The two lines that best indicate the theme of the poem "A Shadow" by Henry Wadsworth Longfellow are:
"Thousands of times has the old tale been told;
The world belongs to those who come the last,"
These lines reflect the theme of continuity and the cyclical nature of life, suggesting that while individual struggles and stories may feel overwhelming, there is an inherent hope that future generations will find their own strength and resilience, just as those before them have done.