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 disproven by the middle of the nineteenth century.
Lazzaro Spallanzani had demonstrated that microbes were present in the air, and could be killed by boiling, in 1768.
In 1861, Louis Pasteur had performed a series of experiments that demonstrated that organisms such as bacteria and fungi do not spontaneously appear in sterile, nutrient-rich media, but only invade them from outside.
Pasteur himself remarks, after a definitive finding in 1864, "Never will the doctrine of spontaneous generation recover from the mortal blow struck by this simple experiment." (Oparin, Aleksandr I. (1953). Origin of Life. Dover Publications, New York. p. 196.)