In short, Nature uses all sorts of conveyances, from the rudest drag to a balloon, but she will get her seeds along in due season.
Suppose one were to take such a boxful of birch seed as I have described into the meeting-house belfry in the fall, and let some of it drop in every wind, but always more in proportion as the wind was stronger, and yet so husband it that there should be some left for every gale even till far into spring; so that this seed might be blown toward every point of the compass and to various distances in each direction. Would not this represent a single birch tree on a hill? Of which trees (though only a part on hills) we have perhaps a million. And yet some feel compelled to suppose that the birch trees which spring up after a burning are spontaneously generated - for want of seed! It is true [it] does not come up in great quantities at the distance I have spoken of, but, if only one comes up there this year, you may have a million seeds matured there a few years hence.