Genetics
Imre Festetics, who published work in German in the first part of the 19th century, was totally forgotten until recently. He described several rules of genetic inheritance in his work  Die genetische Gesätze der Natur, 1819 (The genetic law of nature). His second law is the same as Mendel. In his third law, he developed the basic principles of mutation. None of the histories of genetics published in the 20th century mentions him.