Short answers, pulled from the story.
The Zoellner Quartet consisted of Joseph Zoellner, Sr. on viola, his daughter Antoinette Zoellner on first violin, his son Amandus Carl Zoellner on second violin, and his son Joseph Zoellner, Jr. on cello. Joseph, Sr. was born on the 2nd of February 1862; Amandus was born on the 7th of November 1892; and Joseph, Jr. was born on the 26th of October 1886. All four were natives of Brooklyn, New York.
The Zoellner Quartet was active from approximately 1903 or 1904 through 1925, when it formally retired. Members continued to perform informally afterward; a radio broadcast on station KHJ in Los Angeles was scheduled as late as the 24th of November 1927, and the group played with Albert Einstein at their conservatory in January 1931.
During the 1916-1917 tour season, the Zoellner Quartet played for Helen Keller in Oklahoma City while she held her fingertips on a resonant tabletop. She swayed in time to the music, alternately crying and smiling. Keller described each note as a picture, a fragrance, or the flash of a wing, and said the violins sang of woods, streams, and sun-kissed hills. Joseph Zoellner noted that although no one had told her the Tchaikovsky piece was based on a fisherman's song, Keller described it as evoking the sea and ocean breeze.
The Zoellner Quartet trained in Belgium under the celebrated pedagogue Cesar Thomson, sponsored by Ethel Crocker, wife of San Francisco banking magnate William Henry Crocker. Antoinette studied with Thomson from 1907 to 1912, and Joseph, Jr. graduated from the Royal Conservatory of Brussels in 1910. The group billed itself as the Zoellner Quartet of Brussels in promotional materials because of this formative Belgian connection, despite all four members having been born in Brooklyn, New York.
By 1921, the quartet's total was reported at 1,100 performances within six years of touring. Amandus Zoellner was later said to have participated in more than 2,500 performances during his career with the group. In nine weeks ending in late March 1923 alone, they gave 46 concerts during their twelfth tour of the eastern United States.
Charles Sanford Skilton dedicated his Two Indian Dances to the Zoellner Quartet, which premiered them in January 1916. Arthur Farwell, at Joseph Zoellner's suggestion, wrote a string quartet called The Hako in 1922 and dedicated it to the group. Eugene Goossens also wrote a quartet specifically for the Zoellners. The quartet additionally gave American premieres of Joseph Jongen's Serenade Tendre and Frances Marion Ralston's Fantasie for String Quartet during their 1923 eastern United States tour.