Hadži was born in a Serbian family in Temišvar (today Timișoara, Romania) in what was then Austria-Hungary. He began his career in Zagreb. In 1920, he moved to Ljubljana where he became the head of zoological institute at the then established University of Ljubljana. Between 1951 and 1972, Hadži was the head of the Biological institute at Slovenian Academy of Sciences and Arts (SASA). In 1938, he became a full member of SASA.[1]
Hadži proposed unique theories of animal evolution.[2] He devised a system of classification in which he divided the animal kingdom into six phyla: Protozoa, Parazoa, "Ameria" (animals with no segments), "Oligomeria" (animals with few segments), "Polymeria" (animals with many segments) and Chordata. His choice of characters important for classification was generally discredited by his contemporaries, and the system was never accepted by zoologists.[3] However, due to its simplicity, the system was widely used in science education in the former Yugoslavia. His other major theory was that of the origin of metazoa - he developed an existing hypothesis stating that the first multicellular animals resembling today's flatworms evolved from multinucleateciliates in which cell nuclei became separated by cellular membranes. Again, the theory emphasized similarities of structure while disregarding other important characters, so it was never generally accepted.[4]
Hadži's faunistical work focused on the invertebrate fauna of caves and mountains where he described more than a hundred new species and genera.
^"Zgodovina DZRJL" [History of DZRJL] (in Slovenian). Društvo za raziskovanje jam Ljubljana [Ljubljana Cave Exploration Society]. Retrieved 2019-05-29.
