To H. C. Watson   [after 10 June 1856]¹

And now for my question, which bears on the capital manner in which you compare the relation of Europe & N. America, with what Europe probably would have to E. Asia if intervening tracts were submerged.— It is, whether all, or nearly all (as far as you know) the plants, which are common to Europe & N. America live North of the Arctic Circle.² By rights N. Scandinavia ought to be excepted on account of warmth from Gulf Stream from my question, but this would much complicate my question.— I do not know whether you will care to hear what I am speculating on (one never cares for other people’s speculations) but for the chance, I will write & you can slightly skim over it, if you so like.— My notions, however, are based on the belief that all the plants common to Europe & N. America do not live within the Arctic circle, (especially if northern Scandinavia be excepted) & in this, I imagine lies the difficulty in accounting for those which are in common to old & new worlds.— From reasons not worth explaining I have been led to speculate that about the middle of the pliocene period, when most of the species (at least of shells) were the same as now, & when the climate was rather warmer than now, the productions now inhabiting middle Europe & N. United States, might have lived fairly within the Arctic Circle, say between 65^o & 75^o. As between these Latitudes there is almost continuous land, from Scandinavia, by Asia & N. America & Greenland, would not the Flora in all probability have been pretty nearly uniform? As the climate became cooler, I imagine the productions to have been driven a little further south; & during glacial epoch much further south, & since glacial epoch to have remigrated northward, but not so far north, as during the pliocene period.— I fancy some such idea might do instead of Forbe’s immense subsidences.³ But even if there be any truth in this notion, it would require more working out than I fear I am at all capable of.—⁴ And really I ought to apologise for troubling you with such vague notions. Finally to put my question in another way,—does not the difficulty in understanding the relations of the Floras of Europe & N. America, depend on the plants in common not inhabiting the Arctic circle lands, where the land is nearly continuous?

Can you forgive me!

Your’s most truly obliged | Charles Darwin