To Auguste Forel   15 October 1874

My dear Sir

I have now read the whole of your admirable work & seldom in my life have I been more interested by any book.¹ There are so many interesting facts & discussions, that I hardly know which to specify; but I think, firstly, the newest points to me have been about the size of the brain in the three sexes, together with your suggestion that increase of mind-power may have led to the sterility of the workers.² Secondly about the battles of the ants, & your curious account of the enraged ants being held by their comrades until they calmed down.³ Thirdly the evidence of ants of the same community being the offspring of brothers & sisters.⁴ You admit, I think, that new communities will often be the product of a cross between not-related ants. Fritz Müller has made some interesting observations on this head with respect to Termites.⁵ The case of of Anergates is most perplexing in many ways; but I have such faith in the law of occasional crossing that I believe an explanation will hereafter be found, such as the dimorphism of either sex & the occasional production of winged males.⁶ I see that you are puzzled how ants of the same community recognize each other; I once placed two ants (F. rufa) in a pill-box smelling strongly of assafœtida, & after a day returned them to their homes; they were threatened, but at last recognized. I made this trial thinking that they might know each other by their odour; but this cannot here have been the case, & I have often fancied that they must have some common signal.⁷ Your last chapter is one great mass of wonderful facts & suggestions, & the whole profoundly interesting. I have seldom been more gratified than by the honorable mention of my work

I should like to tell you one little observation which I made with care many years ago; I saw ants (Formica rufa) carrying coccoons from a nest which was the largest I ever saw & which was well known to all the country people near, & an old man apparently about 80 years of age told me that he had known it ever since he was a little boy. The ants carrying the coccoons did not appear to be emigrating; following the line I saw many ascending a tall fir tree still carrying their coccoons. But when I looked closely, I found that all the coccoons were empty cases. This astonished me, & next day I got a man to observe with me, & we again saw ants bringing empty coccoons out of the nest; each of us fixed on one ant & slowly followed it, & repeated the observation on many others. We thus found that some ants soon dropped their empty coccoons; others carried them for many yards, as much as 30 paces, & others carried them high up the fir tree out of sight.⁸ Now here I think we have one instinct in contest with another & mistaken one. The first instinct being to carry the empty coccoons out of the nest, & it would have been sufficient to have laid them on the heap of rubbish, as the first breath of wind would have blown them away. And then came in the contest with the other very powerful instinct of preserving & carrying their coccoons as long as possible; & this they could not help doing although the coccoons were empty. According as the one or other instinct was the stronger in each individual ant, so did it carry the empty coccoon to a greater or less distance. If this little observation should ever prove of any use to you, you are quite at liberty to use it.

Again thanking you cordially for the great pleasure which your Work has given me, I remain with much respect— Yours sincerely | Charles Darwin

P.S | If you read English easily I should like to send you M^r Belt’s book, as I think you would like it as much as did Fritz Müller⁹