“Before I was born out of my mother, generations guided me,” Walt Whitman wrote in Song of Myself, envisioning his unborn self as the product of myriad potentialities converging since the dawn of time — “the nebula cohered to an orb” and “the long, slow strata piled” to make it possible. A century and a half after Whitman, Ross Gay — another poet of uncommon sensitivity to our shared longings and largehearted wonderment at the universe in its manifold expressions…