The genius which sighs for new worlds to conquer beyond that surprising region in which "geometry, algebra, and the theory of numbers melt into one another like sunset tints, or the colours of a dying dolphin," may be of comparatively little service in the cold domain (mostly lighted by the moon, some say) of philosophy. And the more I think of it, the more does our friend seem to me to fall into the position of one of those "verstaendige Leute," about whom he makes so apt a quotation from Goethe. Surely he has not duly considered two points. The first, that I am in no way answerable for the origination of the doctrine he criticises: and the second, that if we are to employ the terms observation, induction, and experiment, in the sense in which he uses them, logic is as much an observational, inductive, and experimental science as mathematics; and that, I confess, appears to me to be a _reductio ad absurdum_ of his argument. Thirdly, the essay "On the Physical Basis of Life" was intended to contain a plain and untechnical statement of one of the great tendencies of modern biological thought, accompanied by a protest, from the philosophical side, against what is commonly called Materialism. The result of my well-meant efforts I find to be, that I am generally credited with having invented "protoplasm" in the interests of "materialism." My unlucky "Lay Sermon" has been attacked by microscopists, ignorant alike of Biology and Philosophy; by philosophers, not very learned in either Biology or Microscopy; by clergymen of several denominations; and by some few writers who have taken the trouble to understand the subject. I trust that these last will believe that I leave the essay unaltered from no want of respectful attention to all they have said. Fourthly, I wish to refer all who are interested in the topics discussed in my address on "Geological Reform," to the reply with which Sir William Thomson has honoured me. And, lastly, let me say that I reprint the review of "The Origin of Species" simply because it has been cited as mine by a late President of the Geological Society. If you find its phraseology, in some places, to be more vigorous than seems needful, recollect that it was written in the heat of our first battles over the Novum Organon of biology; that we were all ten years younger in those days; and last, but not least, that it was not published until it had been submitted to the revision of a friend for whose judgment I had then, as I have now, the greatest respect. Ever, my dear TYNDALL, Yours very faithfully, T.H. HUXLEY LONDON, _June 1870_. CONTENTS. I. PAGE ON THE ADVISABLENESS OF IMPROVING NATURAL KNOWLEDGE. (A Lay Sermon delivered in St. Martin's Hall, on the evening of Sunday, the 7th of January, 1866, and subsequently published in the _Fortnightly Review_) 3 II.