Plant 3Iigration 311 



Darwin was clearly prepared to go further than Hooker m ac- 

 counting for the southern flora by dispersion from the north. Tlius 

 he says: "We must, I suppose, admit that every yard of land has 

 been successively covered with a beech-forest between the Caucasus 

 and Japan ^." Hooker accounted for the dissevered condition of the 

 southern flora by geographical change, but this Darwin could not 

 admit. He suggested to Hooker that the Australian and Cape floras 

 miglit have had a point of connection through Abyssinia-, an idea 

 M'hich was promptly snufted out. Similarly he remarked to Bentham 

 (1869): "I suppose you think that the Restiaceae, Proteaceae, etc., 

 etc. once extended over the whole world, leaving fragments in the 

 soutli^" Eventually he conjectured " that there must have been a 

 Tertiary Antarctic continent, from which various forms radiated to 

 the southern extremities of our present continents*." But character- 

 istically he could not admit any land connections and trusted to 

 " floating ice for transporting seed^" I am far from saying that this 

 theory is not deserving of serious attention, though there seems to 

 be no positive evidence to support it, and it immediately raises the 

 difficulty how did such a continent come to be stocked ? 



We must, however, agree with Hooker that the common origin 

 of the northern and southeni floras must be referred to a remote 

 past. That Darwin had this in his mind at the time of the publication 

 of the Origin is clear from a letter to Hooker. " The view which 

 I should have looked at as perhaps most probable (though it hardly 

 differs from yours) is that the whole world during the Secondary 

 ages was inhabited by marsupials, araucarias (ISIem. — Fossil wood 

 of this nature in South America), Banksia, etc. ; and that these were 

 supplanted and exterminated in the greater area of the north, but 

 were left alive in the south^" Remembering that Araucaria, unlike 

 Banksia, belongs to the earlier Jurassic not to the angiospermous 

 flora, this view is a germinal idea of the widest generality. 



The extraordinary congestion in species of the peninsulas of the 

 Old World points to the long-continued action of a migration south- 

 wards. Each is in fact a cid-de-sac into which they have poured 

 and from which there is no escape. On the other hand the high 

 degree of specialisation in the southern floras and the little power 

 the species possess of holding their own in competition or in adapta- 

 tion to new conditions point to long-continued isolation. " An island 

 ...will prevent free immigration and competition, hence a greater 

 number of ancient forms will survived" But variability is itself 

 subject to variation. The nemesis of a high degree of protected 



» More Letters, ii. p. 9. * Ibid. t. p. 447. » Ibid. i. p. 330. 



* Life and Letter$, ill. p. 231. " More Letters, i. p. IIG. 



« Ibid. I. p. 453. ' Ibid. i. p. 481. 



