LAMARCK, JEAN BAPTISTE PIERRE ANTOINE DE MONET DE (b. Bazentin-le-Petit, Picardy, France, 1 August 1744; d. Paris, France, 28 December 1829), botany, invertebrate zoology and paleontology, evolution.

    Gradually new organs or parts would be formed as acquired modifications were passed on through reproduction.

The great expansion of Lamarck's ideas occurred between 1800 and 1802, when the Recherches sur l'organisation des corps vivans was published. Much of the work was devoted to documenting the “degradations” in organization of the larger groupings of animals, from mammals to polyps. He was, however, more aware of the need to turn this series over so that it would correspond to the order of production in nature and time and thus be a really natural method of classification. Once he began to think in terms of increasing levels of complexity, he needed a mechanism to propel change; otherwise one would have only polyps in the world. He therefore began to talk of a natural tendency in the organic realms toward increasing complexity. Lamarck is not clear or consistent about the manner in which this natural tendency operates; often it seems to function like a moving escalator. At other times it can best be understood by a stairlike construction where descent is more directly tied to the historical past although Lamarck never suggests any dates marking the appearance of particular forms of life.

Lamarck's conception of a natural tendency toward increasing complexity provided a perfect complement to his views of the mineral kingdom with the opposite natural tendency. In both cases a long time span allowed nature to do her work and local circumstances explained irregularities. Among living beings, irregularities included all organisms below the level of the “masses,” which usually meant classes but sometimes was extended to orders and families, never to genera and species.

In 1802 many of the additions were designed to explain in more detail why and how evolution happened; Lamarck's chemistry was essential to those explanations. He had to face the crucial problem of what happened at the lower ends of the vegetable and animal series. Rejecting the vitalist views of such contemporaries as Bichat, Lamarck defined life in physical terms. Life resulted from a particular kind of organization and a general tension maintained by the stimulation of the subtle fluids of his chemistry, especially modified forms of fire.

Spontaneous generation of animals, which Lamarck held was analogous to fertilization, occurred when heat (or caloric), sunlight, and electricity acted on small amounts of unorganized, moist, gelatinous matter to produce the simplest animals. He later specified that the simplest plants were spontaneously generated when the same physical substances organized moist, mucilaginous matter. The first traces of organic organization formed by the subtle fluids were simple structures capable of containing certain fluids, such as water, and more complex substances. From this point on, the natural tendency of organic movement toward increasing complexity could take over. In plants and simple animals, the physical cause of this tendency was the constant agitation of the contained fluids by the subtle fluids of the environment (especially the matter of heat), which were not containable and could penetrate the living organism. The result of this agitation was the gradual hollowing out of passages and tubes and the eventual formation of organs and then primitive systems.

Animals with circulatory systems were less directly dependent on the environment because, Lamarck believed, the matter of heat was constantly disengaging from the blood and thus providing an internal stimulus for greater development. Such was the materialistic explanation he gave for the origin of the larger groupings of plants and animals. All differences below that level were explained by variations in the movement of the containable and subtle fluids due to different circumstances, especially the temperature of the environment, and to changing life styles resulting in new habits. Through the use of parts, containable fluids were concentrated and the formation of new organs was accelerated. With new organs and systems, new faculties appeared. Acquired changes were preserved through inheritance.

In 1802 Lamarck dealt briefly with the upper limit of the animal series—man. He cautiously suggested that man was the result of the same processes that had produced all other living organisms. The major obstacle to the inclusion of man in the evolutionary process was his higher mental faculties. At the end of the Recherches, Lamarck offered a possible solution to this problem. Using his chemistry and comparative anatomy, he attempted to provide a materialistic explanation for the functioning of the nervous system. Following Haller's distinction, he maintained that while all animals exhibit irritability, only those with a nervous system experience sensibility or feeling. The degree to which an animal possessed the latter faculties depended on the level of complexity of the nervous system and the movement of the nervous fluid, which was a modified form of fire, similar to electricity. This subject of the evolution of the higher mental faculties underwent major development in the Philosophie zoologique (1809).

This work is the best-known and most extensive presentation of Lamarck's theory of evolution. An expanded version of the 1802 Recherches, it is divided into three sections. The first is a more elaborate analysis of the evidence for increasing levels of