her subaerial or marine) of the upheaved portion at a rate perhaps
a hundred times faster than plains and plateaux. So, local subsidence
might itself lead to very rapid deposition. Suppose a portion of the
Gulf of Mexico near the mouth of the Mississippi were to subside for a
few thousand years, it might receive the greater part of the sediment
from the whole Mississippi valley, and thus form strata at a very rapid
rate.

4. You quote the Pampas thistles, etc., against my statement of the
importance of preoccupation. 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. For, where could the rich lowland _equatorial_ flora
have existed during a period of general refrigeration sufficient for
this? and what became of the wonderfully rich Cape flora which, if the
temperature of Tropical Africa had been so recently lowered, would
certainly have spread northwards and on the return of the heat could
hardly have been driven back into the sharply defined and _very
restricted area_ in which it now exists?

As to the migration of plants from mountain to mountain not being so
probable as to remote islands, I think that is fully counterbalanced by
two considerations:

(a) The area and abundance of the mountain stations along such a range
as the Andes are immensely greater than those of