An atavism is the ..reappearance of a lost character (morphology or behaviour) typical of remote ancestors and not seen in the parents or recent ancestors of the organisms displaying the atavistic character (Hall, 1984). In humans, hypertrichosis (extensive body hair), the presence of a tail and supernumerary nipples are often quoted as examples (Hall, 1995). However, Louis Bolk (1866–1930) explained these phenomena in another way. He considered human morphology as an unspecialized expression of the mammalian developmental pattern. The latter also encompasses potentialities for unilateral or propulsive development pathways (specializations) that usually remain latent in humans, but can become expressed in other species. According to Bolk, the appearance of so-called atavisms in humans results from the occasional expression of these latencies in Homo sapiens; they do not recapitulate ancestral conditions.