Plant Migration 305 



violent assumption, and in showing that the principle of descent is 

 adequate to explain the ascertained facts. 



It does not, I think, detract from the merit of Darwin's con- 

 clusions that the tendency of modern research has been to show 

 that the effects of the Glacial period were less simple, more localised 

 and less general than he perhaps supposed. He admitted that 

 "equatorial refrigeration... must have been small 1 ." It may prove 

 possible to dispense with it altogether. One cannot but regret that 

 as he wrote to Bates: — "the sketch in the Origin gives a very 

 meagre account of my fuller MS. essay on this subject 2 ." Wallace 

 fully accepted "the eifect of the Glacial epoch in bringing about 

 the present distribution of Alpine and Arctic plants in the northern 

 hemisphere" but rejected " the lowering of the temperature of the 

 tropical regions during the Glacial period " in order to account for 

 their presence in the southern hemisphere 3 . The divergence how- 

 ever does not lie very deep. Wallace attaches more importance to 

 ordinary means of transport. "If plants can pass in considerable 

 numbers and variety over wide seas and oceans, it must be yet more 

 easy for them to traverse continuous areas of land, wherever mountain- 

 chains offer suitable stations 4 ." And he argues that such periodical 

 changes of climate, of which the Glacial period may be taken as a 

 type, would facilitate if not stimulate the process 5 . 



It is interesting to remark that Darwin drew from the facts of 

 plant distribution one of his most ingenious arguments in support 

 of this theory 6 . He tells us, " I was led to anticipate that the species 

 of the larger genera in each country would oftener present varieties, 

 than the species of the smaller genera 7 ." He argues " where, if we 

 may use the expression, the manufactory of species has been active, 

 we ought generally to find the manufactory still in action 8 ." This 

 proved to be the case. But the labour imposed upon him in the 

 study was immense. He tabulated local floras "belting the whole 

 northern hemisphere 9 ," besides voluminous works such as De Can- 

 dolle's Prodromus. The results scarcely fill a couple of pages. This 

 is a good illustration of the enormous pains which he took to base 

 any statement on a secure foundation of evidence, and for this the 

 world, till the publication of his letters, could not do him justice. 

 He was a great admirer of Herbert Spencer, whose "prodigality 

 of original thought " astonished him. " But," he says, " the reflection 

 constantly recurred to me that each suggestion, to be of real value to 

 service, would require years of work 10 ." 



1 More Letters, i. p. 177. 2 Loc. cit. 



3 More Letters, n. p. 25 (footnote 1). * Island Life (2nd edit.), London, 1895, p. 512. 



5 Loc. cit. p. 518. 6 See More Letters, i. p. 424. 



7 Origin, p. 44. 8 Ibid. p. 45. 



9 More Letters, i. p. 107. 10 Ibid. n. p. 235. 



D. 20 



