The introduction to Barack Obama's memoir Dreams from My Father: A Story of Race and Inheritance (1995) contains the passage:
And yet what strikes me most when I think about the story of my family is a running strain of innocence, an innocence that seems unimaginable, even by the measures of childhood. My wife's cousin, only six years old, has already lost such innocence: A few weeks ago he reported to his parents that some of his first grade classmates had refused to play with him because of his dark, unblemished skin.
I fail to understand the meaning of the words "running strain of innocence".
meaning
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