so comprehensive as this must require a wide and varied basis of evidence. It is one of the characteristics of geology to gather evidence from sources which at ﬁrst sight seem far removed from its scope, and to seek aid from almost every other leading branch of science. Thus, in dealing with the earliest conditions of the planet, the geologist must fully avail himself of the labours of the astronomer. \Vhatever is ascertainable by telescope, spectroscope, or chemical analysis, regarding the constitution of other heavenly bodies, has a geological bearing. The experiments of the physicist, undertaken to determine con- ditions of matter and of energy, may sometimes be taken as the starting-points of geological investigation. The work of the chemical laboratory forms the foundation of a vast and increasing mass of geological inquiry. To the botanist, the zoologist, even to the unscientiﬁc, if observant, traveller byoland or sea, the geologist turns for information and assistance. But while thus culling freely from the dominions of other sciences, geology claims as its peculiar territory the rocky framework of the globe. In the materials composing that framework, their composition and arrangement, the pro- cesses of their formation, the changes which they have undergone, and the terrestrial revolutions to which they bear witness, lie the main data of geological history. It is the task of the geologist to group these elements in such a way that they may be made to yield up their evidence as to the march of events in the evolution of the planet. He ﬁnds that they have in large measure arranged themselves in chronological sequence,——the oldest lying at the bottom and the newest at the top. Relics of an ancient sea-ﬂoor are overlaid by traces of a vanished land-surface ; these are in turn covered by the deposits of a former lake, above which once more appear proofs of the return of the sea. Among these rocky records lie the lavas and ashes of long- extinct volcanoes. The ripple left upon the shore, the cracks formed by the sun’s heat upon the muddy bottom of a dried-up pool, the very imprint of the drops of a paSS- ing rain—shower, have all been accurately preserved, and yield their evidence as to geographical conditions widely different from those which exist where such markings are now found. But it is mainly by the remains of plants and animals imbedded in the rocks that the geologist is guided in un- ravelling the chronological succession of geological changes. He has found that a certain order of appearance charac- terizes these organic remains, that each great group of rocks is marked by its own special types of life, and that these types can be recognized, and the rocks in which they occur can be correlated even in distant countries, and where 110 other means of comparison would be possible. At one moment he has to deal with the bones of some large mammal scattered through a deposit of superficial gravel, at another time with the minute foraminifers and ostraeods of an upraised sea-bottom. Corals and crinoids crowded and crushed into a massive limestone where they lived and died, ferns and terrestrial plants matted together into a bed of coal where they originally grew, the scattered shells of a submarine sand-bank, the snails and lizards which lived and died within a hollow tree, the insects which have been imprisoned within the exuding resin of old forests, the footprints of birds and quadrupeds, the trails of worms left upon former shores—these, and innumerable other pieces of evidence, enable the geologist to realize in some measure what the faunas and ﬂoras of successive periods have been, and what geographical changes the site of chl‘y land has undergone. It is evident that to deal successfully with these varied materials, a considerable acquaintance with different branches of science is needful. Especially necessary is a tolerably wide knowledge of the processes now at work in changing the surface of the earth, and of at least those forms of plant and animal life whose remains are apt to be preserved in geological deposits, or which in their structure