Spontaneous generation refers both to the supposed processes by which different types of life might repeatedly emerge from specific sources other than seeds, eggs, or parents, and also to theoretical principles presented in support of any such phenomena. Crucial to this doctrine are the ideas that life comes from non-life and that no causal agent, such as a parent, is needed. The hypothetical processes by which life routinely emerges from nonliving matter on a time scale of minutes, weeks, or years (e.g. in the supposed seasonal generation of mice and other animals from the mud of the Nile) are sometimes referred to as "abiogenesis". Such ideas have no operative principles in common with the modern hypothesis of abiogenesis, which asserts that life emerged in the early ages of the planet, over a time span of at least millions of years, and subsequently diversified, and that there is no evidence of any subsequent repetition of the event.