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 a history of the 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 belching 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 phenomena 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 organisation 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 phenomena the
     subject matter of an analogous telluric physiology, to which we
     sometimes give the name of meteorology; sometimes of physical
     geography, sometimes that of geology.  Again, the earth has a
     place in space and 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 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.