Henslow is staying here; I have had some talk with him; he is in much the same state as Bunbury (The late Sir Charles Bunbury, well-known as a Palaeo-botanist.), and will go a very little way with us, but brings up no real argument against going further. He also shudders at the eye! It is really curious (and perhaps is an argument in our favour) how differently different opposers view the subject. Henslow used to rest his opposition on the imperfection of the Geological Record, but he now thinks nothing of this, and says I have got well out of it; I wish I could quite agree with him. Baden Powell says he never read anything so conclusive as my statement about the eye!! A stranger writes to me about sexual selection, and regrets that I boggle about such a trifle as the brush of hair on the male turkey, and so on. As L. Jenyns has a really philosophical mind, and as you say you like to see everything, I send an old letter of his. In a later letter to Henslow, which I have seen, he is more candid than any opposer I have heard of, for he says, though he CANNOT go so far as I do, yet he can give no good reason why he should not. It is funny how each man draws his own imaginary line at which to halt. It reminds me so vividly what I was told (By Professor Henslow.) about you when I first commenced geology--to believe a LITTLE, but on no account to believe all. Ever yours affectionately, C. DARWIN. CHARLES DARWIN TO ASA GRAY. Down, February 18th [1860]. My dear Gray, I received about a week ago two sheets of your Review (The 'American Journal of Science and Arts,' March, 1860. Reprinted in 'Darwiniana,' 1876.); read them, and sent them to Hooker; they are now returned and r-read with care, and to-morrow I send them to Lyell. Your Review seems to me ADMIRABLE; by far the best which I have read. I thank you from my heart both for myself, but far more for the subject's sake. Your contrast between the views of Agassiz and such as mine is very curious and instructive. (The contrast is briefly summed up thus: "The theory of Agassiz regards the origin of species and their present general distribution over the world as equally primordial, equally supernatural; that of Darwin as equally derivative, equally natural."--'Darwiniana,' page 14.) By the way, if Agassiz writes anything on the subject, I hope you will tell me. I am charmed with your metaphor of the streamlet never running against the force of gravitation. Your distinction between an hypothesis and theory seems to me very ingenious; but I do not think it is ever followed. Every one now speaks of the undulatory THEORY of light; yet the ether is itself hypothetical, and the undulations are inferred only from explaining the phenomena of light. Even in the THEORY of gravitation is the attractive power in any way known, except by explaining the fall of the apple, and the movements of the Planets? It seems to me that an hypothesis is DEVELOPED into a theory solely by explaining an ample lot of facts.