At the Museum, he also developed an interest in
paleontology and
geology. The curator, professor
Van Breda, approached him to describe the
fossil fishes from his own and Teylers' collections. This set Winkler up at the museum and his work, well received, was published in the
Verhandelingen ('Transactions') of Teylers' Society in 1859. This was followed up by further work on fishes from the German
Solnhofen limestone, and completion of the catalogue of the museum's fossil fish collection. This achievement so impressed the Museum's directors that they approached Winkler to do the same for their other fossil and mineral collections. A year later he finished the catalogue, despite being forced to work in an unheated room and continuing with his general medical practice. In 1864 Winkler was asked to become curator of Teylers' paleontological and
mineralogical cabinet, a post he kept until his death in 1897. He immediately set to work to catalogue the Museum's entire collection of fossils, which at the time was unnumbered and, frequently, undocumented. On the advice of the prominent
Utrecht natural historian Pieter Harting, he applied a numerical system in which the fossils were divided into Periods (
Paleozoic,
Mesozoic and
Caenozoic) and sorted from 'high' to 'low'. This system, and the way in which Winkler applied it, already showed the influence of
Darwin's theory of
evolution. Completing this catalogue would take until 1896, by which time six volumes and five supplements had been published, documenting a total of 15,458 fossils. Winkler also catalogued the museum's
mineral collection. ==Publications==