Now there

is one property that animals are found to have in common
with plants. For some plants are generated from the seed of plants,
whilst other plants are self-generated through the formation of some
elemental principle similar to a seed; and of these latter plants some
derive their nutriment from the ground, whilst others grow inside
other plants, as is mentioned, by the way, in my treatise on Botany.
So with animals, some spring from parent animals according to their
kind, whilst others grow spontaneously and not from kindred stock; and
of these instances of spontaneous generation some come from putrefying
earth or vegetable matter, as is the case with a number of insects,
while others are spontaneously generated in the inside of animals
out of the secretions of their several organs.