“Having regard to this general scheme of geological knowledge
     and thought, it is obvious that geological speculation may be, so
     to speak, anatomical and developmental speculation, so far as it
     relates to points of stratigraphical arrangement which are out of
     reach of direct observation; or, it may be physiological
     speculation so far as it relates to undetermined problems
     relative to the activities of the earth; or, it may be
     distributional speculation, if it deals with modifications of the
     earth’s place in space; or, finally, it will be aetiological
     speculation if it attempts to deduce the history of the world, as
     a whole, from the known properties of the matter of the earth, in
     the conditions in which the earth has been placed.”

Huxley then proceeded to shew that uniformitarianism and catastrophism had neglected this last and most important branch of geology, the attempt to trace the interaction of causes which had brought the world into its present condition.  He gave a striking display of the wide knowledge of his reading by going back to the foundation of this branch of modern science, and giving a masterly account of the then little-known treatise of Immanuel Kant, who in 1775 had written An Attempt to Account for the Constitutional and Mechanical Origin of the Universe upon Newtonian Principles.  Next he declared that evolution embraced all that was sound in both catastrophism and uniformitarianism while rejecting the arbitrary limits and assumptions of both.

Finally he came to the great question to which these observations upon the existing schools of geology had led.  The most distinguished physicist of the age, then Sir William Thomson, now Lord Kelvin, and Huxley’s immediate successor in the Presidential Chair of the Royal Society, had stated that the English school of geology had assumed an impossible age for the earth.  By physical reasonings, Thomson stated that he was able to prove “That the existing state of things on the earth—­all geological history showing continuity of life—­must be limited within some such period of time as one hundred million years.”  This pronouncement had been received with acclamation by those who feared the geological and biological sciences, as a sign of internal dissensions within the house of science.  Huxley, then, as all through the latter part of his life, at once constituted himself the champion of science, and, taking Thomson’s arguments one by one, shewed by a series of masterly deductions from known facts that there was a great deal to be said for the other side, and that physicists were as little certain as geologists could be of the exact duration of time that had elapsed since the dawn of life.  His plea for more time since the cooling of the globe than physicists were willing to allow remains one of the classics of geological literature.  But he carried the question much farther.  The