But is the earth nothing but a cooling mass, “like a hot-water jar such as is used in carriages,” or “a globe of sandstone,” and has its cooling been uniform?  An affirmative answer to both these questions seems to be necessary to the validity of the calculations on which Sir W. Thomson lays so much stress.

Nevertheless it surely may be urged that such affirmative answers are purely hypothetical, and that other suppositions have an equal right to consideration.

For example, is it not possible that, at the prodigious temperature which would seem to exist at 100 miles below the surface, all the metallic bases may behave as mercury does at a red heat, when it refuses to combine with oxygen; while, nearer the surface, and therefore at a lower temperature, they may enter into combination (as mercury does with oxygen a few degrees below its boiling-point), and so give rise to a heat totally distinct from that which they possess as cooling bodies?  And has it not also been proved by recent researches that the quality of the atmosphere may immensely affect its permeability to heat; and, consequently, profoundly modify the rate of cooling the globe as a whole?

I do not think it can be denied that such conditions may exist, and may so greatly affect the supply, and the loss, of terrestrial heat as to destroy the value of any calculations which leave them out of sight.

My functions as your advocate are at an end.  I speak with more than the sincerity of a mere advocate when I express the belief that the case against us has entirely broken down.  The cry for reform which has been raised without, is superfluous, inasmuch as we have long been reforming from within, with all needful speed.  And the critical examination of the grounds upon which the very grave charge of opposition to the principles of Natural Philosophy has been brought against us, rather shows that we have exercised a wise discrimination in declining, for the present, to meddle with our foundations.

XI

PALAEONTOLOGY AND THE DOCTRINE OF EVOLUTION

[1870]

It is now eight years since, in the absence of the late Mr. Leonard Horner, who then presided over us, it fell to my lot, as one of the Secretaries of this Society, to draw up the customary Annual Address.  I availed myself of the opportunity to endeavour to “take stock” of that portion of the science of biology which is commonly called “palaeontology,” as it then existed; and, discussing one after another the doctrines held by palaeontologists, I put before you the results of my attempts to sift the well-established from the hypothetical or the doubtful.  Permit me briefly to recall to your minds what those results were:—­

1.  The living population of all parts of the earth’s surface which have yet been examined has undergone a succession of changes which, upon the whole, have been of a slow and gradual character.