ts 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,"
Volume II., page 410.))

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 M