4 o 4 MY LIFE 



their immobility, produce cumulative effects ; and thus a low- 

 ering of temperature of a few degrees may lead to a country 

 being ice-clad which before was ice-free. This is a vital 

 point which is of the very essence of the problem of glacia- 

 tion ; yet it has been altogether neglected in the various mathe- 

 matical or physical theories which have recently been put 

 forward. My own discussion of the problem in chapter viii. 

 of " Island Life ' has never, so far as I know, been contro- 

 verted, and I still think it constitutes the most complete ex- 

 planation of the phenomena yet given. 



During a discussion in Nature, so late as 1896, Professor 

 G. H. Darwin and Mr. E. P. Culverwell adduced some new 

 calculations as to the amount of diminished sun-heat due to 

 eccentricity, as invalidating Croll's arguments; whereupon I 

 pointed out that their facts had not the importance they sup- 

 posed, because they took no account of the cumulative effects 

 of snow and ice above referred to (Nature, vol. liii. p. 220). 

 Sir Robert Ball also, quite independently, made the same ob- 

 jection as myself. 



8. In 1880 I published my " Island Life," and the last 

 chapter but one is " On the Arctic Element in South Temperate 

 Floras," in which I gave a solution of the very remarkable 

 phenomena stated by Sir Joseph Hooker in his " Introductory 

 Essay on the Flora of Australia." My explanation is founded 

 on known facts as to the dispersal and distribution of plants, 

 and does not require those enormous changes in the climate 

 of tropical lowlands during the glacial period on which Darwin 

 founded his explanation, and which, I believe, no biologist 

 well acquainted either with the fauna or the flora of the 

 equatorial zone has found it possible to accept. I am informed 

 by my friend Mr. Francis Darwin that this chapter was es- 

 pecially noticed in Germany at the time of its first appearance, 

 but he can hear of no detailed criticism of it, except one by 

 H. von Jhering in Engler's Botan-Jahrbilcher (vol. xvii., 

 I ^93), of which he has kindly sent me a translation of the 

 more important passages. This is not the place to reply to 

 the criticism, which would require a chapter. I can only say 



