For Heaven's sake never speak of boring me, as it would be the greatest pleasure to aid you in the slightest degree and your letter has interested me exceedingly. I will go through your points seriatim, but I have never attended much to the history of any subject, and my memory has become atrociously bad. It will therefore be a mere chance whether any of my remarks are of any use.
Your idea, to show what travellers have done, seems to me a brilliant and just one, especially considering your audience.
1. I know nothing about Tournefort's works.
2. I believe that you are fully right in calling Humboldt the greatest scientific traveller who ever lived, I have lately read two or three volumes again. His Geology is funny stuff; but that merely means that he was not in advance of his age. I should say he was wonderful, more for his near approach to omniscience than for originality. Whether or not his position as a scientific man is as eminent as we think, you might truly call him the parent of a grand progeny of scientific travellers, who, taken together, have done much for science.
3. It seems to me quite just to give Lyell (and secondarily E. Forbes) a very prominent place.
4. Dana was, I believe, the first man who maintained the permanence of continents and the great oceans... When I read the 'Challenger's' conclusion that sediment from the land is not deposited at greater distances than 200 or 300 miles from the land, I was much strengthened in my old belief. Wallace seems to me to have argued the case excellently. Nevertheless, I would speak, if I were in your place, rather cautiously; for T. Mellard Reade has argued lately with some force against the view; but I cannot call to mind his arguments. If forced to express a judgment, I should abide by the view of approximate permanence since Cambrian days.
5. The extreme importance of the Arctic fossil-plants, is self-evident. Take the opportunity of groaning over [our] ignorance of the Lignite Plants of Kerguelen Land, or any Antarctic land. It might do good.
6. I cannot avoid feeling sceptical about the travelling of plants from the North EXCEPT DURING THE TERTIARY PERIOD. It may of course have been so and probably was so from one of the two poles at the earliest period, during Pre-Cambrian ages; but such speculations seem to me hardly scientific seeing how little we know of the old Floras.
I will now jot down without any order a few miscellaneous remarks.
I think you ought to allude to Alph. De Candolle's great book, for though it (like almost everything else) is washed out of my mind, yet I remember most distinctly thinking it a very valuable work. Anyhow, you might allude to his excellent account of the history of all cultivated plants.