Wolf soon became the illustrator of choice for all the books published by returning adventurers like David Livingstone, Alfred Russel Wallace and Henry Walter Bates (for instance Bates' 1863 book "The Naturalist on the River Amazons"). Wolf joined an association called the German Athenaeum which was founded in 1869 and members met for scientific, literary and musical evenings. For their exhibitions he worked on a range of compositions often with natural elements. His favourite medium was charcoal and ink. Wolf became treasurer to a fund for German widows during the Franco-Prussian War (War of 1870.) After the war, he met Daniel Giraud Elliot in Paris and visited a battlefield. He rendered the image in a design called "Peace and War" with turtle doves on a bush over a soldier's helmet. He also produces some cartoon like illustrations including "Lecture on Embryology" in which he taunts certain men of science. When Charles Darwin began his study of animal expressions, he was introduced by Abraham Dee Bartlett, the zoo superintendent, to the abilities of Wolf in illustrating minute details of animals in action. Darwin requested Wolf to make some illustrations from photographs and living animals in the zoological garden. Wolf held his own opinions on the reliability of others' observations and even doubted Darwin's interpretation of the face of a monkey as a "laugh". Darwin visited him on several occasions and Wolf appreciated him for being very approachable, someone that even "a child could talk to". Wolf's abilities were widely acclaimed even in his lifetime. Wolf established wildlife art as a genre and his observation of living birds allowed him to produce illustrations in very accurate and lifelike stances. On occasion he would come back from a trip and produce very accurate sketches from memory. He was very careful in his observation of feather patterns and when he read the works of Sundevall and Nitzsch on pterylography, he had nothing new to learn. The zoologist Alfred Newton called him "the greatest of all animal painters", while Landseer said that Wolf must have been a bird before he became a man. Wolf made numerous drawings in pen and charcoal as well as lithographs for scholarly societies such as the Zoological Society of London (he produced 340 "attractive" colour plates for the ZSL Proceedings in the course of 30 years), and a very large number of illustrations for books on natural history and travel published from various countries; and was considerably successful as a painter as well. Until 1946, the cover of the journal "Ibis" carried a woodcut by Wolf of an Ibis against a background with ruins, a pyramid and a rising sun.