Let it advance first either from north or south till the Tropics became slightly cooled, and a few temperate forms reached the Silla of Caracas and the mountains of Brazil.

You would say, I suppose, that nearly all the tropical productions would be killed; and that subsequently, after the cold had moderated, tropical plants immigrated from the other non-chilled parts of the world.

But this is impossible unless you bridge over the tropical parts of the Atlantic--a doctrine which you know I cannot admit, though in some respects wishing I could.

Oswald Heer would make nothing of such a bridge.

When the Glacial period affected the Old World, would it not be rather rash to suppose that the meridian of India, the Malay Archipelago, and Australia were refrigerated, and Africa not refrigerated?
But let us grant that this was so; let us bridge over the Red Sea (though rather opposed to the former almost certain communication between the Red Sea and the Mediterranean); let us grant that Arabia and Persia were damp and fit for the passage of tropical plants: nevertheless, just look at the globe and fancy the cold slowly coming on, and the plants under the tropics travelling towards the equator, and it seems to me highly improbable that they could escape from India to the still hot regions of Africa, for they would have to go westward with a little northing round the northern shores of the Indian Ocean.