To F. C. Donders   7 July 1874

My dear Prof Donders.

My son George writes to me that he has seen you, & that you have been very kind to him for which I return you my cordial thanks.—¹ He tells me on your authority— of a fact which interests me in the highest degree, & which I much wish to be allowed to quote. It relates to the action of one millionth of a grain of Atropine on the eye.— Now will you be so kind whenever you can find a little leisure to tell me whether you yourself have observed this fact, or believe it on good authority— I also wish to know what proportion by weight the atropine bore to the water of solution, & how much of the solution, was applied to the eye— The reason why I am so anxious on this head is that it gives some support to certain facts repeatedly observed by me—with respect to the Action of Phosphate of Ammonia on Drosera— The $\frac{1}{\mathrm{4,000,000}}$ of a grain absorbed by a gland clearly makes the tentacle which bears this gland become inflected; & I am fully convinced that $\frac{1}{\mathrm{20,000,000}}$ of a grain of the Crystallized salt. (i  e containing about $\frac{1}{3}$ of its weight of water of crystallization) does the same.² Now I am quite unhappy at the thought of having to publish such a statement   It will be of great value to me to be able to give any analogous facts in support.³ The case of Drosera is all the more interesting as the absorption of the salt or any other stimulant applied to the gland causes it to transmit a motor influence to the base of the tentacle which bears the gland—

Pray forgive me for troubling you, & do not trouble yourself to answer this until your health is fully reestablished—

Pray believe me. | your’s very sincerely— | Charles Darwin.