Trees seem in this respect to behave rather differently from other plants.
It would be a very curious point, but I fear you would think it out of your essay, to compare the list of European plants in Tierra del Fuego (in Hooker) with those in North America; for, without multiple creation, I think we must admit that all now in T.del Fuego must have travelled through North America, and so far they do concern you.
The discussion on social plants (vague as the terms and facts are) in De Candolle strikes me as the best which I have ever seen: two points strike me as eminently remarkable in them; that they should ever be social close to their extreme limits; and secondly, that species having an extremely confined range, yet should be social where they do occur: I should be infinitely obliged for any cases either by letter or publicly on these heads, more especially in regard to a species remaining or ceasing to be social on the confines of its range.
There is one other point on which I individually should be extremely much obliged, if you could spare the time to think a little bit and inform me: viz., whether there are any cases of the same species being more variable in United States than in other countries in which it is found, or in different parts of the United States?
Wahlenberg says generally that the same species in going south become more variable than in extreme north.

Even still more am I anxious to know whether any of the genera, which have most of their species horribly variable (as Rubus or Hieracium are) in Europe, or other parts of the world, are less variable in the United States; or, the reverse case, whether you have any odious genera with you which are less odious in other countries?
Any information on this head would be a real kindness to me.
I suppose your flora is too great; but a simple list in close columns in small type of all the species, genera, and families, each consecutively numbered, has always struck me as most useful; and Hooker regrets that he did not give such list in introduction to New Zealand and other Flora.

I am sure I have given you a larger dose of questions than you bargained for, and I have kept my word and treated you just as I do Hooker.

Nevertheless, if anything occurs to me during the next two months, I will write freely, believing that you will forgive me and not think me very presumptuous.
How well De Candolle shows the necessity of comparing nearly equal areas for proportion of families! I have re-read this letter, and it is really not worth sending, except for my own sake.

I see I forgot, in beginning, to state that it appeared to me that the six heads of your Essay included almost every point which could be desired, and therefore that I had little to say.
LETTER 326.