In the past, people believed that certain creatures could spontaneously generate. This belief was based on the theory of spontaneous generation, which stated that creatures like fleas and maggots could form spontaneously from certain types of matter. This theory was believed for centuries, and only began to be disproven in the late 1600s. Experiments by Louis Pasteur in the 1850s finally disproved the theory of spontaneous generation and it fell out of the popular consciousness.