What we call a 'weed' is, in fact, an attempt at survival by a particular species which took Nature millions of years to create and develop. The flower was fertilised at the expense of innumerable insects, it was transformed into seed, the wind scattered it over the fields round about and so - because it was not planted in just one place, but in many - its chances of surviving until next spring are that much greater. If it was concentrated in just one place, it would be vulnerable to being eaten, to flood, fire and drought.