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.” 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?” Wallace 
fully accepted “the effect 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*®. 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.” 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®. 
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®. 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’.” 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®” 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®,” 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.” 
1 More Letters, 1. p. 177. 2 Loc. cit. 
3 More Letters, 11. p. 25 (footnote 1). 4 Island Life (2nd edit.), London, 1895, p. 512. 
5 Loe. cit. p. 518. 6 See More Letters, 1. p. 424. 
7 Origin, p. 44. 8 Ibid. p. 45. 
® More Letters, 1. p. 107. 10 Ibid. 11. p. 235. 
D. 20 
