Plant Migration 311 
Darwin was clearly prepared to go further than Hooker in ac- 
counting for the southern flora by dispersion from the north. Thus 
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 
might have had a point of connection through Abyssinia’, an idea 
which was promptly snuffed 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 
south.” 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®.” Iam 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 southern 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 (Mem.—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 cul-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 survive’.” But variability is itself 
subject to variation. The nemesis of a high degree of protected 
1 More Letters, 1. p. 9. 2 Ibid. 1. p. 447. 3 Ibid. 1. p. 380. 
4 Life and Letters, 1. p. 231. 5 More Letters, 1. p. 116. 
8 Ibid. 1. p. 453. 7 Ibid. 1. p. 481. 
