The possibility of spontaneous generation cannot be denied, but that it takes place is against all human experience.
It was not possible to attain any considerable knowledge of the bacteria discovered by Loewenhoeck until more perfect instruments for studying them were devised.

Lenses for studying objects were used in remote antiquity, but the compound microscope in which the image made by the lens is further magnified was not discovered until 1605, and when first made was so imperfect that the best simple lenses gave clearer definition.

With the betterment of the microscope, increasing the magnifying power and the sharpness of the image of the object seen, it became possible to classify the minute organisms according to size and form and to study the separate species.

The microscope has now reached such a degree of perfection that objects smaller than one one hundred thousandth of an inch in diameter can be clearly seen and photographed.
Great impetus was given to the biological investigation of disease by the discoveries which led to the formulation of the cell theory in 1840 and the brilliant work of Pasteur on fermentation,[1] but it was not until 1878 that it was definitely proved that a disease of cattle called anthrax was due to a species of bacteria.

What should be regarded as such proof had been formulated by Henle in 1840.