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 produetions 

 must have been cotisiderablp 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 Darmn'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 ignotmn 

 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 



^ Life and Letters, ii. p. 136. 



"^ Linn. Trans, xxiii. 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 AnimalSy 

 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 Schroter, of Ziirich, for an 

 examination of his earlier papers with an equally negative result. 



