Theodor Wilhelm (Theo) Kassner, geologist and plant collector, was the son of Friedrich Wilhelm Kassner, farmer, part-time mayor and notary, and his second wife, Christiane Emilie Pauline Hainich. He was a headstrong, impulsive and devil-may-care youngster, ran away from home at the age of 17 and worked his way via Hamburg and England to South Africa to seek his fortune. In 1890 he collected plants in the vicinity of Cape Town and Tulbagh. Thereafter he worked on the Barberton goldfields and in 1899 published a Geological sketch map of the De Kaap goldfields, scale 1:30 000. This was the first geological map of the goldfields and included the Barberton region.
From Barberton he moved to the Witwatersrand where he worked for some time as a mine manager. In December 1893 he travelled to Lydenburg and back with the botanist F.R. Rudolph Schlechter*, colleting insects. During the Anglo-Boer War (1899-1902) he was in England, for on 18 July 1900 he married Alice Caroline Rowbotham in London and they went on a seven month honeymoon and business trip to France, Italy, Austria and Germany. In 1902 he published a book in London, Gold-seeking in South Africa: A handbook of hints for intending explorers, prospectors, and settlers. With a chapter on the agricultural prospects of South Africa. It contained much information about all the gold reefs in the country, with practical instructions on how to prospect and mine for gold.
Early in 1902 Kassner met with the director of the British Museum in London, who wanted him to collect plants, insects and fossils in Africa. Towards the end of January 1902 he left for East Africa and went on a prospecting expedition in Tanganyika (now Tanzania), returning to Johannesburg and his family in October that year. He then continued his prospecting and mapping work, mainly on the Witwatersrand and westwards to Potchefstroom and Klerksdorp, and compiled several maps: Geological map of the west Witwatersrand goldfields (1903), Geological map, showing the relationship of the East, Central and Western Witwatersrand, and Geological map of the southern Transvaal goldfields.
Kassner and his family returned to Europe late in 1904 and settled in Cologne, Germany. He was elected a Fellow of the Royal Geographical Society (FRGS) in March 1907, came to South Africa again in August, and in November that year set out with a scientific party on a walking expedition of sixteen months, from southern Africa to Egypt. The party included 'university men of known ability' in geology, zoology, botany, analytical chemistry, entomology, natural history, and astronomy, and the venture came to be known as the 'Kassner Expedition'. They travelled from South Africa to Bulawayo by train, arriving on 9 December 1907, and met Father Goetz* there. The train journey continued to the Victoria Falls. From present Zambia they travelled on foot, with carriers, but the other members of the expedition soon abandoned the enterprise and Kassner continued on his own. He collected plants in the Cape Colony, Transvaal, Zimbabwe, and countries further north and published an account of the journey in the form of a book, My journey from Rhodesia to Egypt, including an ascent of Ruwenzori and a short account of the route from Cape Town to Broken Hill and Lado to Alexandria (London, 1911). In June 1910 he set off on a similar journey to the less explored parts of the Congo, returning to his family in Britain in April 1911. The next year he published another book, Die Zukunft Afrikas: Ratschlaege fuer die Kolonisation (The future of Africa: Suggestions for colonisation; Leipzig, 1912).
The family lived in Germany during World War I (1914-1918) and in 1921 returned to Johannesburg where Kassner took up prospecting again and went into business as a consulting geologist. During 1924-1925 he worked for the South African Phosphates Exploration Syndicate at their phosphate mine near Bandolierkop, Limpopo, and in 1933 was still involved in phosphate mining (and cotton farming) in the area. In 1927 he and J.F. Ludorf signed a prospecting contract with the municipality of Pietersburg (now Polokwane) in regard of the farm Weltevreden No.140. During the nineteen-thirties he prospected also for phosphates in Mozambique, for asbestos near Carolina, and for uranium in Gordonia, and was involved in the opening up of the gold fields in the Heidelberg district. In July 1937 he attended the 17th International Geological Congress in Moscow. Two years later he visited the United States and Canada, returning to Johannesburg when World War II (1939-1945) broke out.
Plants collected by Kassner are housed in the Bolus Herbarium, University of Cape Town; the National Herbarium, Pretoria; the herbaria of the Botanical Museum, Berlin; the Royal Botanical Gardens, Edinburgh; the Natural History Museum, London; Kew Gardens; the University of Zurich; and others. The species Periglossum kassnerianum was named after him by Schlechter. His geological specimens were presented to the University of the Witwatersrand, some probably after his death and others after the death of his son Felix.