LAMARCKISMO. — His fame rests chiefly on the evolutionary theory that bears his name (lamarckism), set forth in the Philosophie zoologique. He is also the author of a system of classification that he modified and perfected on successive occasions.
According to lamarckism, life began on earth through spontaneous generation; from the primordial beings thus generated, through successive complications and improvements, all the animals and plants that exist and have existed on the surface of the globe would have derived. For L., the “exercise of life” continually tends to complicate organization. Life constantly 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 unceasing, regular, and linear, so that one species could be transformed into another, but only once, and the number of species would remain constant or at least increase only very slowly. Two factors intervene to disturb such a rectilinear course and to cause the various individuals originating from the same stock to diverge from one another: the influence of external agents and the use and disuse of organs.
External influences can act considerably upon the morphology of living beings. If the environment induces a certain animal to use one particular organ more than the others, the former will acquire greater development, while the latter will atrophy. Thus, for example, some birds became accustomed to living along the banks of waters and attempted to swim. In this attempt, making extensive use of their feet, the organic fluids were driven more actively into them, and little by little the interdigital membranes developed, producing web-footed birds.
However, for lamarckism to explain evolution, it would be necessary for such characteristics, acquired by an organism during its lifetime, to be transmissible to its descendants, a proposition that is far from proven.
As a careful observer of nature, in his writings he affirms God as creator and wise ordainer; on the other hand, he had no clear understanding of the nature and immortality of the human soul.