Many thanks for your interesting letter. From the serene elevation of my old age I look down with amazement at your youth, vigour, and indomitable energy. With respect to Hooker and the axis of the earth, I suspect he is too much overworked to consider now any subject properly. His mind is so acute and critical that I always expect to hear a torrent of objections to anything proposed; but he is so candid that he often comes round in a year or two. I have never thought on the causes of the Glacial period, for I feel that the subject is beyond me; but though I hope you will own that I have generally been a good and docile pupil to you, yet I must confess that I cannot believe in change of land and water, being more than a subsidiary agent. (506/2. In Chapter XI. of the "Origin," Edition V., 1869, page 451, Darwin discusses Croll's theory, and is clearly inclined to trust in Croll's conclusion that "whenever the northern hemisphere passes through a cold period the temperature of the southern hemisphere is actually raised..." In Edition VI., page 336, he expresses his faith even more strongly. Mr. Darwin apparently sent his MS. on the climate question, which was no doubt prepared for a new edition of the "Origin," to Sir Charles. The arrival of the MS. is acknowledged in a letter from Lyell on March 10th, 1866 ("Life of Sir Charles Lyell," II., page 408), in which the writer says that he is "more than ever convinced that geographical changes...are the principal and not the subsidiary causes.") I have come to this conclusion from reflecting on the geographical distribution of the inhabitants of the sea on the opposite sides of our continents and of the inhabitants of the continents themselves. LETTER 507. TO C. LYELL. Down, September 8th [1866].