LAMARCKISM. – Its fame is chiefly tied to the evolutionary theory that bears his name (Lamarckism) and which is expounded in the *Philosophie zoologique*. He is also the author of a system of classification that he modified and perfected in successive revisions.
According to Lamarckism, life would have begun on earth through spontaneous generation; from the primordial beings thus generated, all animals and plants that exist and have existed on the surface of the globe would have derived through successive complications and perfections. For Lamarck, the “exercise of life” continually tends to complicate organization. Life continuously accelerates and varies the movement of fluids in the tissues, in which the various organs thus develop. If there were no disturbing causes, the progression of complication would be incessant, regular, and linear, so that one species could transform into another, but only once, and the number of species would remain constant or at least increase only very slowly. To disrupt such a straight-line course and cause divergence among the various individuals descended from the same stock, two factors intervene: the influence of external agents and the use and disuse of organs. External influences can act significantly on the morphology of living beings. If the environment impels a certain animal to use a particular organ more than others, the former will acquire greater development, while the latter will diminish. Thus, for example, some birds have become accustomed to living on the banks of rivers and have attempted to swim. In this effort, by using their feet extensively, the organic fluids were pushed more actively into them, and little by little interdigital membranes developed, giving rise to web-footed birds.
As a careful observer of nature, in his writings he affirms God as the wise creator and ordainer; he did not, however, have a clear understanding of the nature and immortality of the human soul.