the same bed, some portions consisting almost exclusively of them, while in others they are sparingly diffused or entirely absent. Some limestones many feet in thickness are made up of shells or corals in countless numbers, cemented together by the carbonate of lime. But the greatest numbers of individuals accumulated in a limited space are presented by the most minute animals. Soldani obtained from a fragment of rock in Tuscany, which weighed one ounce and a half, ten thousand four hundred and fifty-four marine shells, resembling the Nautilus. One thousand of them weighed one grain. Some of the Wealden strata in England one thousand feet thick, and deposits in Auvergne seven hundred feet thick, covering an area eighty miles long and twenty wide, are filled with microscopic crustaceous animals. In Germany a bed fourteen feet thick, is made up of fossil animalcules so minute that, according to Ehrenberg, fortyone thousand millions of them occupy but a cubic inch. The total number of organic relics in the rocks is inconceivably great since more than two-thirds of the earth's surface is covered by fossiliferous deposits, many of which are several thousand feet thick. They are found at an elevation of at least seventeen thousand feet above the level of the sea, in strata of the Himalayas and Andes; and are obtained in excavations more than two thousand feet below the same level.

132. The medium through which fossils have been deposited has usually been the ocean; and by far the largest number are those which lived in the sea, and hence are designated marine. Some, however, lived in brackish water, and the strata in which they are found are termed estuary deposits, as was the case in the coal measures, and the Wealden strata. A part of the Tertiary series contains

fresh water animals and plants. Comparatively few terrestrial relics occur: but the Wealden, Tertiary, and Diluluvial deposits contain some remains of land plants and animals.

133. Fossils usually occur in the situations in which they were at the time of their death, and hence often have their most delicate parts preserved. Some of the shells of the Wealden retain their epidermis, and ligaments of their valves, showing that they have not been subjected to friction. But there are immense reefs of broken shells and corals, some of which are worn to fine dust, indicating the violence to which they have been subjected.

134. In order to a full appreciation of the import of fossils, we must be enabled to compare them with each other at different geological periods, and with the existing races. This can be effected only by means of the distinctions and principles of classification used in Natural History.

A species is a group of individuals that are alike in every character not affected by accidental circumstances. Species are continued from generation to generation without the slightest change of character. Those variable changes which are produced by accidental causes, constitute varieties. Species are not convertible into each other. We know of no power adequate to the production of a species, short of the direct act of the Creator. Species have been obliterated in thousands of instances in the former ages of the world, and in a few known cases since the creation of man. (§ 86.)

A genus is an assemblage of species having certain characters in common. Genera usually embrace a considerable number of species, but a single species differing widely from all others may constitute a genus.

An order is an assemblage of genera, as a class is of

orders, while the whole are included in a kingdom. The terms family and tribe are used to designate groups intermediate between genera and orders.

135. We have seen (§ 87) that the plants and animals of the present period are distributed in floras and faunas, limited by climate and other circumstances. The examination of fossils in their beds shows that similar laws of distribution prevailed also at former periods. Classes and orders were, as they now are, very widely diffused, and genera often covered extensive areas, but species were restricted within comparatively narrow limits. In the periods of deposition of the earliest fossiliferous rocks, the temperature of the globe appears to have been higher than at present, and the faunas and floras less numerous, but covering a wider range. A formation existing in different parts of the world, very rarely contains the same species throughout its extent. The fossils of the chalk in England are in very few instances specifically identical with the American chalk fossils: the genera are the same, and the species analagous but not identical.

136. Examination of the successive formations of the fossiliferous rocks, shows that species of plants and animals have been introduced at different periods, continued for a time, then obliterated and their places supplied by others. The periods of duration of families, genera and species, are very variable. Species rarely extend through more than a single formation, but genera are often found in several successive formations, and in a few instances have survived all the mutations exhibited by the fossiliferous strata, having representatives in the existing races. Genera, however, and even orders are sometimes limited to a single formation; in other instances they reappear after having ceased

through several formations. Those genera and species which survive through several formations, are such as have a wide geographical distribution, and seem to be endowed with a hardihood which enables them to endure changes of climate and other circumstances. Those species which live in deep water, with a more equable temperature are also more permanent. Shellfish and crustaceous animals are more enduring than vertebrate animals: no species of fish has yet been found in two successive formations.

137. Geologists presume plants and animals to have been introduced at the period of the deposition of the lowest strata in which their remains are found, though it is admitted they may have lived at an earlier period, and all trace of the existence of those first created, may have ceased in consequence of their frailty or the action of heat upon the lowest strata. Negative evidence on this subject is by no means conclusive. The failure to find the remains of a particular animal in a formation, does not satisfactorily prove that the animal was not living during that period: continued search may yet discover it; or its mode of life may have been such as to render the preservation of its remains after death, very improbable. But if from the remains found in a formation, we can infer the adaptation of circumstances to a certain class of animals, we do not expect to find in that formation animals whose organization require other circumstances. Thus in the coal formation whose plants clearly indicate the prevalence of a climate more elevated than that of the tropics at the present time, we do not expect to find indications of the existence of plants adapted to a temperate or arctic climate.

138. The close of geological periods appears to have been the time at which most of the changes in species oc

spe

curred; in some instances the change-obliteration of the old species and introduction of the new-is sudden, abrupt, and in others gradual. The introduced species are in no sense modifications of, or descendants from the species that preceded them. The facts of Geology disprove the hypothesis which supposes that animated existences were all introduced as mere points of vitality—monads-from which by a natural process of development, without the interposition of creative agency, they advance through continuous, regular grades, to the highest state of perfection and complexity. Mr. Hugh Miller has shown, on the contrary, that in certain orders there has been a degradation of cies.* There was a time when reptiles represented the carnivorous and herbivorous quadrupeds, but such magnificent reptiles have not existed since. There was a time when birds seem to have been the sole representatives of the warm-blooded animals, but their foot-prints in the sandstone show, that in size no birds of the present day can compare with them. Although species remain constant, in their characters, and changes are effected only by creative energy introducing new forms, still progress is discernible in the beings which have successively peopled the surface of the earth-progress in their approach to the present races. In the vertebrate animals the order of succession has been, fishes, reptiles, birds, mammiferous quadrupeds, and finally man. The relation between animals and the periods at which they live, appears to be a part of the plan of the Creator, as truly as is their distribution.

139. By a comparison of the fossils of different formations, we learn that those which are most unlike are most

Foot-Prints of the Creator.