LETTER 506. TO C. LYELL. Down, March 8th [1866].

(506/1. In a letter from Sir Joseph Hooker to Mr. Darwin on February 21st, 1866, the following passage occurs: "I wish I could explain to you my crude notions as to the Glacial period and your position towards it. I suppose I hold this doctrine: that there was a Glacial period, but that it was not one of universal cold, because I think that the existing distribution of glaciers is sufficiently demonstrative of the proposition that by comparatively slight redispositions of sea and land, and perhaps axis of globe, you may account for all the leading palaeontological phenomena." This letter was sent by Mr. Darwin to Sir Charles Lyell, and the latter, writing on March 1st, 1866, expresses his belief that "the whole globe must at times have been superficially cooler. Still," he adds, "during extreme excentricity the sun would make great efforts to compensate in perihelion for the chill of a long winter in aphelion in one hemisphere, and a cool summer in the other. I think you will turn out to be right in regard to meridional lines of mountain-chains by which the migrations across the equator took place while there was contemporaneous tropical heat of certain lowlands, where plants requiring heat and moisture were saved from extinction by the heat of the earth's surface, which was stored up in perihelion, being prevented from radiating off freely into space by a blanket of aqueous vapour caused by the melting of ice and snow. But though I am inclined to profit by Croll's maximum excentricity for the glacial period, I consider it quite subordinate to geographical causes or the relative position of land and sea and the abnormal excess of land in polar regions." In another letter (March 5th, 1866) Lyell writes: "In the beginning of Hooker's letter to you he speaks hypothetically of a change in the earth's axis as having possibly co-operated with redistribution of land and sea in causing the cold of the Glacial period. Now, when we consider how extremely modern, zoologically and botanically, the Glacial period is proved to be, I am shocked at any one introducing, with what I may call so much levity, so organic a change as a deviation in the axis of the planet...' (see Lyell's "Principles," 1875, Chapter XIII.; also a letter to Sir Joseph Hooker printed in the "Life of Sir Charles Lyell,"