Sect. 1.—Objects of this Science.
PERHAPS in extending the term Geological Dynamics to the causes of changes in organized beings, I shall be thought to be employing a forced and inconvenient phraseology. But it will be found that, in order to treat geology in a truly scientific manner, we must bring together all the classes of speculations concerning known causes of change; and the Organic Dynamics of Geology, or of Geography, if the reader prefers the word, appears not an inappropriate phrase for one part of this body of researches.
As has already been said, the species of plants and animals which are found embedded in the strata of the earth, are not only different from those which now live in the same regions, but, for the most part, different from any now existing on the face of the earth. The remains which we discover imply a past state of things different from that which now prevails; they imply also that the whole organic creation has been renewed, and that this renewal has taken place several times. Such extraordinary general facts have naturally put in activity very bold speculations.
But it has already been said, we cannot speculate upon such facts in the past history of the globe, without taking a large survey of its present condition. Does the present animal and vegetable population differ from the past, in the same way in which the products of one region of the existing earth differ from those of another? Can the creation and diffusion of the fossil species be explained in the same manner as the creation and diffusion of the creatures among which we live? And these questions lead us onwards another step, to ask,—What are the laws by which the plants and animals of different parts of the earth differ? What was the manner in which they were originally diffused?—Thus we have to include, as portions of our subject, [562] the Geography of Plants, and of Animals, and the History of their change and diffusion; intending by the latter subject, of course, palætiological history,—the examination of the causes of what has occurred, and the inference of past events, from what we know of causes.
It is unnecessary for me to give at any length a statement of the problems which are included in these branches of science, or of the progress which has been made in them; since Mr. Lyell, in his Principles of Geology, has treated these subjects in a very able manner, and in the same point of view in which I am thus led to consider them. I will only briefly refer to some points, availing myself of his labors and his ideas.
Sect. 2.—Geography of Plants and Animals.
With regard both to plants and animals, it appears,[78] that besides such differences in the products of different regions as we may naturally suppose to be occasioned by climate and other external causes; an examination of the whole organic population of the globe leads us to consider the earth as divided into provinces, each province being occupied by its own group of species, and these groups not being mixed or interfused among each other to any great extent. And thus, as the earth is occupied by various nations of men, each appearing at first sight to be of a different stock, so each other tribe of living things is scattered over the ground in a similar manner, and distributed into its separate nations in distant countries. The places where species are thus peculiarly found, are, in the case of plants, called their stations. Yet each species in its own region loves and selects some peculiar conditions of shade or exposure, soil or moisture: its place, defined by the general description of such conditions, is called its habitation.
[78] Lyell, Principles, B. iii. c. v.
Not only each species thus placed in its own province, has its position further fixed by its own habits, but more general groups and assemblages are found to be determined in their situation by more general conditions. Thus it is the character of the flora of a collection of islands, scattered through a wide ocean in a tropical and humid climate, to contain an immense preponderance of tree-ferns. In the same way, the situation and depth at which certain genera of shells are found have been tabulated[79] by Mr. Broderip. Such general inferences, if [563] they can be securely made, are of extreme interest in their bearing on geological speculations.