Variation, as I understand it, can only be a slight modification in the offspring of that which exists in the parent.  The question whether pigs could possibly develop wings is absurd, and altogether beside the question, which is, solely, so far as direct evidence goes, as to the means by which the change from one species to another closely allied species has been brought about.  Those who want to begin by discussing the causes of change from a dog to a seal, or from a cow to a whale, are not worth arguing with, as they evidently do not comprehend the A, B, C of the theory.

Darwin’s ineradicable acceptance of the theory of heredity of the effects of climate, use and disuse, food, etc., on the individual led to much obscurity and fallacy in his arguments, here and there.—­Yours very sincerely,

ALFRED R. WALLACE.

* * * * *

TO PROF.  POULTON Parkstone, Dorset.  February 14, 1897.

My dear Poulton,—­Thanks for copy of your British Association Address,[28] which I did not read in Nature, being very busy just then.  I have now read it with much pleasure, and think it a very useful and excellent discussion that was much needed.  There is, however, one important error, I think, which vitiates a vital part of the argument, and which renders it possible so to reduce the time indicated by geology as to render the accordance of Geology and Physics more easy to effect.  The error I allude to was made by Sir A. Geikie in his Presidential Address[29] which you quote.  Immediately it appeared I wrote to him pointing it out, but he merely acknowledged my letter, saying he would consider it.  To me it seems a most palpable and extraordinary blunder.  The error consists in taking the rate of deposition as the same as the rate of denudation, whereas it is about twenty times as great, perhaps much more—­because the area of deposition is at least twenty times less than that of denudation.  In order to equal the area of denudation, it would require that every bed of every formation should have once extended over the whole area of all the land of the globe!  The deposition in narrow belts along coasts of all the matter brought down by rivers, as proved by the Challenger, leads to the same result.  In my “Island Life,” 2nd Edit., pp. 221-225, I have discussed this whole matter, and on reading it again I can find no fallacy in it.  I have, however, I believe, overestimated the time required for deposition, which I believe would be more nearly one-fortieth than one-twentieth that of mean denudation; because there is, I believe, also a great overestimate of the maximum of deposition, because it is partly made up of beds which may have been deposited simultaneously.  Also the maximum thickness is probably double the mean thickness.

The mean rate of denudation, both for European rivers and for all the rivers that have been measured, is a foot in three million years, which is the figure that should be taken in calculations.—­Believe me yours very truly,