Fourthly, I do not know whether you will discuss in your book on the 'Mind of Animals' any of the more complex and wonderful instincts. It is unsatisfactory work, as there can be no fossilised instincts, and the sole guide is their state in other members of the same order and mere probability. But if you do discuss any (and it will perhaps be expected of you) I should think that you could not select a better case than that of the sand-wasps, which paralyse their prey, as formerly described by Fabre in his wonderful paper in 'Annales des Sciences,' and since amplified in his admirable 'Souvenirs.' Whilst reading this latter book, I speculated a little on the subject. Astonishing nonsense is often spoken of the sand-wasp's knowledge of anatomy. Now will anyone say that the Gauchos on the plains of La Plata have such knowledge, yet I have often seen them prick a struggling and lassoed cow on the ground with unerring skill, which no mere anatomist could imitate. The pointed knife was infallibly driven in between the vertebræ by a single slight thrust. I presume that the art was first discovered by chance, and that each young Gaucho sees exactly how the others do it, and then with a very little practice learning the art. Now I suppose that the sand-wasps originally merely killed their prey by stinging them in many places (see p. 129 of Fabre, 'Souvenirs,' and page 241), on the lower and softer side of the body, and that to sting a certain segment was found by far the most successful method, and was inherited, like the tendency of a bull-dog to pin the nose of a bull, or of a ferret to bite the cerebellum. It would not be a very great step in advance to prick the ganglion of its prey only slightly, and thus to give its larvæ fresh meat instead of old dry meat. Though Fabre insists so strongly on the unvarying character of instinct, yet it shows that there is some variability, as on pp. 176, 177. I fear that I shall have utterly wearied you with my scribbling and bad handwriting. My dear Romanes, Yours very sincerely, CH. DARWIN. _From G. J. Romanes to C. Darwin, Esq._ 18 Cornwall Terrace, Regent's Park, N.W.: April 17, 1881. Your long letter has been most refreshing to me in every way. I am looking forward with keen interest to the appearance of your book on Worms, and am unexpectedly glad to hear that my letter was of any use. I should very much like to see the book you mention, and from what you say about sending it I shall not order it. But there is no need to send it soon, as I have already an accumulation of books to review for 'Nature.'