"Answer to an Insisting Friend" is a sort of introduction to some of the other sections in Specimen Days which detail Whitman's early life and family history.
Of the influence of his forebears on his work, he says, "I have often thought of the meaning of such things -- that one can only encompass and complete matters of that kind by exploring behind, perhaps very far behind, themselves directly, and so into their genesis, antecedents, and cumulative stages."
This statement is reminiscent of several stanzas in "Song of Myself":
"Rise after rise bow the phantoms behind me,
Afar down I see the huge first Nothing, the vapor from the nostrils of death,
I know I was even there...I waited unseen and always,
And slept while God carried me through the lethargic mist,
And took my time...and took no hurt from the foetid carbon.
Before I was born out of my mother generations guided me,
My embryo has never been torpid...nothing could overlay it;
For it the nebula cohered to an orb...the long slow strata piled to rest it on
...vast vegetables gave it sustenance,
Monstrous sauroids transported it in their mouths and deposited it with care."
"Sauroids" certainly qualify as "very far behind." Whitman traces the seeds of his existence from his "mother generations" all the way back through a vast network of evolving species, and God's imagination, and the void. If at other times in "Song of Myself" he identifies with everyone alive and dead, in these stanzas he extends that into being united with all of the diverse life forms of history. There is no shortage of resources for inspiration to draw from there.