Alliteration

alliteration: the repetition of initial consonant sounds in two or more neighboring words (as in "She sells seashells..."). The repetition can reinforce meaning, unify ideas, and/or supply a musical sound.

"He who sells my sister, for purposes of prostitution, stand forth as the pious advocate of purity."
                                                           -Narrative of the Life of Frederick Douglass
                                                                            by Frederick Douglass

Douglass' autobiography is as much a collection of speeches as it is a written narrative. As such, much of the material is riddled with the use of devices such as this. Alliteration punctuates the words in this passage. Douglass uses the alliteration to emphasize words. The repetition of the "p" is powerful and causes each each word to sort of "ring." Douglass wants his message to be clear and stay with the audience so that they feel compelled to do something to end slavery.