To F. C. Donders   19 June 1871

My dear Professor Donders

I cannot thank you too much for your very kind letter & valuable information.¹ The result about the divergence is very curious, and no doubt gives that peculiar appearance to the eyes, when we are lost in thought, which is so readily recognized without our knowing, as in so many other cases, what is the cause. I find your letters especially useful & very different from many others which I receive, because you tell me plainly the points which are not known.

I will give your results in my essay, but when this will be published I can hardly say, for I have now to leave off work for a time, to prepare a new edit. of the Origin, which always requires much correction, owing to the Railroad speed at wch. Nat: Hist: progresses.² When reading over your several letters, the thought has often crossed my mind how incomparably better an essay on expression you could have written than that which I shall be able to produce. I did receive your kind present of your work & read the parts referred to & some others with interest.³ Indeed I should have liked much to have read all that I could understand, had time permitted.

Whenever you visit England I shall be delighted to see you.⁴ Again I thank you, not only for y^r. valuable assistance, but for the extremely kind manner with which you give it.

believe me with sincere respect | yours very truly | Ch. Darwin