I thought it was in Sir G.Grey, but it is not.

Could it have been in Eyre's book?
LETTER 357.

J.D.HOOKER TO CHARLES DARWIN.

[November 1862].
...I have speculated on the probability of there having been a post-Glacial Arctic-Norwego-Greenland in connection, which would account for the strong fact, that temperate Greenland is as Arctic as Arctic Greenland is--a fact, to me, of astounding force.

I do confess, that a northern migration would thus fill Greenland as it is filled, in so far as the whole flora (temperate and Arctic) would be Arctic,--but then the same plants should have gone to the other Polar islands, and above all, so many Scandinavian Arctic plants should not be absent in Greenland, still less should whole Natural Orders be absent, and above all the Arctic Leguminosae.