5. My Chapter XXIII. is no doubt very speculative, and I cannot wonder at your hesitating at accepting my views. To me, however, your theory of hosts of existing species migrating over the tropical lowlands from the N. temperate to the S. temperate zone appears more speculative and more improbable. For where could the rich lowland equatorial flora have existed during a period of general refrigeration sufficient for this? and what became of the wonderfully rich Cape flora, which, if the temperature of tropical Africa had been so recently lowered, would certainly have spread northwards, and on the return of the heat could hardly have been driven back into the sharply defined and very restricted area in which it now exists. As to the migration of plants from mountain to mountain not being so probable as to remote islands, I think that is fully counterbalanced by two considerations:-- a. The area and abundance of the mountain stations along such a range as the Andes are immensely greater than those of the islands in the N. Atlantic, for example. b. The temporary occupation of mountain stations by migrating plants (which I think I have shown to be probable) renders time a much more important element in increasing the number and variety of the plants so dispersed than in the case of islands, where the flora soon acquires a fixed and endemic character, and where the number of species is necessarily limited. No doubt direct evidence of seeds being carried great distances through the air is wanted, but I am afraid can hardly be obtained. Yet I feel the greatest confidence that they are so carried. Take, for instance, the two peculiar orchids of the Azores (Habenaria sp.) What other mode of transit is conceivable? The whole subject is one of great difficulty, but I hope my chapter may call attention to a hitherto neglected factor in the distribution of plants. Your references to the Mauritius literature are very interesting, and will be useful to me; and I again thank you for your valuable remarks. LETTER 397. TO J.D. HOOKER. (397/1. The following letters were written to Sir J.D. Hooker when he was preparing his Address as President of the Geographical Section of the British Association at its fiftieth meeting, at York. The second letter (August 12th) refers to an earlier letter of August 6th, published in "Life and Letters," III., page 246.) 4, Bryanston Street, W., Saturday, 26th [February, 1881].