Don't you see what this means?
Bagon dreamt of flying, and would leap off cliffs in an attempt to grow wings and leave the ground.
Instead it became Shelgon; not so it may fly, but so it can simply better endure its cliff jumping endeavours. At that point it looked as though flight would always remain a fantasy.
Little did it know, from inside that bulky, deficient shell, it grew in silence and attained the power it needed to burst out as the winged Salamence, and at last it achieved its dream of flight.
Now it has evolved once more.
It has forgone the need for legs. It is now all wings. At last in can sever its ties with the earth, and become one with the sky. And I think that's rather sweet.