In "Sailing to Byzantium," how are themes of mortality and immortality developed?

Here is the text of the
poem:

That is
no country for old men. The young
In one another's arms, birds in the
trees
- Those dying generations - at their song,
The salmon-falls,
the mackerel-crowded seas,
Fish, flesh, or fowl, commend all summer
long
Whatever is begotten, born, and dies.
Caught in that sensual
music all neglect
Monuments of unageing intellect.

An aged
man is but a paltry thing,
A tattered coat upon a stick, unless
Soul
clap its hands and sing, and louder sing
For every tatter in its mortal
dress,
Nor is there singing school but studying
Monuments of its own
magnificence;
And therefore I have sailed the seas and come
To the
holy city of Byzantium.

O sages standing in God's holy
fire
As in the gold mosaic of a wall,
Come from the holy fire, perne
in a gyre,
And be the singing-masters of my soul.
Consume my heart
away; sick with desire
And fastened to a dying animal
It knows not
what it is; and gather me
Into the artifice of
eternity.

Once out of nature I shall never take
My bodily
form from any natural thing,
But such a form as Grecian goldsmiths
make
Of hammered gold and gold enamelling
To keep a drowsy Emperor
awake;
Or set upon a golden bough to sing
To lords and ladies of
Byzantium
Of what is past, or passing, or to
come.

The poem has invited a
great deal of literary analysis and interpretation. Clearly Yeats develops a contrast
between mortality and immortality and between flesh and intellect. These are established
in the first stanza. The young embrace "in one another's arms. Caught up in the "sensual
music" of life, they give no thought to "[m]onuments of unageing intellect." They glory
in their mortal existence, giving no thought to immortality. Yeats' reference to
"monuments" can be inferred to mean works of art born from intellect. Through these, the
artist becomes immortal; his intellect, the unique part of his being, will not age and
therefore will not die. All that is "begotten, born" dies in "dying generations." Flesh
is mortal; through art, however, immortality can be
achieved.

The remainder of the poem expresses Yeats'
thoughts and feelings about this contrast. Byzantium becomes symbolic; it is an immortal
place of artistic and intellectual culture where creativity can flourish. By choosing to
sail to "the holy city of Byzantium," he wishes to gain immortality through his
poetry.