If the difficulty of grafting was as great as the difficulty of crossing, and as regular, I admit it would be a most serious objection.  But it is not.  I believe many distinct species can be grafted while others less distinct cannot.  The regularity with which natural species are sterile together, even when very much alike, I think is an argument in favour of the sterility having been generally produced by Natural Selection for the good of the species.

The other difficulty, of unequal sterility of reciprocal crosses, seems none to me; for it is a step to more complete sterility, and as such would be useful and would be increased by selection.

I have read Sir C. Lyell’s second volume with great pleasure.  He is, as usual, very cautious, and hardly ever expresses a positive opinion, but the general effect of the whole book is very strong, as the argument is all on our side.

I am in hopes it will bring in a new set of converts to Natural Selection, and will at all events lead to a fresh ventilation of the subject.—­Believe me yours very faithfully,

ALFRED R. WALLACE.

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4 Chester Place, Regent’s Park, N.W.  March 27, 1868.

My dear Wallace,—­My son has failed in your problem, and says that it is “excessively difficult”:  he says you will find something about it in Thomson and Tait, “Natural Philosophy” (art. 649).  He has, however, sent the solution, if the plate rested on a square rim, but he supposes this will not answer your purpose; nevertheless, I have forwarded it by this same post.  It seems that the rim being round makes the problem much more difficult.

I enclose my photograph, which I have received from Down.  I sent your answer to George on his objection to your argument on sterility, but have not yet heard from him.  I dread beginning to think over this fearful problem, which I believe beats the plate on the circular rim; but I will sometime.  I foresee, however, that there are so many doubtful points that we shall never agree.  As far as a glance serves it seems to me, perhaps falsely, that you sometimes argue that hybrids have an advantage from greater vigour, and sometimes a disadvantage from not being so well fitted to their conditions.  Heaven protect my stomach whenever I attempt following your argument!—­Yours most sincerely,

C. DARWIN.

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Down, Bromley, Kent.  April 6, 1868.

My dear Wallace,—­I have been considering the terrible problem.  Let me first say that no man could have more earnestly wished for the success of Natural Selection in regard to sterility than I did, and when I considered a general statement (as in your last note) I always felt sure it could be worked out, but always failed in detail, the cause being, as I believe, that Natural Selection cannot effect what is not good for the individual, including in this term a social community.  It would take a volume to discuss all the points; and nothing is so humiliating to me as to agree with a man like you (or Hooker) on the premises and disagree about the result.