158 GEOLOGY [CHAP. IX 



Letter 506 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 redis- 

 tribution 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 LyelFs Principles, 1875, Chap. XIII.; 

 also a letter to Sir Joseph Hooker printed in \hzLife of Sir Charles Lyell, 

 Vol. II., p. 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. 1 I have come to this 

 conclusion from reflecting on the geograph. distribution of the 

 inhabitants of the sea on the opposite sides of our continents 

 and of the inhabitants of the continents themselves. 



1 In Chapter XI. of the Origin, Ed. V., 1869, p. 451, Darwin 

 discusses Croll's theory, and is clearly inclined to trust in Croll's con- 

 clusion that " whenever the northern hemisphere passes through a cold 

 period the temperature of the southern hemisphere is actually raised . . ." 

 In Ed. VI., p. 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 loth, 1866 

 (Life of Sir Charles Lyell, II. p. 408), in which the writer says that 

 he is " more than ever convinced that geographical changes . . . are 

 the principal and not the subsidiary causes." 



