discussion on migration during the Glacial period; but I remember that the distribution of mammalia, and the very regular relation of the Alpine plants to points due north (alluded to in "Origin"), seemed to indicate continuous land at close of Glacial period.
LETTER 352.

TO J.D.HOOKER.

Down, March 18th [1861].
I have been recalling my thoughts on the question whether the Glacial period affected the whole world contemporaneously, or only one longitudinal belt after another.

To my sorrow my old reasons for rejecting the latter alternative seem to me sufficient, and I should very much like to know what you think.

Let us suppose that the cold affected the two Americas either before or after the Old World.