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.

    “complication” observed in the major classificatory groupings of animals and plants. It also presents in more detail Lamarck's two-factor theory of evolution: the natural tendency toward organic complexity as a way of explaining the hierarchical organization of the “masses” and the influence of the environment as the factor responsible for all variations from this norm. In the second part of the Philosophie zoologique, Lamarck developed his views on the physical nature of life, its spontaneous production resulting in simple cellular tissue, and its characteristics at the simplest level, the lower ends of the plant and animal series. While these two parts were very important in summarizing many of his evolutionary views, they do not differ significantly from the positions of 1802.

The third part contains the most important additions to the earlier theories. In this section Lamarck deals in great detail with the problem of a physical explanation for the emergence of the higher mental faculties. Some of the eighteenth-century materialists, such as Maupertuis, had attempted to avoid the question of emergence by making thought a property of matter. Some religious figures went to the other extreme and limited thought to man and his soul. Lamarck's breakthrough was tying a progressive development of higher mental faculties in a physical way to structural development of the nervous system. He had already advanced explanations for the evolution of new structures and systems, and the theories on the nervous system were an extension of these earlier views. Higher mental faculties could emerge precisely because they were a product of increased structural complexity, and in all this a physically defined nervous fluid was crucial. For Lamarck one of the most important events in the evolutionary process was the development of the nervous system, particularly the brain, because at that point animals began to form ideas and control their movements.

There has been great misunderstanding of Lamarck's concept of sentiment intérieur, or inner feeling, as a directing factor in the functioning and evolution of higher animals. Lamarck never believed that the giraffe has a longer neck because it consciously wanted one. Rather, he observed that higher animals were capable of voluntary motion which might become habit (as in a search for food or avoidance of danger) and of involuntary motion, or what we would call reflex action. Lamarck attempted to account for such behavior through the mechanism of the sentiment intérieur, an internal physical feeling resulting from agitation of the nervous fluid. The brain of an animal with an internal physical need, such as hunger, would direct the nervous fluid so as to cause muscular motion to satisfy that need. If this action were constantly repeated, new organs would eventually result. On the other hand, a sudden, strong stimulus, such as a loud noise, would produce a reflex action because of a particular perturbation of the nervous fluid.

The concept of the sentiment intérieur included not only the direct interaction with the physical world but also a more sophisticated level. It could be affected, particularly in human beings, by ideas or moral sensations. Such a view was in keeping with an extension of Condillac's sensationalist psychology and epistemology, especially as expressed by Cabanis and the Idéologues. Moral and aesthetic reactions were thus as physically caused as instinctive or reflex ones; the only difference was that between primary or secondary causation. It is not surprising that Lamarck has many references to Cabanis on the relationship between physique and morale. Lamarck felt he had provided a materialistic account for all the activities involving the nervous system, including instinct, will, memory, judgment, understanding, and imagination. He further developed these views in his last publication, Système analytique des connaissances positives de l'homme (1820).

Next to the Philosophie zoologique, Lamarck's best-known work dealing with evolution is the 1815 “Introduction” to his impressive seven-volume Histoire naturelle des animaux sans vertèbres (1815-1822). In this work he summarized his evolutionary views in four laws. The first law concerns his principle of the natural tendency toward increasing organic complexity as observed in the larger groupings of the plant and animal series. The other three laws explain how changes occurred and account for irregularities below the class level. The second law deals with the way new organs evolve by the indirect influence of the environment on an animal. The use-disuse principle, or third law, accounts for changes in the body as a result of new habits; this principle was not new with Lamarck but was generally accepted. The last law, dealing with the inheritance of acquired characteristics, was necessary after positing a slow, gradual evolution; without it Lamarck would have been unable to explain cumulative change and the emergence of new structures. Too much energy has been spent attacking this last law; because it represents an assumption not believed today, it has been said that this disproves Lamarck's whole theory of evolution. The historical context of Lamarck's thought has been forgotten. Most of his contemporaries believed in the inheritance of acquired characteristics, so much so that they rarely felt any need to offer proof of it. The above summary of the four 1815 laws shows that the basic features of Lamarck's evolutionary theory remained relatively unchanged from 1802.