This bears on chance of birds by their muddy feet transporting fresh-water plants.
This would not be a bad dodge for a collector in country when plants were not in seed, to collect and dry mud from ponds.
LETTER 585. TO ASA GRAY. Down [1857].
I am very glad to hear that you think of discussing the relative ranges of the identical and allied U. States and European species, when you have time. Now this leads me to make a very audacious remark in opposition to what I imagine Hooker has been writing (585/1. See Letter 338, Volume I.), and to your own scientific conscience. I presume he has been urging you to finish your great "Flora" before you do anything else. Now I would say it is your duty to generalise as far as you safely can from your as yet completed work. Undoubtedly careful discrimination of species is the foundation of all good work; but I must look at such papers as yours in Silliman as the fruit. As careful observation is far harder work than generalisation, and still harder than speculation, do you not think it very possible that it may be overvalued? It ought never to be forgotten that the observer can generalise his own observations incomparably better than any one else. How many astronomers have laboured their whole lives on observations, and have not drawn a single conclusion; I think it is Herschel who has remarked how much better it would be if they had paused in their devoted work and seen what they could have deduced from their work. So do pray look at this side of the question, and let us have another paper or two like the last admirable ones. There, am I not an audacious dog!
You ask about my doctrine which led me to expect that trees would tend to have separate sexes. I am inclined to believe that no organic being exists which perpetually self-fertilises itself. This will appear very wild, but I can venture to say that if you were to read my observations on this subject you would agree it is not so wild as it will at first appear to you, from flowers said to be always fertilised in bud, etc. It is a long subject, which I have attended to for eighteen years. Now, it occurred to me that in a large tree with hermaphrodite flowers, we will say it would be ten to one that it would be fertilised by the pollen of its own flower, and a thousand or ten thousand to one that if crossed it would be crossed only with pollen from another flower of same tree, which would be opposed to my doctrine. Therefore, on the great principle of "Nature not lying," I fully expected that trees would be apt to be dioecious or monoecious (which, as pollen has to be carried from flower to flower every time, would favour a cross from another individual of the same species), and so it seems to be in Britain and New Zealand. Nor can the fact be explained by certain families having this structure and chancing to be trees, for the rule seems to hold both in genera and families, as well as in species.
I give you full permission to laugh your fill at this wild speculation; and I do not pretend but what it may be chance which, in this case, has led me apparently right. But I repeat that I feel sure that my doctrine has more probability than at first it appears to have. If you had not asked, I should not have written at such length, though I cannot give any of my reasons.
The Leguminosae are my greatest opposers: yet if I were to trust to observations on insects made during many years, I should fully expect crosses to take place in them; but I cannot find that our garden varieties ever cross each other. I do NOT ask you to take any trouble about it, but if you should by chance come across any intelligent nurseryman, I wish you would enquire whether they take any pains in raising the varieties of papilionaceous plants apart to prevent crossing. (I have seen a statement of naturally formed crossed Phaseoli near N. York.) The worst is that nurserymen are apt to attribute all varieties to crossing.
Finally I incline to believe that every living being requires an occasional cross with a distinct individual; and as trees from the mere multitude of flowers offer an obstacle to this, I suspect this obstacle is counteracted by tendency to have sexes separated.