I know you do not like theory, so I shall return to fact. There can be no doubt whatever that the seat of spontaneity is as much localised in the margin as the sensibility to stimulus is diffused throughout the bell. There _must_, therefore, be some structural difference in the tissue here to correspond to this great functional difference. Agassiz is very positive in describing a chain of cells running round the inner part of the marginal canal. Now, although I sometimes see a thin cord-like appearance here, I should not dare to say it was nervous. Gold certainly stains it, but it also stains many other parts of the tissue, and until I can see _cells_ here I cannot be sure about a _visible_ nervous cord. The cord I do see may be the wall of the marginal canal. I intend to persevere, however, trying your suggestions, also osmic acid. I can get no indications of electrical disturbance during contraction in the way you suggest--at least not with Sarsia; but I intend to try with some of the larger Medusæ. Some apparatus is coming from Cambridge to enable me to test for electrotonus and Pflüger's law. I shall apply it to the luminous Medusæ also, whose light, I forgot to say, is seen under the microscope in the dark to proceed not only from the margin alone, but from that particular part of the margin where Agassiz describes his chain of nervous cells. GEO. J. ROMANES. _From C. Darwin to G. J. Romanes._ Down, Beckenham, Kent: July 18, 1875. I have been much interested by your letter, and am truly delighted at the prospect of success. Such energy as yours is almost sure to command victory. The world will be much more influenced by experiments on animals than on plants. But in any case I think a large number of successful results will be necessary to convince physiologists. It is rash to be sanguine, but it will be splendid if you succeed. My object in writing has been to say that it has only just occurred to me that I have not sent you a copy of my 'Insectivorous Plants;' if you would care to have a copy, and do not possess one, send me a postcard, and one shall be sent. If I do not hear, I shall understand. Yours very sincerely, CH. DARWIN. Dunskaith, Nigg P.O., Ross-shire, N.B.: July 20, 1875.