It is years since I have read a purely geological paper which has interested me so greatly. I was all the more interested, as in the Cordillera I often speculated on the sources of the deluges of submarine porphyritic lavas, of which they are built; and, as I have stated, I saw to a certain extent the causes of the obliteration of the points of eruption. I was also not a little pleased to see my volcanic book quoted, for I thought it was completely dead and forgotten. What fine work will Mr. Judd assuredly do!... Now I have eased my mind; and so farewell, with both E.D.'s and C.D.'s very kind remembrances to Miss Lyell. Yours affectionately, CHARLES DARWIN. [Sir Charles Lyell's reply to the above letter must have been one of the latest that my father received from his old friend, and it is with this letter that the volumes of his published correspondence closes.] CHARLES DARWIN TO AUG. FOREL. Down, October 15, 1874. My dear Sir, I have now read the whole of your admirable work ('Les Fourmis de la Suisse,' 4to, 1874.) and seldom in my life have I been more interested by any book. There are so many interesting facts and 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. Secondly about the battles of the ants, and your curious account of the enraged ants being held by their comrades until they calmed down. Thirdly, the evidence of ants of the same community being the offspring of brothers and sisters. You admit, I think, that new communities will often be the product of a cross between not-related ants. Fritz Muller has made some interesting observations on this head with respect to Termites. The case 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 and the occasional production of winged males. I see that you are puzzled how ants of the same community recognize each other; I once placed two (F. rufa) in a pill-box smelling strongly of asafoetida and after a day returned them to their homes; they were threatened, but at last recognized. I made the trial thinking that they might know each other by their odour; but this cannot have been the case, and I have often fancied that they must have some common signal. Your last chapter is one great mass of wonderful facts and suggestions, and the whole profoundly interesting. I have seldom been more gratified than by [your] honourable mention of my work.