( ?) 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.
LETTER 48.

TO J.D.HOOKER.

Down, July 5th [1856].
I write this morning in great tribulation about Tristan d'Acunha.