Origin of Species Variorum

native ranks, and have reached or even crossed the equator. The invasion would, of course, have been greatly favoured by high land, and perhaps by a dry climate; for Dr. Falconer informs me that it is the damp with the heat of the tropics which is so destructive to perennial plants from a temperate climate. On the other hand, the most humid and hottest districts would have afforded an asylum for the .. natives. The mountain-ranges north-west of the Himalaya, and the long line of the Cordillera, seem to have afforded two great lines of invasion: and it is a striking fact, .. communicated to me by Dr. Hooker, that all the flowering plants, about forty-six in number, common to Tierra del Fuego and to Europe, still exist in North America, which must have lain on the line of march. We might of course speculate on the land having been formerly higher than at present in various parts of the tropics, where temperate forms apparently have crossed; but as the lines of migration have been so numerous, such speculations would be rash. Hence I am forced to believe that in certain regions, as in India, some temperate productions entered and crossed even the lowlands of the tropics at the period when the cold was most intense,— when arctic forms in Europe had migrated over at least twenty-five degrees of latitude, ... and covered the land at the foot of the Pyrenees. At this period of extreme cold, I believe that the climate under the equator at the level of the sea was about the same with that now felt there at the height of from five to six thousand feet. During this the coldest period, ... large spaces of the tropical lowlands were probably clothed with a mingled tropical and temperate vegetation, like that now growing with strange luxuriance at the base of the Himalaya, at the height of four or five thousand feet, as so graphically described by Hooker.