Which two lines in the poem indicate its theme?A Shadow by Henry Wadsworth Longfellow

I said unto myself, if I were dead,
What would befall these children? What would be
Their fate, who now are looking up to me
For help and furtherance? Their lives, I said,
Would be a volume wherein I have read
But the first chapters, and no longer see
To read the rest of their dear history,
So full of beauty and so full of dread.
Be comforted; the world is very old,
And generations pass, as they have passed,
A troop of shadows moving with the sun;
Thousands of times has the old tale been told;
The world belongs to those who come the last,
They will find hope and strength as we have done.