<P> 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 . </P> <P> According to this theory, living things came forth from nonliving things because the nonliving material contained pneuma, or "vital heat". The creature generated was dependent on the proportions of this pneuma and the five elements he believed comprised all matter . While Aristotle recognized that many living things emerged from putrefying matter, he pointed out that the putrefaction was not the source of life, but the byproduct of the action of the "sweet" element of water . </P> <P> Animals and plants come into being in earth and in liquid because there is water in earth, and air in water, and in all air is vital heat so that in a sense all things are full of soul . Therefore living things form quickly whenever this air and vital heat are enclosed in anything . When they are so enclosed, the corporeal liquids being heated, there arises as it were a frothy bubble . </P> <P> Numerous forms were attributed to various sources . The testaceans (shelled molluscs) are characterized by forming by spontaneous generation in mud, but differ based upon the material they grow in--for example, clams and scallops in sand, oysters in slime, and the barnacle and the limpet in the hollows of rocks . Some reddish worms form from long - standing snow which has turned reddish . Another grub was said to grow out of fire . </P>

Compare the theory of spontaneous generation with the theory of biogenesis