If I study a living being, under what heads does the knowledge I obtain fall?  I can learn its structure, or what we call its ANATOMY; and its DEVELOPMENT, or the series of changes which it passes through to acquire its complete structure.  Then I find that the living being has certain powers resulting from its own activities, and the interaction of these with the activities of other things—­the knowledge of which is PHYSIOLOGY.  Beyond this the living being has a position in space and time, which is its DISTRIBUTION.  All these form the body of ascertainable facts which constitute the status quo of the living creature.  But these facts have their causes; and the ascertainment of these causes is the doctrine of AETIOLOGY.

If we consider what is knowable about the earth, we shall find that such earth-knowledge—­if I may so translate the word geology—­falls into the same categories.

What is termed stratigraphical geology is neither more nor less than the anatomy of the earth; and the history of the succession of the formations is the history of a succession of such anatomies, or corresponds with development, as distinct from generation.

The internal heat of the earth, the elevation and depression of its crust, its belchings forth of vapours, ashes, and lava, are its activities, in as strict a sense, as are warmth and the movements and products of respiration the activities of an animal.  The phaenomena of the seasons, of the trade winds, of the Gulf-stream, are as much the results of the reaction between these inner activities and outward forces, as are the budding of the leaves in spring and their falling in autumn the effects of the interaction between the organization of a plant and the solar light and heat.  And, as the study of the activities of the living being is called its physiology, so are these phaenomena the subject-matter of an analogous telluric physiology, to which we sometimes give the name of meteorology, sometimes that of physical geography, sometimes that of geology.  Again, the earth has a place in space and in time, and relations to other bodies in both these respects, which constitute its distribution.  This subject is usually left to the astronomer; but a knowledge of its broad outlines seems to me to be an essential constituent of the stock of geological ideas.

All that can be ascertained concerning the structure, succession of conditions, actions, and position in space of the earth, is the matter of fact of its natural history.  But, as in biology, there remains the matter of reasoning from these facts to their causes, which is just as much science as the other, and indeed more; and this constitutes geological aetiology.