↑(på engelsk)Hagenbeck Tierpark und Tropen-Aquarium. Zoo and Aquarium Visitor. Hentet 2008-07-22. "The founder and his idea Carl Hagenbeck built what no other dared dream of. In 1907, the Hamburg man opened the first barless zoo in the world. As early as the end of the nineteenth century, this son of a fishmonger had the idea of showing animals no longer caged up but in open viewing enclosures. In his zoo of the future, nothing more than unseen ditches were to separate wild animals from members of the public. Carl Hagenbeck patented this idea in 1896. Nine years later his dream was to come true in Hamburg-Stellingen. The revolutionary open viewing enclosures and panoramas were in fact ridiculed in professional circles but took the public's breath away. Hagenbeck's zoo is considered to have prepared the way for today's wildlife adventure parks."