A pivotal moment in The Hairy Ape, of course, is that instant in which a vociferous, cursing Yank, with a black-smudged back on which rivulets of sweat run, around whose eyes "the coal dust sticks like black make-up, giving [him] a queer, sinister expression" turns in hot rage over the whistling from the bridge only to be vis-a-vis with a "white apparition" that is horrified by the sight of him, calling him "a hairy ape." But, the real terror is that she is involved in an existential confrontation.
For all her nonchalant allusions to her grandfather intended more to irritate her supercilious aunt than to express any admiration for her ancestor, the pale, vacuous young woman realizes that she does not possess the mettle or earthy honesty of her ancestor:
I would like to be sincere; to touch life somewhere. But I'm afraid I have neither the vitality nor integrity. All that was burnt out in our stock before I was born. Grandfather's blast furnaces, flaming to the sky, melting steel, making millions--then father keeping those home fires burning, making more millions--and little me at the tail end of all. I'm a waste in the Bessemer process.
When, therefore, Mildred is confronted by Yank, the primordial creature of dead generations, "the hairy ape," she faces what her grandfather may have been closer to. It is a brutality and a raw strength, but it is also honest and real. And, this existential realization terrifies Mildred because she is weak and pretentious and truly what her aunt has called her, though for a different reason: "poser."