Mystery Poem. Who Wrote the following?

ONE’S-SELF I sing—a simple, separate Person;
Yet utter the word Democratic, the word En-masse.

Of Physiology from top to toe I sing;
Not physiognomy alone, nor brain alone, is worthy for the muse—I say the Form complete is worthier far;
The Female equally with the male I sing.

Of Life immense in passion, pulse, and power,
Cheerful—for freest action form’d, under the laws divine,
The Modern Man I sing.