Analysis of A Song of Derivations

I come from nothing; but from where
Come the undying thoughts I bear?
Down, through the long links of death and birth,
From the past poets of the earth,
My immortality is there.

I am like the blossom of an hour.
But long, long vanished sun and shower
Awoke my breath i' the young world's air;
I track the past back everywhere
Through seed and flower and seed and flower.

Or I am like a stream that flows
Full of the cold springs that arose
In morning lands, in distant hills;
And down the plain my channel fills
With melting of forgotten snows.

Voices, I have not heard, possessed
My own fresh songs; my thoughts are blessed
With relics of the far unknown.
And mixed with memories not my own
The sweet streams throng in my breast.

Before this life began to be,
The happy songs that wake in me
Woke long ago and far apart.
Heavily on this little heart
Presses this immortality.