Now think of hides and wool (and wool exported largely over Europe), and plants introduced, and samples of corn; and I must think that if Australia had been the old country, and Europe had been the Botany Bay, very few, very much fewer, Australian plants would have run wild in Europe than have now in Australia.
The case seems to me much stronger between La Plata and Spain.
Nevertheless, I will put in my one sentence on this head, illustrating the greater migration during Glacial period from north to south than reversely, very humbly and cautiously.

(343/4.

"Origin of Species," Edition I., page 379.

Darwin refers to the facts given by Hooker and De Candolle showing a stronger migratory flow from north to south than in the opposite direction.

Darwin accounts for this by the northern plants having been long subject to severe competition in their northern homes, and having acquired a greater "dominating power" than the southern forms.