How does Shelley differentiate the song of the skylark from the earthly songs?

P.B. Shelley is a romantic revolutionary poet. He always wants to escape from this sorrowful mundane world and arrive in a world of peace and rest. Here, in the poem, "To A Skylark" the poet uses the name of skylark as the inhabitant of the imaginary world of happiness.

         He says that the skylark is a 'blithe spirit' and "an unbodied joy" who pours his "strains of unpremidiated art". The skylark's song is spontaneous and there is no stress of sorrow and trobles, because it is a dweller of the imaginary world and it does absolutely not know the tribul actions of this mundane world because it lives in "heaven, or near it."

        But the human beings are the inhabitant of this sorrowful mundane world. They are always busy in thinking about their sorrows, sufferings, frustration and so on. It is said that the "chorus hymeneal" and "triumphant chaunt" are the most beautiful songs of the human world. But according to our poet, compared with the spontaneous song of the skylark, which is nothing but the divine rapture these songs are 'empty vaunt.' Because these odes lack divine inspiration. And these songs are 'the expression of a vast unfulfilment.' The human beings always seem to be haunted by a desire of achieving excessive physical happiness. So, the poet says that " Our sweetest songs are those that tell of saddest thought" And defines the song of the skylark as the "rain of melody."