My dear Wallace

My son has failed in your problem & says that it is “excessively difficult”: he says you will find something about it in Thompson & Tait Nat Philos (Art. 649).² 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.³

I sent your answer to George on his objections to your argument on sterility but have not yet heard from him.—⁴ 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, & sometimes a disadvantage from not being so well fitted to their conditions.— Heaven protect my stomach whenever I attempt following your argument.—

Your’s most sincerely | C. Darwin