Numerous large districts of the earth contain immense deposits of marine shells, which must therefore once have formed the bed of the ocean above which they are now elevated ; and as they exhibit an unbroken level, we infer that they were gradually elevated without disturbance by similar slowly-acting subterranean causes, such as have been shown to produce elevating forces now gradually raising parts of existing continents.
Again ; in other districts we trace the marks of sudden and violent local inundations at remote epochs : precisely such inundations have been known to be produced by sub marine volcanic action. Such effects may clearly be supposed to have taken place upon a larger scale where the phenomena indicate it, but we are still not departing from just analogies.
All the changes of which we have evidence in past epochs have been manifestly local; just as the operation of existing causes is confined to a series of the like partial and local alterations. Thus no sound inductive geologist at the present day can admit anything like a universal simultaneous formation, or sudden action, applying at once to the entire surface of the present dry land. One small portion after another has been successively deposited, elevated, peopled with animal and vegetable life, again in the course of profoundly-adjusted changes to be obliterated and over whelmed, while another has been in progressive advance. Just and sober inductive science, applied to the examina tion of the actual structure of the earth's crust, enables us with satisfaction and certainty to trace the changes which have taken place on the surface of a globe possessing the same general nature as the existing earth, and in the structure and habits of organised beings analogous to those now inhabiting the world. It investigates the alterations which have been effected by physical agents resembling those now in operation, and in accordance with general laws the same as those now recognised in the economy of nature. But it does not and cannot rise to the disclosure of what might have occurred under a different state of things, or owing to the action of causes of a different order from those now discovered by physical research. It cannot show a chaos, or trace the evolution of a world out of it. It cannot reason upon a supposed state of universal confusion and ruin, and the immediate reduction of it into order and arrangement. It can investigate the changes of things, but not their origin. In a word, sound geology will never aspire to the character of cosmogony. Yet geology is peculiarly distinguished from other branches of physical science, in this, that, while they teach us only the existing order of nature, it carries us back in time, and shows a period when the present races of organised beings did not exist, and by consequence establishes the fact of their having in some way received a commencement of being, and in truth the Occurrence of many such events; and these not brought about at any one marked period, or extending to all animated nature at once, but by the slow and gradual introduction of each new species while yet the older partially remained ; and each in turn thus progressively yielding its place to be filled up with fresh forms of organisation. All that geology
establishes in respect to organised life is the fact of the gradual origination of new species, but by no means the particular method or pr...:ess by which it was brought about.
It is true there have not been wanting theories to explain these processes on supposed natural principles : yet these have not been altogether satisfactory or free from material objections. Physical research, indeed, in its nature, cannot bring us to any distinct conception of what we term an act of creation. If we consider the simple case of the introduc tion of a single species, or even an individual of a new species, there is an obvious limit imposed on our speculations. On the other hand, it is certainly quite open to the physiological inquirer to trace, as closely as he can, the secondary means, if any, as far as the nature of the case admits, by which it is conceivable that such changes may have been brought about or modified. Such inquiries may produce no satisfactory results, but certainly it is the only legitimate channel open to the inductive inquirer, to examine carefully all the possible effects which different combinations of natural conditions, as temperature, domestication, crossing of breeds, and the like, may produce. Theories, indeed, of this kind have been proposed and carried out by some to a most singular and preposterous extent, and a series of transmutations of species imagined which seem more like the hallucinations of insanity than the sober deductions of science. Yet the broad question respecting the immutability of species, and the abstract possibility of a transition from one into another, of the modifications of intermediate races being perpetuated, of new species being thus eventually introduced, have fairly formed subjects of debate among physiologists. At all events, if natural science ever should be able to conduct us to any satisfactory knowledge on such a subject, it can only be by some such route as this. But in comparing what may have occurred in remote epochs with the analogous facts of modern observation on the modifica tions of species, there is one point most carefully to be remembered—the limited time during which existing operations have been contemplated—from which it would be unsafe to argue what may have taken place in the vast and almost unlimited periods of past duration.