tion.

As to "epoch" and "period," I use them as synonyms to avoid repeating
the same word.

3. Rate of deposition and geological time. Here no doubt I may have
gone to an extreme, but my "28 million years" may be anything under 100
millions, as I state. There is an enormous difference between mean and
maximum denudation and deposition. In the case of the great faults
the upheaval along a given line would itself facilitate the denudation
(whether sub-aerial or marine) of the upheaved portion at a rate perhaps
a hundred times above the average, just as valleys have been denuded
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 mouths of the Mississippi, were to
subside for a few thousand years, it might receive the greater portion
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.

5. My Chapter 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
N. temperate to the S. 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