There was a considerable interest in "Less Observed Analogies of the Three Kingdoms of Nature" (1815) and On the "Propagation of Mankind in the Various Parts of the Earth" (1818). In the first he proposed — 44 years before Charles Darwin — a theory of gradual evolution. According to his proposition, the Creator organized nature not into three totally different kingdoms (animal, mineral, vegetable), but as a continuous chain of organization, without sudden steps or breaks, into «three species of life, that is, the formative, vegetative and sensitive life; so that the first, destined to minerals, participates to some degree in the second, typical of plants, and this, in the third, assigned to animals. Thus, for example, crystalline minerals precede the simplest plant forms. And complex plant organizations give way to animals. In the second mentioned work, Molina proposed the thesis that the observable physical differences in the human race are due to climatic and geographical factors.