LAMARCKISM. - Its fame rests above all on the evolutionary theory that goes by his name (Lamarckism) and that is set out 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 there would have derived, through successive complications and perfectings, all the animals and all the plants that exist and that existed on the surface of the globe. For L., the "exercise of life" tends ceaselessly to complicate organization. Life continually accelerates and varies the movement of the 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 be transformed into another, but all at once, and the number of species would be constant or at least would increase only very slowly. Two factors intervene to disturb such a rectilinear course, to make the various individuals sprung from the same stock diverge from one another: the influence of external agents and the use and disuse of organs.
External influences can act notably on the morphology of living beings. If the environment prompts a certain animal to use a particular organ more than the others, the former will acquire a greater development, while the latter will be reduced. Thus, for example, some birds became accustomed to living on the banks of waters and attempted to swim. In this attempt, making much use of their feet, the organic fluids were driven more actively into them and little by little the interdigital membranes developed and web-footed birds came about.
However, in order for Lamarckism to be able to explain evolution, it would be necessary that such characters, acquired by an organism during its life, could be transmitted to its descendants, which is far from proven.
As an accurate observer of nature, in his writings he affirms God as creator and wise orderer; he did not, however, have a clear knowledge of the nature and immortality of the human soul.