In those rocks, of whatever date, which are of igneous origin, or show marks of having undergone fusion, if organic remains ever existed, it is clear they must have been destroyed, so that we can argue nothing from their non appearance.
With reference to the present question, it will be readily apparent that our knowledge of the subject can go no higher than the evidence of fossil remains carries us.
In the earliest rock in which any organic remains have yet been found, these remains are not those of/Slants, but of animals, and these not absolutely of the lowest kind ; and from this first observed origin of organic life there is no break in the vast chain of organic development till we reach the existing order of things—no one geological period, long or short—no one series of stratified rocks everywhere devoid of traces of life : the world, once inhabited, has apparently never, for any ascertainable period, been totally despoiled of its living wonders ; but there have been many changes in individual forms, great alterations in generic assemblages, entire revolutions in the relative number and development of the several classes. The systems of life have been varied from time to time, to suit the altered condition of the globe, but never extinguished.
The proportionate number of species has gone on ing in the successive generations up to the multitudes of existing species. The change in organic structure also has been in some degree proportioned to the time elapsed ; but we cannot lay down any distinct principle as to the by which its progression, its greater or less complexity or perfection in the scale of existence, can be decided ; though generally we may say that the higher forms of life are not found till we come to the more recent strata.
Throughout the whole we trace one unbroken continuity of plan and design : different races of animals and plants have successively arisen as others disappeared, the disappear ance of the one and the introduction of the other being each coincident with changes in the state of the globe.
The existing forms of life resemble those of times gone by, as the general aspect of the physical conditions of the world has always been analogous ; and they differ from them as the co-relations of life and physical conditions are strict and necessary : so that all the variations of these conditions are represented in the phases of organic structure, while all their general agreements are also represented by the conformity of the great principles of structure in the creatures of every geological age, and the often-repeated analogies and par allelisms of series of forms between different geological periods, which we find as a law of nature, when comparing the most distant regions with each other. We are not then
in a different system of nature, properly so called, from those which have been created and have been suffered to pass away before the origin of the human race ; but in an advanced part of the same system, whose law of progression is fixed, though from time to time the signification of the term varies. The full and complete system of organic life now on the globe includes all the effects of sea and land, warmth and cold, divided regions, and all other things which are the diversifying causes of nature ; and it is no wonder if, before the present land was raised from the deep, and the present distinction of natural regions was produced, there was not the same extreme variety of natural productions which we now witness, and which is not without its end in rendering the globe a more fitting residence for intellectual beings.
Looking to the very latest periods to which geology refers, we find detached portions of the surface composed of beds containing remains of species nearly the same as those now existing ; and every indication presented by the nature, form, structure, and obvious mode of formation, deposition, and elevation of these beds, is precisely similar to what is now found actually going on, and especially to the results of exactly similar modes of action which we trace in operations which have gone on within the period of the existing order of things. The imbedding of existing races of animals and plants in ancient peat-bogs, in dried-up lakes, in new-formed deltas, and shoals, and the destruction of other portions of the actual surface and its productions, by the action of the sea, landslips, and submergencies ; as well as, above all, the exact identity of the action of modern earthquakes and volcanoes with those of old formations—all attest the unbroken uniformity of the chalet of causation which unites the present state of things with all those varying conditions which we trace in earlier epochs, and which have only appeared to some to present so much more strongly-marked vicissitudes, because we are apt to crowd those events together in the perspective, and measure them too much according to our narrow ideas of duration. Thus, whether we look at these changes in time or in space, we find in the one no definite assignable period at which we can fix any one grand revolution or distinct era—no one portion of the earth's surface which we can say was all produced, with its organised inhabitants, at one time. All the epochs of change Were gradual ; the different orders of things passed by insensible gradations from one into another ; all parts of the globe were brought into their present state by small local instalments.