Theory of biogenesis – life originates only from pre-existing life.
Strengths – it asserts that living things can only be produced by another living thing, and not by a non-living thing.
By the middle of the 19th century, the theory of biogenesis had accumulated so much evidential support, due to the work of Pasteur and others, that the alternative theory of spontaneous generation had been effectively disproved.
Louis Pasteur performed a series of careful experiments which proved that organisms such as bacteria and fungi do not appear in nutrient rich media of their own accord in no-living material and which supported cell theory.
Obviously this can be traced far enough back to when there was no life, which raises debates as to how life was first formed.
Weakness – although Pasteur had demonstrated that modern organisms do not generate spontaneously in non living nutrients, his experiments were limited to a smaller system, and for a shorter time, than the open surface of the planet over millions or billions of years.