304 Geographical Distribution of Plants 
epoch the whole world was colder; I inferred that,...from erratic 
boulder phenomena carefully observed by me on both the east and 
west coast of South America. Now I am so bold as to believe that 
at the height of the Glacial epoch, and when all Tropical productions 
must have been considerably distressed, several temperate forms 
slowly travelled into the heart of the Tropics, and even reached the 
southern hemisphere ; and some few southern forms penetrated in 
a reverse direction northward'.” Here again it is clear that though 
he credits Agassiz with having called vivid attention to the Glacial 
period, he had himself much earlier grasped the idea of periods of 
refrigeration. 
Putting aside the fact, which has only been made known to us 
since Darwin’s death, that he had anticipated Forbes, it is clear 
that he gave the theory a generality of which the latter had no 
conception. This is pointed out by Hooker in his classical paper 
On the Distribution of Arctic Plants (1860). “The theory of a 
southern migration of northern types being due to the cold epochs 
preceding and during the glacial, originated, I believe, with the late 
Edward Forbes; the extended one, of the trans-tropical migration, 
is Mr Darwin’s.” Assuming that local races have derived from a 
common ancestor, Hooker’s great paper placed the fact of the migra- 
tion on an impregnable basis. And, as he pointed out, Darwin has 
shown that “such an explanation meets the difficulty of accounting 
for the restriction of so many American and Asiatic arctic types to 
their own peculiar longitudinal zones, and for what is a far greater 
difficulty, the representation of the same arctic genera by most closely 
allied species in different longitudes.” 
The facts of botanical geography were vital to Darwin’s argument. 
He had to show that they admitted of explanation without assuming 
multiple origins for species, which would be fatal to the theory of 
Descent. He had therefore to strengthen and extend De Candolle’s 
work as to means of transport. He refused to supplement them by 
hypothetical geographical changes for which there was no inde- 
pendent evidence: this was simply to attempt to explain ignotum 
per ignotius. He found a real and, as it has turned out, a far- 
reaching solution in climatic change due to cosmical causes which 
compelled the migration of species as a condition of their existence. 
The logical force of the argument consists in dispensing with any 
1 Life and Letters, m. p. 136. 
2 Linn. Trans, xxu1. p. 253. The attempt appears to have been made to claim for Heer 
priority in what I may term for short the arctic-alpine theory (Scharff, European Animals, 
p. 128). I find no suggestion of his having hit upon it in his correspondence with Darwin 
or Hooker. Nor am I aware of any reference to his having done so in his later 
publications. I am indebted to his biographer, Professor Schréter, of Ziirich, for an 
examination of his earlier papers with an equally negative result. 
