Wilhelm Gueinzius (also sometimes spelled Guenzius, Quinzius, Queinzius), a German apothecary and naturalist, attended a school in Halle, near his birthplace, and subsequently qualified as a druggist assistant in 1833. During 1836-1837 he was employed at the pharmacy of a hospital in Berlin and then at a pharmacy in Dessau. At this time he met Professor E.F. Poepping, founder of the zoological museum in Leipzig, who supported his wish to become a collector of natural history specimens. As a result Guienzius came to the Cape Colony in 1839, where he requested permission to practise as an apothecary, chemist and druggist. He was duly registered, and still appeared (as William Guenzius) in the list of Cape apothecaries, chemists and druggists published in the Cape Town directory for 1865.
At least some of the natural history specimens Gueinzius collected at the Cape were sent to Poepping. Among these was the fungus Secotium gueinzii, which was named in his honour by G. Kunze in 1840. After some time as a teacher in Hottentots Holland, where he resided with a certain Field-cornet Morkel, he proceeded to Port Natal (now Durban) by sea in April 1841. There he became the first resident plant collector in KwaZulu-Natal, living in a hut in what came to be known as Stella Bush. For some time he was paid to collect for the museum of the University of Leipzig, sending his plants to Poepping, who incorporated them in his herbarium under his own name. Poepping described some of his specimens of reptiles, but the manuscript was never published.
During 1842-1843 Gueinzius returned briefly to the Cape when there was a clash between the British settlers and the Voortrekkers at Congella. That same year he requested permission from the Cape government to obtain two bontebok for the museum of the King of Saxony. After the withdrawal of the Voortrekkers he was one of four persons sent to convey to the government of the South African Republic (Transvaal) the terms under which the Voortrekkers could remain in Natal. The ferns he had collected in the Cape and Natal were described by G. Kunze in 1844. In 1848 he applied for permission to practise as an apothecary in Natal, but he is not listed as a licensed chemist in the Natal Almanac for 1872. At this time he broke his connection with Professor Poepping and began collecting for the firm W. Schlueter in Halle and for museums in Leipzig, Stettin and Dresden. In 1849 William Tuck* lived with him for a short time. By 1850 he owned a plot on the Umbilo River, where he still resided in 1857. In 1853 he applied for the position of curator of the Durban Botanic Gardens, but was not successful. In later years he lived as a recluse, with three pet pythons, in a dilapidated shed at Posselt's Mission Station, New Germany (near Pinetown).
One "Gueinzius" (no initials, but presumably Wilhelm) published two papers in the Transactions and Proceedings of the Entomological Society of London for 1858-1861: "Notes on the habits of Paussidae", and "On the habits of the Hymenoptera of Natal". Later a paper by W. Gueinzius, "Aus den Vogelleben Sued-Afrikas" appeared in the Journal fuer Ornithologie (1873). In March 1868 Gueinzius exhibited an excellent collection of rare moths at the first conversazione of the Natural History Association of Natal (1868-1871). In addition to moths and plants (including fungi and lichens) he collected birds, snakes and butterflies, many of which he raised from caterpillars. He was the first resident of KwaZulu-Natal known to have collected shells and sent specimens to Leipzig. Amongst other species he appears to have discovered the land snail Archachatina semidecussata. He was a tall, thin man with a long beard, considered eccentric but very knowledgeable about natural history by his contemporaries. Several trees and other plant species were named after him, including Combretum gueinzii and Rhus gueinzii (both by O.W. Sonder*), Psoralea gueinzii (by W.H. Harvey*), Asplenium gueinzii, and Fabronia gueinzii. His plant specimens ended up in various herbaria, including that of the South African Museum.