The context for this proto-evolutionary thought is de Maillet's belief that space contained the seeds of life, invisible spores always available for seeding. This theory of the origin of life is called panspermia. The really important observation he made was that the lower layers of sedimentary rocks contained animals and plants different from those of today, and some types that were unrecognizable. He appreciated that this was a critical issue and offered several explanations, none of which reach the present idea of evolution.