Hence I should infer that we ought to have in the warm-temperate S.hemisphere more representative or modified forms, and fewer identical species than in comparing the colder regions of the N.and S.I have expressed this very obscurely, but you will understand, I think, what I mean.

It is a parallel case (but with a greater difference) to the species of the mountains of S.Europe compared with the arctic plants, the S.European alpine species having been isolated for a longer period than on the arctic islands.

Whether there are many tolerably close species in the warm-temperate lands of the S.and N.I know not; as in La Plata, Cape of Good Hope, and S.Australia compared to the North, I know not.

I presume it would be very difficult to test this, but perhaps you will keep it a little before your mind, for your argument strikes me as by far the most serious difficulty which has occurred to me.

All your criticisms and approvals are in simple truth invaluable to me.