But I am referring especially to St.
Helena, and to plants naturally introduced from the adjacent continents.
Surely, if a certain number of African plants reached the island and
became modified into a complete adaptation to its climatic conditions,
they would hardly be expelled by other African plants arriving
subsequently. They might be so conceivably, but it does not seem
probable. The cases of the Pampas, New Zealand, Tahiti, etc., are very
different, where highly developed _aggressive_ plants have been
artificially introduced. Under nature it is these very aggressive
species that would first reach any island in their vicinity, and, being
adapted to the island and colonising it thoroughly, would then hold
their own against other plants from the _same_ country, mostly less
aggressive in character. I have not explained this so fully as I should
have done in the book. Your criticism is therefore useful.
My Chap. XXIII. is no doubt very speculative, and I cannot wonder at
your hesitating at accepting my views. To me, however, your theory of
hosts of existing species migrating over the tropical lowlands from the
North Temperate to the South Temperate zone appears more speculative and
more improbable.
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