Spontaneous generation or
anomalous generation is an obsolete body of thought on the ordinary formation of living organisms without descent from similar organisms. Typically, the idea was that certain forms such as fleas could arise from inanimate matter such as dust, or that
maggots could arise from dead flesh. A variant idea was that of
equivocal generation, in which species such as
tapeworms arose from unrelated living organisms, now understood to be their
hosts. Doctrines supporting such processes of generation held that these processes are commonplace and regular. Such ideas are in contradiction to that of
univocal generation: effectively exclusive reproduction from genetically related parent(s), generally of the same species.