Let us take as examples a single little set of instances, familiar to
everybody, but far commoner in the world at large than the inhabitants
of towns are at all aware of: I mean, the winged seeds, that fly about
freely in the air by means of feathery hairs or gossamer, like
thistle-down and dandelion. Of these winged types we have many hundred
varieties in England alone. All the willow-herbs, for example, have
such feathery seeds (or rather fruits) to help them on their way
through life; and one kind, the beautiful pink rose-bay, flies about so
readily, and over such wide spaces of open country, that the plant is
known to farmers in America as fireweed, because it always springs up
at once over whole square miles of charred and smoking soil after every
devastating forest fire. It travels fast, for it travels like Ariel. In
much the same way, the coltsfoot grows on all new English railway
banks, because its winged seeds are wafted everywhere in myriads on the
winds of March. All the willows and poplars have also winged seeds: so
have the whole vast tribe of hawkweeds, groundsels, ragworts, thistles,
fleabanes, cat's-ears, dandelions, and lettuces. Indeed, one may say
roughly, there are very few plants of any size or importance in the
economy of nature which don't deliberately provide, in one way or
another, for the dispersal and dissemination of their fruits or
seedlings.

Why is this? Why isn't the plant content just to let its grains or
berries drop quietly on to the soil beneath, and there shift for
themselves as best they may on their own resources?

The answer is a more profound one than you would at first imagine.
Plants discovered the grand principle of the rotation of crops long
before man did. The farmer now knows that if he sows wheat or turnips
too many years running on the same plot, he 'exhausts the soil,' as we