individual might see a chain of hills rise, and rise with it, much [more] a
species--and those islands which J. Hooker describes as covered with New
Zealand plants three hundred (?) miles to the N.E. (?) of New Zealand may
have been separated from the mainland two or three or four generations of
Washingtonia ago.

If the identity of the land-shells of all the hundreds of British Isles be
owing to their having been united since the Glacial period, and the
discordance, almost total, of the shells of Porto Santo and Madeira be
owing to their having been separated [during] all the newer and possibly
older Pliocene periods, then it gives us a conception of time which will
aid you much in your conversion of species, if immensity of time will do
all you require; for the Glacial period is thus shown, as we might have
anticipated, to be contemptible in duration or in distance from us, as
compared to the older Pliocene, let alone the Miocene, when our
contemporary species were, though in a minority, already beginning to
flourish.

The littoral shells, according to MacAndrew, imply that Madeira and the
Canaries were once joined to the mainland of Europe or Africa, but that
those isles were disjoined so long ago that most of the species came in
since. In short, the marine shells tell the same story as the land shells.
Why do the plants of Porto Santo and Madeira agree so nearly? And why do
the shells which are the same as European or African species remain quite
unaltered, like the Crag species, which returned unchanged to the British
seas after being expelled from them by glacial cold, when two millions (?)
of years had elapsed, and after such migration to milder seas? Be so good
as to explain all this in your next letter.