XL] MY NEW IDEAS 387 



and ice, by their immobility, produce cumulative effects ; and 

 thus a lowering 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 

 glaciation ; yet it has been altogether neglected in the various 

 mathematical 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 con- 

 troverted, and I still think it constitutes the most complete 

 explanation of the phenomenon 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 

 supposed, 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 objection 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 expla- 

 nation 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 especially 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 Englers 

 Botan-Jahrbiicher (vol. xvii., 1893), 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 



