for some weeks, till I have done with crossing; but I have not been able to stop myself meditating on your powerful objection to the mundane cold period (334/1.
See Letter 49.), viz.

that MANY-fold more of the warm-temperate species ought to have crossed the Tropics than of the sub-arctic forms.

I really think that to those who deny the modification of species this would absolutely disprove my theory.

But according to the notions which I am testing--viz.

that species do become changed, and that time is a most important element (which I think I shall be able to show very clearly in this case)--in such change, I think, the result would be as follows.
Some of the warm-temperate forms would penetrate the Tropics long before the sub-arctic, and some might get across the equator long before the sub-arctic forms could do so (i.e.always supposing that the cold came on slowly), and therefore these must have been exposed to new associates and new conditions much longer than the sub-arctic.