I have been very much pleased to see how boldly and clearly you speak out on the modification of species. I thank you for giving me the pages of reference; but they were superfluous, for I found so many original and profound remarks that I have carefully looked through all the papers. I hope that your discovery about the Cynips (177/1. "On Dimorphism in the hymenopterous genus Cynips," "Proc. Entom. Soc. Philadelphia," March, 1864. Mr. Walsh's view is that Cynips quercus aciculata is a dimorphous form of Cynips q. spongifica, and occurs only as a female. Cynips q. spongifica also produces spongifica females and males from other galls at a different time of year.) will hold good, for it is a remarkable one, and I for one have often marvelled what could be the meaning of the case. I will lend your paper to my neighbour Mr. Lubbock, who I know is much interested in the subject. Incidentally I shall profit by your remarks on galls. If you have time I think a rather hopeless experiment would be worth trying; anyhow, I should have tried it had my health permitted. It is to insert a minute grain of some organic substance, together with the poison from bees, sand-wasps, ichneumons, adders, and even alkaloid poisons into the tissues of fitting plants for the chance of monstrous growths being produced. (177/2. See "Life and Letters," III., page 346, for an account of experiments attempted in this direction by Mr. Darwin in 1880. On the effects of injuring plant-tissues, see Massart, "La Cicatrisation, etc." in Tome LVII. of the "Memoires Couronnes" of the Brussels Academy.) My health has long been poor, and I have lately suffered from a long illness which has interrupted all work, but I am now recommencing a volume in connection with the "Origin." P.S.--If you write again I should very much like to hear what your life in your new country is. What can be the meaning or use of the great diversity of the external generative organs in your cases, in Bombus, and the phytophagous coleoptera? What can there be in the act of copulation necessitating such complex and diversified apparatus? LETTER 178. TO W.H. FLOWER. Down, July 11th, 1864.