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 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 the islands in the N.
Atlantic, for example.

b. The temporary occupation of mountain stations by migrating plants
(which I think I have shown to be probable) renders time a much more
important element in increasing the number and variety of the plants so
dispersed than in the case of islands, where the flora soon acquires a
fixed and endemic character, and where the number of species is necessarily
limited.